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University of Groningen

The reactivity of rare-earth metallocenes towards alkynesQuiroga Norambuena, Victor

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Citation for published version (APA):Quiroga Norambuena, V. (2006). The reactivity of rare-earth metallocenes towards alkynes: mechanismand synthetic applications [Groningen]: s.n.

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Rare-earth metallocene propargyl/allenyls

13

2. Rare-earth metallocene propargyl/allenyls

2.1. Introduction Organo rare-earth metal chemistry has witnessed a tremendous growth during the last two decades.1

This spectacular growth in research activities can be attributed to that the fact that many of these compounds exhibit not only unique structural and physical properties, but are highly active catalysts in a variety of catalytic processes as well. Among the different classes of organo rare-earth metal complexes, the hydride and alkyl derivatives are the most studied, as they often display activities dramatically higher than that of comparable transition metal complexes. Other classes, such as alkynyl, aryl and allyl derivatives, have been investigated less extensively, but their preparation, molecular structure and properties are well-documented.

In contrast to allenyl and propargyl derivatives of transition metals, alkali metals and alkaline earth metals, which play an important role in organic synthesis,2 relatively little is known about the corresponding rare-earth metal congeners. η1-Allenyl (I) and -propargyl (III) metal complexes are known to exhibit interesting structural features and an unusual reactivity (Scheme 2-1).3 An aspect that has received much recent attention is their tautomeric behavior. Metal η1-propargyls have been shown to undergo both reversible and irreversible tautomerization into metal η1-allenyls. In addition, well-characterized delocalized π-bonded η3-allenyl/propargyl complexes (II) are known as well.4

To the best of our knowledge, the first report of rare-earth metal allenyl and propargyl derivatives is the reaction of Cp*2LuMe with allene and 2-butyne in the early 1980s.5 A facile 1,3-metal shift of the propargyl to yield the observed allenyl was put forward in a review article (Scheme 2-2), but no experimental data were given in the references. In 1990 Heeres et al. reported a series of spectroscopically characterized alkyl- and silyl-substituted lanthanidocene propargyls Cp*2LnCH2CCR (Ln = La, Ce).6 The reactivity of these derivatives remained largely unexplored and the exact nature of the bonding was unknown, as structural data were lacking.

Scheme 2-2. Early examples of propargyl and allenyl lanthanides.

Lu CH3 + . Lu.

+ CH4

Lu CH3 + Lu.

+ Lu

Scheme 2-1. Structural isomerism in allenyl/propargyl metal complexes.

.R

M

HR

HR

R

M

I II III

R HR

M

Chapter 2

14

Since then, only three studies involving well-defined allenyl/propargyl lanthanide species have appeared in literature, including the spectroscopically characterized yttrium η1-allenyl species A, which is reported to be in a rapid equilibrium with its η1-propargyl isomer A’,7 the structurally characterized η3-propargyl samarium half-sandwhich complexes (e.g. B), incorporating a chelating allenyl/propargyl ligand system,8 and a divalent ytterbium η3-propargyl (C) which has also been structurally characterized (Scheme 2-3).9 The only other reports regarding propargyl and/or allenyl rare-earth metal complexes are reactive intermediates in synthetic organic reactions, prepared in situ via transmetalation reactions of organopalladium intermediates10 or Grignard reagents.11

Lanthanide propargyl derivatives Cp*2LnCH2CCR have been proposed to be the active catalyst in the catalytic cyclodimerization of alkyl- and silyl-substituted 1-methylalk-2-ynes CH3CCR in the presence of Cp*2LnCH(SiMe3)2 (Ln = La, Ce) (Scheme 2-4).6 Motivated by the possibility to apply this reaction to bifunctional aromatic substrates and thereby prepare a novel class of polymers (Chapter 3), a study of the synthesis, structure and reactivity of rare-earth metallocene aryl-substituted propargyls was initiated. The investigations described in this chapter represent the first systematic study of allenyl/propargyl derivatives of group 3 and lanthanide metals and include the first crystallographic structure determination of these derivatives having the common trivalent oxidation state and non-chelating allenyl/propargyl ligands.

2.2. Synthesis of substrates Introduction

In order to explore the scope of propargylic C-H activation of 1-methylalk-1-ynes by rare-earth metallocene derivatives, a series of 1-aryl-1-propynes was prepared. Ortho-substituted phenyl derivatives were prepared to study steric effects and a pentafluorophenyl derivative was prepared to address electronic effects. 1-(2-methylphenyl)-1-propyne (2)

The synthesis of 2-propynyltoluene (2) was straightforward and accomplished by palladium-catalyzed cross-coupling of the corresponding iodoarene and propynylzinc bromide (the Negishi reaction, Scheme 2-5).12 The zinc reagent was prepared in situ from commercially available propynylmagnesium bromide, propyne gas and anhydrous zinc bromide.

Scheme 2-3. Other examples of propargyl and allenyl lanthanides.

SmII

LiNN

SiPh3

Si

SiMe3Me3Si

BH

NNN

NN NYb

SiMe3Me3Si

A A' B C

Y

NN N

MeMe

Y

NN N

MeMe

.

Scheme 2-4. The catalytic cyclodimerization of 1-methylalk-1-ynes.

Cp*2LnCH(SiMe3)2R

R

R

R

R

+

Ln = La, CeR = Me, Et,nPr


15

1-(2,6-dimethylphenyl)-1-propyne (3) The preparation of 2,6-dimethylpropynylbenzene (3) was achieved analogously to 2-propynyltoluene,

but started with the aqueous iododediazotization of 2,6-dimethylaniline, according to a published procedure.13 When 2,6-dimethyliodobenzene was cross-coupled via the Negishi protocol, the corresponding 1-aryl-1-propyne was obtained in high yield and purity. 1-(2,6-diisopropylphenyl)-1-propyne (4)

Unfortunately, the application of a similar strategy in the preparation of 1-(2,6-diisopropylphenyl)-1-propyne (4) was not successful. When 2,6-diisopropylaniline was subjected to the standard iododediazotization methodology, a mixture was obtained which contained 2,6-diisopropylchlorobenzene (41%, GC), 2,6-diisopropylphenol (7%), 2,6-diisopropyliodobenzene (21%) and an unknown compound (m/z 352, 11%), according to GC-MS analysis.14 It was believed that the low solubility of 2,6-diisopropylaniline in aqueous solution allowed for the occurrence of competing reactions.15 The crude yield of the iodo compound (47%, GC) was increased by the addition of ethanol and the use of aqueous hydrogen iodide solution. Surprisingly, the use of copper(I) iodide or copper bronze was also found to increase the crude yield of the desired iodo compound (60-65%, GC).16 Subsequent column chromatography (silica, hexanes) and vacuum distillation (120-140 °C, 5 mmHg) furnished samples of desired aryliodide in reasonable purity (95%, GC). Nonetheless, samples of the corresponding propynylarene (97% pure, GC), prepared via cross-coupling with propynylcopper (vide infra), were unsuitable for the present reactivity studies involving organo rare-earth metallocenes, presumably due to inseparable, unidentified impurities of the iododiazotization reaction.

It should be noted that the synthesis of 2,6-diisopropyliodobenzene (by aqueous iododiazotization of 2,6-diisopropylaniline) has been reported after preparation of this manuscript. In all of these cases, however, experimental details are lacking17 and/or the isolated yields are quite low (35%).18 The synthesis of 2,6-diisopropylbromobenzene is also documented in literature (albeit in low yields and purity either via the

Scheme 2-5. The synthesis of propynylarenes 2 and 3.

2

I85%

(ii)

387%

(i)NH2 I

78%

(ii)

Reaction conditions: (i): CH3CCMgBr (1.2 equiv), ZnBr2 (1.2 equiv), Pd(PPh3)4 (5 mol%), THF, 50 °C. (ii): (1) NaNO2, HCl; (2) KI.

Scheme 2-6. The synthesis of 2,6-diisopropyliodobenzene.

(i)NH2 N2BF4 I

97%

(ii)

78%

(iii)NH2 I

Reaction conditions: (i): HBF4, n-BuONO, 0 °C, EtOH/H2O; (ii): KI, I2, copper bronze, DMSO, 50 °C; (iii) n-BuONO, KI, I2, copper bronze, DMSO, 50-60 °C.

Chapter 2

16

Sandmeyer reaction19 or via aprotic diazotization20), but was not considered in this study, as iodides are generally more reactive than bromides and triflates in transition metal-catalyzed cross-coupling reactions with organometallics.21 In view of the low solubility of 2,6-diisopropylaniline in aqueous hydrogen halide solutions, continued efforts to prepare 2,6-diisopropyliodobenzene were directed along the lines of aprotic diazotization.22

Several reports involving the in situ aprotic diazotization of 2,6-diisopropylanilines (followed by chlorination23a or cyanation23b) and the preparation of substituted iodobenzenes24 (by in situ aprotic diazotization of the corresponding anilines in the presence of iodine) suggested that in situ aprotic iododiazotization of 2,6-diisopropylaniline should, in principle, be feasible, but initial attempts based on these protocols were unsuccessful.25 Based on experiments with 2,6-diisopropylbenzenediazonium tetrafluoroborate,26 it was possible to develop a convenient procedure in which 2,6-diisopropylaniline could directly be transformed into the corresponding iodide by treatment with n-butylnitrite in the presence of iodine, potassium iodide and copper bronze under an inert atmosphere and using dry DMSO as a solvent (Scheme 2-6). The only by-product was 1,3-diisopropylbenzene (<1%, GC/GC-MS) which could completely be separated by column chromatography (silica, hexanes) or vacuum distillation (120-140 °C, 5 mmHg).

In contrast to 2,6-dimethyliodobenzene, its diisopropyl analogue did not react with propynylmagnesium bromide and zinc bromide in the presence of Pd(PPh3)4, not even after a prolonged reaction time or after increasing the reaction temperature and catalyst loading.27 This lack of reactivity can plausibly be ascribed to the steric hindrance of the two ortho-isopropyl groups.28 Other attempts involving the cross-coupling of 2,6-diisopropyliodobenzene with a propynylboronic ester29 (the Suzuki-Miyaura reaction30) and the use of 2,6-diisopropylphenyl triflate were equally disappointing.31

An early report in which the substitution of 2,4,6-triisopropyliodobenzene was achieved by a copper(I) acetylide32, albeit in low yield, prompted us to explore this reaction (the Castro reaction) in the present context.33 Motivated by the well-recognized highly explosive nature of copper(I) acetylides,34 attempts were undertaken to develop a convenient one-pot procedure.35 These attempts failed, however, and the Castro reaction had to be performed with isolated propynylcopper (prepared in ammonia from propyne gas, according to a modified published procedure36). The potentially explosive copper compound was kept moist with ether after isolation and added directly in a large excess to a solution of the 2,6-diisopropyliodobenzene in dry pyridine. Heating the suspension at reflux under an inert nitrogen atmosphere for several days afforded the desired propynylarene (4) in good yield (Scheme 2-7). The only contaminants in the crude product mixture were 1,3-diisopropylbenzene (4%, GC/GC-MS) and 2,6-diisopropyliodobenzene (5%) which could be removed to a large extent after repeated column chromatography (silica, hexanes).37 The final sample of 4 contained small amounts of 1,3-diisopropylbenzene (<2%, 1H NMR). As this compound was considered to be relatively inert towards the rare-earth metallocene derivatives in subsequent reactivity studies, no further attempts were undertaken to obtain a sample of higher purity. 1-Pentafluorophenyl-1-propyne (5)

In order to investigate electronic effects in the propynylaromatic substrates as well, it was decided to prepare pentafluoro-propynylbenzene (5). According to a modification of a general procedure, pentafluorophenylpropyne (5) was prepared in a low yield (24%) from the reaction of propynyllithium with excess hexafluorobenzene in THF at low temperature (Scheme 2-8).38 Propynyllithium was generated in situ from propyne gas and n-butyllithium in THF at -70 °C. The product mixture was evaporated to dryness after

Scheme 2-7. The synthesis of 1-(2,6-diisopropylbenzene)-1-propyne 4.

CH3C CH(i)

CuC CCH3

4

I

76%

(ii)+ CuC CCH3

CuSO4 +

Reaction conditions: (i): NH2OH·HCl, NH3, RT (aq); (ii): pyridine, 110-120 °C.


17

which the product was conveniently isolated by sublimation. The major by-product of the reaction was identified as 1,4-dipropynylhexa-fluorobenzene.

2.3. Synthesis of propargyl/allenyls

2.3.1. Introduction

As synthetic routes in organo rare-earth metal chemistry starting from halide derivatives are known to be plagued by the undesired incorporation of alkali metal salts and solvents, the most straightforward synthetic strategy towards Cp*2LnCH2CCR complexes via salt metathesis reaction of a halide precursor and a propargylic metal salt was a priori considered to be problematic.39 Owing to this tendency to form “ate” complexes, Lewis-base free complexes are typically prepared from alkyl or hydride precursors in the organometallic chemistry of rare-earth metals.

The alkyl derivative of the type Cp*2LnCH(SiMe3)2 represents the most commonly used and well-characterized entry into the synthetic and reactive chemistry of Lewis-base free, 14-electron lanthanidocenes.40 Attempts to prepare Lewis-base free, monomeric propargyl/allenyl rare-earth metal complexes started therefore with reactivity studies of Cp*2LnCH(SiMe3)2 (6) and [Cp*2Ln(µ-H)]2 (7) (Ln = La, Y) towards 1-methylalk-1-ynes.

2.3.2. Reactions of alkyls with 1-aryl-1-propynes

1-Phenyl-1-propyne (1) and ortho-substituted 1-phenyl-1-propynes (2-4) When an equimolar amount of 1-phenyl-1-propyne (1) and Cp*2LaCH(SiMe3)2 (6a) was allowed to

react in benzene-d6, no reaction was observed by 1H NMR spectroscopy after several days. The clean, but slow formation of the desired propargyl species Cp*2LaCH2CCPh (7a) was apparent upon warming the reaction mixture to 80 °C. For example, 2% of 6a was converted into 7a and CH2(SiMe3)2 after 3 hours at 80 °C (Scheme 2-9). The rate of propargylic metalation was only moderately increased by increasing the reaction temperature. For example, 14% of 6a was converted into 7a and CH2(SiMe3)2 after 20 hours at 100 °C. The use of an excess amount of substrate, on the other hand, resulted in a significant acceleration of propargylic metalation, accompanied by the formation of several organic products, as indicated by 1H NMR spectroscopy and GC-MS analysis after quenching the reaction mixture with methanol-d4 (Section 2.5.6). For example, the use of a 10-fold molar excess of substrate led to the complete conversion of 6a into 7a, CH2(SiMe3)2 and several organic products after 2 hours at 100 °C.

Analogous reactions of Cp*2YCH(SiMe3)2 (6b) with 1-phenyl-1-propyne (1) and Cp*2LaCH(SiMe3)2 (6a) with CH3CCC6H4Me-2 (2), CH3CCC6H4Me2-2,6 (3) and CH3CCC6H4

iPr2-2,6 (4) gave similar results, but the corresponding propargyl species Cp*2LnCH2CCAr (7-10) was formed at a considerably lower rate (Scheme 2-9). For example, a stoichiometric mixture of 6b and 1 gave 5% conversion of the alkyl derivative (6b) into the propargyl derivative (7b) after 2 days at 100 °C.

These reactions allowed for the spectroscopic characterization of the corresponding propargyls (7-10) by means of multinuclear 1D and 2D NMR spectroscopy. In all cases, the Cp* ligands were magnetically

Scheme 2-8. The synthesis of propynylarene 5.

5

F

F

FF

F

F F

F

F

F

F24%

(i)+ LiC CCH3

Reaction conditions: (i): THF, -60 °C, 2 h.

Chapter 2

18

equivalent and one CH2 signal was observed in the 1H NMR spectrum, suggesting that the propargyls Cp*2Ln(η3-CH2CCAr) in solution adopt either a static η3-propargyl structure or exist as a rapid equilibrium mixture of the η1-propargyl and η1-allenyl tautomers. Variable-temperature 1H and 13C NMR studies of Cp*2LaCH2CCPh (7a), Cp*2YCH2CCPh (7b) and Cp*2LaCCC6H3Me2-2,6 (9a) in toluene-d8 down to -80 °C and up to 120 °C indicated that these complexes are nonfluxional on the 1H and 13C NMR time scale in this temperature range. The details of NMR and infrared spectroscopic parameters are discussed in Section 2.4.

These initial reactivity studies demonstrated that the reactions of the alkyl derivatives Cp*2LnCH(SiMe3)2 (6) with 1-aryl-1-propynes (1-4) proceed via slow and selective propargylic C-H activation of the 1-aryl-1-propynes, even at elevated temperatures. The slow rate of propargylic metalation renders the preparation of propargyl derivatives Cp*2LnCH2CCAr (7-10) via the reactions of 6 with 1-aryl-1-propynes inconvenient. Although the rate of reaction was considerably increased by the use of an excess amount of substrate, the concomitantly formed organic products impeded the isolation of 7 by fractional crystallization and extraction. As a consequence, the preparation of Cp*2LnCH2CCAr (7-10) from the reaction of 6 and 1-4 were found to be impractical and other routes towards these propargyls were explored. 1-Pentafluorophenyl-1-propyne (5)

In marked contrast to the reactions of Cp*2LnCH(SiMe3)2 6 (Ln = La, Y) with phenylacetylene and ortho-substituted analogues, such as 2-ethynyltoluene and (2,6-dimethylphenyl)acetylene, that proceed via rapid acetylenic metalation, even at low temperatures (Chapter 4), the present reactions of 6 with 1-phenyl-1-propyne and ortho-substituted analogues were found to proceed via slow propargylic metalation. It seems natural to ascribe the lower rate of propargylic C-H activation relative to acetylenic C-H activation to the lower (kinetic) acidity of the propargylic C-H bond. A study of the lithiation reactions of a series of 1-aryl-1-propynes revealed that the rate of propargylic lithiation of 1-methyl-1-alkynes increases with the electron-withdrawing capacity of the aromatic group.41 Accordingly, the reaction of 6 and 1-pentafluorophenyl-1-propyne (5) was conducted to investigate whether propargylic metalation by Cp*2LnCH(SiMe3)2 (6) could be increased upon increasing the acidity of the propargylic C-H bond in this system.42 The electron-withdrawing and carbanion-stabilizing capacity of the pentafluorophenyl group is well established.43

When a mixture of Cp*2LaCH(SiMe3)2 (6a) and a stoichiometric amount of CH3CCC6F5 (5) was heated in benzene-d6 to 50 °C for 1 day, no reaction was observed by 1H NMR spectroscopy. The slow formation of the propargyl Cp*2LaCH2CCC6F5 (11a) and CH2(SiMe3)2 was observed after heating the reaction mixture for several hours at 80 °C. For example, 12% of 6a was converted after 52 hours at 80 °C. Complete conversion of 6a into 11a and CH2(SiMe3)2 was observed after heating the reaction mixture for 13 days at 80 °C, albeit less selectively as observed previously for 1-phenyl-1-propyne and its ortho-substituted analogues (vide supra). Although the reaction of Cp*2LaCH(SiMe3)2 (6a) and CH3CCC6F5 (5) yielded 11a as the major organometallic product, based on the presence of one major Cp* 1H NMR resonance at δ 1.80 ppm, the yield was only 47%, based on integration versus CH2(SiMe3)2.

Scheme 2-9. The reactions of 6 with 1-5.

Ln = La (a), Y(b)

6 1-5 7-11

∆Cp*2LnCH2C CArCp*2LnCH(SiMe3)2 + + CH2(SiMe3)2CH3C CAr

Ar

F F

F

FF

CH3CCAr 1 2 3 4 5 Cp*2LnCH2CCAr 7 8 9 10 11


19

Multinuclear NMR spectroscopy and GC/GC-MS analysis of the reaction mixture after quenching with H2O and D2O plausibly ruled out the occurrence of H/D scrambling at the Cp* methyl groups and C-F activation of 5, but failed to shed light on the identities of the formed organometallic by-products. Because quantitative 1H NMR analysis indicated that 0.90 equiv. of 5 was converted relative to CH2(SiMe3)2 after complete conversion of 6a, it is believed that the low yield of Cp*2LaCH2CCC6F5 (11a) is the result of the limited thermal stability of 11a. Additional support for this view is provided by the observed lower yield of 11a (25%), when the analogous reaction was conducted at 100 °C.

The validity of the linear relationship between the rate of proton transfer from a carbon acid and the first-order carbon-hydrogen coupling constant of the corresponding carbon (1JCH) has been demonstrated by many studies.44 The rationale for this behavior is found in the relative amount of s-character of the hybrid orbital on the carbon atom. As the s-character increases, the hybrid orbital is more tightly bound to the carbon atom and the C-H bond becomes more polar, thereby increasing the acidity of the hydrogen atom. Conversely, the formed anion is the most stable in the orbital with the highest s-character. Hence, the following relative order of increasing kinetic acidity was found: 3 ≈ 4 < 1 < 5, on the basis of the first-order carbon-hydrogen coupling constant of the propargylic methyl group (1JCH).45

It seems that the anticipated rate increase of propargylic C-H activation is only modest upon increasing the kinetic acidity of the propargylic methyl group by substituting the phenyl group in 1-pheny-1-propyne with a pentafluorophenyl group. In addition, experminents indicated that the corresponding propargyl Cp*2LaCH2CCC6F5 (11a) was thermally less stable than its non-fluorinated analogues (7-10), decomposing into unidentified compounds after prolonged times at relatively high temperatures. Thus, both the relative slow rate and thermal instability limit the utility of the present reaction as a convenient preparative route towards propargyl derivatives of rare-earth metallocenes.

2.3.3. Reactions of hydrides with 1-aryl-1-propynes

1-Phenyl-1-propyne (1) Because the formation of the corresponding propargyl was slow from the reaction of alkyl derivatives

Cp*2LnCH(SiMe3)2 (6) with 1-aryl-1-propynes (1-5), analogous reactions of [Cp*2Ln(µ-H)]2 (12) were studied as a more convenient preparative route towards propargyl derivatives of rare-earth metallocenes Cp*2LnCH2CCAr (7-10). When 0.5 equiv. of 1-phenyl-1-propyne was added to a solution of [Cp*2La(µ-H)]2 (12a) in benzene-d6, a complex reaction mixture formed with a time-dependent composition, as indicated by in situ 1H NMR spectroscopy. After 5 min at room temperature, the hydride was converted completely into three major Cp*2La species (on the basis of Cp* 1H NMR resonances at δ 1.92, 1.90 and 1.88 ppm) of which the propargyl species Cp*2LaCH2CCPh (δ 1.92 ppm) constituted only 31% of the total intensity of the Cp* 1H NMR

Table 2-1. NMR and IR spectral data of the 1-aryl-1-propynes.a

ArCCH3C321

Entry Ar C-1 (1JCH)

C-2 (2JCH)

C-3 (3JCH)

H-1 νC≡ C (cm-1)

1 Ph (1)

3.92 (131.4)

80.46 (4.8)

86.08 (10.2)

1.64 2250

2 C6H3Me2-2,6 (3)

4.18 (131.0)

78.08 (4.5)

94.20 (10.5)

1.74 2243

3 C6H3iPr2-2,6

(4) 4.26

(131.0) 77.19 (4.7)

93.59 (10.5)

1.75 2243

4 C6F5 (5)

4.02 (133.0)

64.34 (4.6)

99.84 (10.8)

1.50 2254

a NMR measurements performed in C6D6 at 25 °C under a nitrogen atmosphere. Chemical shifts are reported in ppm and coupling constants in Hz. Repeated experiments suggested that 13C chemical shifts were accurate to ± 0.02 ppm, 1H chemical shifts to ± 0.01 ppm and 1JCH coupling constants to ±0.2 Hz. IR measurements as liquid films or nujol mulls.

Chapter 2

20

resonances present. In addition, cis-1-phenylprop-1-ene was identified in the reaction mixture and its concentration increased slowly in time.

(2.1)

[Cp*2La(µ-H)]2 + 2

Cp*2LaCH2C CPh + + + PhPh

LaCp*2

Ph

Cp*2La

CH3C CPh12a

7a 13a 14a

Interestingly, the propargyl derivative became the major organometallic product present (75%, 1H

NMR) after standing for 1 day at room temperature. GC/GC-MS and 1H NMR analysis of the major organic products in the reaction mixture after addition of methanol-d4 indicated the presence of cis-1-phenylprop-1-ene, cis-1-phenylprop-1-ene-d1, phenylallene-d1 and 1-phenyl-1-propyne-d1. The latter two compounds are formed from deuterolyis of Cp*2LaCH2CCPh (7a) (Section 2.5.2). The use of toluene-d8 as solvent gave similar results, thereby indicating that the reactions described above do not involve solvent molecules, as previously observed for lanthanide hydrides and alkyls in aromatic solvents.46

These results can plausibly be rationalized by proposing three separate pathways by which a monomeric hydride derivative reacts with 1-phenyl-1-propyne (Scheme 2-10). Although no spectroscopic evidence for Cp*2LaH was obtained in this study, ample evidence exist in literature that hydride derivatives of rare-earth metallocenes [Cp*2Ln(µ-H)]2 (12) are present in solution as an equilibrium between dimer and monomer.47 The formed monomeric hydride derivative may, in principle, undergo both alkyne insertion into the La-H bond and propargylic C-H activation. Propargylic metalation of 1-phenyl-1-propyne by Cp*2LaH yields 7a and dihydrogen. The reversible nature of propargylic metalation was demonstrated by the independent reaction of 7a with excess dihydrogen (1 atm.). In this reaction, 7a was consumed within several hours, giving rise to phenylpropane and [Cp*2La(µ-H)2] (12a) over the course of 1 day at room temperature. Although the relatively low rate of the reaction of 7a with excess H2 may suggest that reversible metalation of Cp*2LaH by 1-phenyl-1-propyne is only of minor importance in the present system, efficient hydrogen capture is not without precedent.48

Insertion of the carbon-carbon triple bond into the La-H bond of the monomeric hydride derivative may take place both in a 2,1- and 1,2-manner, affording the corresponding 1-phenylprop-1-en-1-yl 12a and 1-phenylprop-1-en-2-yl derivatives 13a, respectively. Clearly, the ratio of these insertion products is determined by

Scheme 2-10. The proposed reaction sequences in the reaction of 12a with 1.

13a 14a

Cp*2LaH

HLaCp*2

Cp*2La H

CH3C CPh

H2

CH3

HCp*2La

PhCH3C CPh

PhH2

Cp*2LaCH2C CPh

CH3C CPhPh

HCp*2La

CH3

CH3C CPh

Ph

Ph

H2

7a

12a

CH3C CPh

H2

CH3C CPh Ph


21

the electronic and steric properties of the substituents at the carbon-carbon triple bond and those of the metal complex and is difficult to predict a priori. 1,2-Insertion may be favored under steric control,49 but 2,1-insertion of 1-arylalk-1-ynes and 1-arylalk-1-enes is commonly observed in rare-earth metal chemistry (Chapter 4).50 It should also be noted that no evidence for reversible alkyne insertion was obtained in the reaction of [Cp*2Ln(µ-H)]2 with diphenylacetylene.51

The transient alkenyl species Cp*LaC(Ph)=C(CH3)H (13a) and Cp*LaC(CH3)=C(Ph)H (14a) were inferred from the presence of cis-1-phenylprop-1-ene and the formation of cis-1-phenylprop-1-ene-d1 upon quenching the reaction mixture with methanol-d4. However, the relative preference for 2,1- or 1,2-insertion in this system could not be determined by means of NMR spectroscopy or GC-MS analysis. Attempts to spectroscopically identify the proposed 1-phenylprop-1-en-1-yl and 1-phenylprop-1-en-2-yl derivatives were hampered by the complexity of the 1H NMR spectrum and overlapping signals.

The reaction of the formed propenyl derivatives with 1-phenyl-1-propyne may give rise to the observed cis-1-phenylprop-1-ene via propargylic C-H activation. Evidence for this hypothesis was provided by the reaction of Cp*2LaC(Ph)=C(Ph)H (15a) with 1-phenyl-1-propyne, affording Cp*2LaCH2CCPh (7a) and cis-diphenylethene (Scheme 2-11). This alkenyl derivative was conveniently prepared from ([Cp*2La(µ-H)]2 and diphenylethyne. Another route towards the observed cis-1-phenylprop-1-ene may plausibly involve the reaction of the propenyl derivatives with dihydrogen formed from propargylic metalation of 1-phenyl-1-propyne, as implicated by the reaction of Cp*2LaCH2CCPh with excess dihydrogen, giving rise to phenylpropane and [Cp*2La(µ-H)]2.52

Because the reaction of the hydride [Cp*2La(µ-H)]2 (12a) and 1-phenyl-1-propyne (1) was not found to be clean at room temperature, it was decided to investigate whether the proposed competition between propargylic C-H activation and alkyne insertion could be influenced in the favor of propargylic C-H activation by lowering the reaction temperature. Thus, the reaction of 12a and 1 (2 equiv.) in toluene-d8 at -60 °C was monitored with 1H NMR spectroscopy. The 1H NMR spectrum of the reaction mixture was more complex, however, exhibiting nine Cp* resonances of which the propargyl represented only 13% of the total Cp* 1H NMR resonance intensities. No significant changes in the composition were observed upon slowly warming to room temperature and further standing. The presence of cis-1-phenylprop-1-ene-d1, phenylallene-d1, 1-phenyl-1-propyne-d1 and several unidentified compounds was indicated by NMR spectroscopy and GC-MS analysis after quenching with methanol-d4. The occurrence of insertion reactions of unsaturated substrates into La-C bonds is ruled out by the observed masses of the reaction products. These considerations suggest that alternative C-H activation sequences compete more effectively with propargylic C-H activation at lower reaction temperatures. Examples of lanthanidocene derivatives undergoing allylic, vinylic and aromatic C-H activation are well-documented.53 The above reaction sequences imply that alkyne insertion is competitive with propargylic C-H activation due to reversible propargylic metalation in a closed atmosphere. Following this line of reasoning, it was put forward that the removal of dihydrogen gas may render propargylic metalation irreversible, thereby suppressing alkyne insertion under the above reaction conditions. This notion was confirmed by performing the reactions under dynamic vacuum, affording the propargyl in high yields (~90%, 1H NMR). Even so, the presence of cis-1-phenyl-1-propene indicated that alkyne insertion was still competitive with propargylic C-H activation under these reaction conditions. The desired propargyl Cp*2LaCH2CCPh (7a) was successfully prepared by performing the reaction of the hydride 12a in an aliphatic (e.g. pentane, hexane) or aromatic solvent at room temperature under dynamic vacuum. Also, the use of an excess amount of substrate was found to be necessary to compensate for the seemingly unavoidable hydrogenation of the substrate. A preparative more convenient method that avoids the

Scheme 2-11. The preparation of Cp*2LaC(Ph)=C(Ph)H (15a) and its reaction with 1-phenyl-1-propyne.

12a

Ph

HCp*2La

Ph

CH3C CPh+PhPh

[Cp*2La(µ-H)]2 + PhPh

Ph

HCp*2La

PhCp*2LaCH2C CPh +

7a

15a

Chapter 2

22

handling of the highly reactive hydride involves the in situ formation of the hydride [Cp*2Ln(µ-H)]2 from the alkyl Cp*2LnCH(SiMe3)2 (Scheme 2-12). Cp*2LaCH2CCPh (7a) is extremely soluble in alkane and aromatic solvents and was initially isolated as a red oil which did not easily form crystals.54,55 A crystalline solid was obtained after complete in vacuo removal of the organic compounds in the crude product mixture (e.g. cis-1-phenyl-1-propene, CH2(SiMe3)2, unreacted 1-phenyl-1-propyne) and prolonged cooling at -80 °C. Recrystallization from toluene afforded single-crystals suitable for an X-ray crystallographic study in which 7a was identified as an η3-propargyl/allenyl complex in the solid state. Structural data is discussed in more detail in Section 2.4.4. A similar methodology also afforded the yttrium analogue Cp*2YCH2CCPh (7b) in good isolated yields and single-crystals suitable for X-ray analysis were obtained analogously. Ortho-substituted 1-phenyl-1-propynes (3-4) When [Cp*2La(µ-H)]2 (12a) was allowed to react in benzene-d6 at room temperature with 2 equiv. of CH3CCC6H4Me2-2,6 (3), an instantaneous reaction was observed with 1H NMR spectroscopy. Although propargylic C-H activation was found to be the major pathway, forming the propargyl Cp*2LaCH2CCC6H4Me2-2,6 (9a) quite selectively (>90%, 1H NMR), hydrogenation was still operative, as indicated by the presence of cis-1-(2,6-dimethylphenyl)prop-1-ene (1H NMR, GC-MS) in the reaction mixture. When the reaction mixture was allowed to stand at room temperature for several days, no significant changes were observed with 1H NMR spectroscopy. The complex Cp*2LaCH2CCC6H4Me2-2,6 (9a) was isolated on a preparative scale by employing the above methodology consisting of the in situ preparation of the hydride derivative and the application of dynamic vacuum. However, the isolated yield was low (22%), most likely due incomplete removal of organic products from the reaction mixture, thereby hampering the crystallization of the propargyl. Unfortunately, attempts to obtain single-crystals suitable for X-ray analysis were unsuccessful. The reaction of [Cp*2La(µ-H)]2 (12a) with 0.5 equiv of CH3CCC6H4

iPr2-2,6 (4) in benzene-d6 gave two major Cp* resonances (δ 2.22, 1.92) in the 1H NMR spectrum after 1 h at room temperature of which the propargyl was the major species (δ 1.92, 58%). Again, the substrate was partially hydrogenated, as indicated by the presence of cis-1-(2,6-diisopropylphenyl)-1-propene (1H NMR, GC-MS). After 2 days at room temperature both the concentration of the propargyl (74%) and hydrogenated substrate increased. Unfortunately, no crystalline product could be isolated at a preparative scale from the reaction mixture of Cp*2LaCH(SiMe3)2 and 4, following the above procedure. The organic products were difficult to remove in vacuo and unidentified organometallic reaction products could not be separated by extraction with appropriate solvents. 1-Pentafluorophenyl-1-propyne (5)

When the prop-1-ynylpentafluorobenzene (5) (1.0 equiv per La) was added to a solution of 12a in benzene-d6 at rom temperature, a complex time-dependent reaction mixture formed, as indicated by 1H NMR spectroscopy. After 10 min the 1H NMR resonances of the hydride and substrate had completely disappeared and four major Cp* 1H NMR resonances were observed (δ 1.88, 1.84, 1.83 and 1.80 ppm) of which the propargyl Cp*2LaCH2CCC6F5 (11a: δ 1.88) represented only 7% of the total Cp* 1H NMR resonance intensities. Attempts to identify the major organometallic products by means of NMR spectroscopy failed. After two days standing at room temperature, the concentration of 11a increased to 14%, while more Cp* 1H NMR resonances appeared concomitantly. 19F NMR spectroscopy indicated the presence of at least four different pentafluorophenyl groups in a 0.4:1.0:0.3:0.3 ratio. When the reaction mixture was quenched with methanol-d4 after 2 days at room temperature, NMR and GC/GC-MS analysis indicated the presence of two major organic products that were plausibly identified as 1-pentafluorophenyl-1-propene-d1 and the monodeuterated coupling product C6F5C(D)=C(Me)C(C6F5)=CHMe in a ratio of 1.00:0.33, respectively. The geometry of the double bonds in the latter double insertion product could not be determined unequivocally and the proposed structure is based on the

Scheme 2-12. The preparation of Cp*2LnCH2CCPh (7) (Ln = Y b, La a) from the reaction of the hydride precursor, generated via in situ hydrogenolysis, and 1-phenyl-1-propyne.

Cp*2LnCH(SiMe3)2 Cp*2LnCH2C CPh + H2

Ln = La (91%), Y (92%)

H2 CH3C CPh

-CH2(SiMe3)2 vacuum


23

well-recognized tendency of organo rare-earth metal complexes to give 2,1-insertion products of 1-aryl-1-alkenes and 1-aryl-1-alkynes into metal-carbon and metal-hydrogen bonds ( Scheme 2-13).50

Based on the inferred presence of the propenyl derivative, Cp*2LaC(C6F5)=CH(CH3) (16a) could be characterized with 1H and 19F NMR spectroscopy (δ 7.67, q, J = 6.7 Hz, CH, 1 H; 1.84, s, Cp*, 30 H; 1.51, d, J = 6.7 Hz, CH3, 3 H). This assignment is further supported by the close resemblance of its 1H NMR spectral data with that of Cp*2LaC(Ph)=CH(Ph) (i.e. the vinylic proton resonates at δ 7.69 ppm and the Cp* ligands appear as one singlet at δ 1.84 ppm) (vide supra). The complexity of the 1H NMR spectrum thwarted attempts to characterize Cp*2LaC(C6F5)=C(CH3)C(C6F5)=CH(CH3) (17a) unambiguously.

2.4. Structural characterization of propargyl/allenyls

2.4.1. Introduction

The Cp*2LnCH2CCAr complexes exhibit equivalent Cp* and CH2 resonances in the 1H and 13C NMR spectrum indicative of either a static η3-propargyl structure or a rapid equilibrium mixture of the η1-propargyl and η1-allenyl species. Because no 1H and 13C NMR resonances attributable to distinct η1-propargyl and η1-allenyl derivatives of Cp*2LaCH2CCPh (7a), Cp*2YCH2CCPh (7b) and Cp*2LaCCC6H3Me2-2,6 (9a) were observed at -80 °C, the former picture is presently favored. To investigate this issue further, these compounds were also characterized by 13C NMR and infrared spectroscopy and, in some cases, by X-ray crystallography.

Furthermore, spectroscopic parameters are likely to reflect structural changes in the present Cp*2LnCH2CCAr complexes resulting from differences in the steric and electronic properties of the aryl-substituted ligands employed in this study. If the Cp*2LnCH2CCAr complexes are static η3-propargyls (E), it is believed that substitution at the ortho-positions of the phenyl ring will shift the η3-propargyl structure closer to a more η1-propargyl-like structure (D) in the continuum of canonical structures, due to the larger steric hindrance between the Cp* ligands and the ortho-substituent in the η1-allenyl-like structure (F) (Scheme 2-14). Alternatively, if the Cp*2LnCH2CCAr complexes are engaged in a rapid equilibrium mixture between the η1-

Scheme 2-13. The proposed reaction sequences of the reaction of 12a with 5.

[Cp*2La(µ-H)]2 HCp*2La

C6F5

+ +

Cp*2La

C6F5

C6F5

H

CD3OD

CD3OD

HD

C6F5

D

C6F5

C6F5

HC6D625 °C

16a

17a

2 CH3C CC6F5

Scheme 2-14. The possible limiting structures of Cp*2LnCH2CCAr.

D E F

Ln

HH

RRLn

.H

H

RR

Ln

H

R

R

H

Chapter 2

24

propargylic (D) and η1-allenylic tautomer (F), the formation of F is likely to be disfavored upon ortho-substitution and more D will be present in the equilibrium mixture.

2.4.2. Spectroscopic properties of the propargyl/allenyls

The 1H and 13C NMR resonances of the rare-earth metallocene propargyl derivatives Cp*2LnCH2CCAr are shown in Table 2-2. The CH2

proton resonances are observed between δ 3.08 and 2.71 ppm. These values suggest a considerable contribution from the propargylic structure, as η1-bound allenylic methylene protons typically resonate within the region of δ 3.9-4.3 ppm and those associated with the propargylic tautomer lie at higher field between δ 1.75 and 3.0 ppm (Table 2-3).3c

The 13C NMR spectra exhibit downfield shifted CH2 propargyl carbon resonances (C-1, δ 47.7-57.8 ppm) relative to the neutral 1-phenyl-1-propyne ligand (δ 3.92 ppm, see Table 2-1). The chemical shifts are in the range of values observed for other substituted, sp3 α-carbons in rare-earth metallocenes, such as Cp*2Y[η3-CH(Ph)CH2Ph]56 (δ 56.40 ppm, 1JCH = 128 Hz), Cp*2La[η3-CH(Ph)CH2Ph]56 (δ 49.20 ppm, 1JCH = 131 Hz) and Cp*2YCH2C6H3Me2-3,5 (δ 47.35 ppm, 1JCH = 133.4 Hz).57 The first-order carbon-hydrogen coupling constants 1JCH (156.7-162.5 Hz), on the other hand, are significantly larger than those observed in the above complexes and reveal that the C-H bonds of the propargylic carbon have considerably more s character. The values are close to that of a sp2 carbon (typically 156.4 Hz) and may be interpreted as to discard η1-propargyl bonding (sp3 hybridization is expected for the propargylic carbon in an η1-propargyl, Scheme 2-16). However, such an unambiguous interpretation is complicated by the fact that inductive/field effects originating from polar substituents influence 1JCH values to such an extent that the 1JCH values mostly do not reflect the true hybridization state of the carbon.58 The CH2CC central carbon resonances (C-2, δ 145.6-165.2 ppm) are closer to the central carbon atom of the neutral phenylallene (δ 209.9 ppm) than that of 1-phenyl-1-propyne (δ 79.9 ppm), thereby suggesting a high degree of allenylic character for the central carbon. Also, the CH2CC carbon resonances (C-3, δ 100.3-112.8 ppm) are closer to that of the corresponding carbon atom of phenylallene than to that of 1-phenyl-1-propyne (δ 85.9 ppm).

Extensive studies in organolithium chemistry have shown that the central carbon (C-2, δ 165-180 ppm for allenyl) and the propargyl carbon (C-1, δ 90-100 ppm) are very sensitive indicators of allenylic and propargylic structures.59 Carbon NMR resonances and associated 1JCH values have also been used to differentiate between η1-allenyl and η1-propargyl structures for other metal complexes, but it is presently difficult to make generalizations based on the limited data reported in literature.3c-d Nevertheless, when the NMR spectral data of the present rare-earth metal complexes are compared to that of reported (phenyl-substituted) allenyl and propargyl metal complexes, it can be seen that the observed spectral parameters point to an η3-allenyl/propargyl

Table 2-2. Spectroscopic data of the Cp*2LnCH2CCAr complexes.a Entry Ln, Ar CH2 Cp* C-1

(1JCH, 1JYC) C-2 C-3

(1JYC) νC≡ C

(cm-1) 1 Y, Ph

(7b) 2.82 1.93 49.33

(159.1, 5.2) 155.09 106.79

(11.9) 1920

2 La, Ph (7a)

2.82 1.91 54.00 (158.3)

152.19 112.75 1944

3 La, C6H3Me2-2,6 (9a)

2.71 1.92 49.00 (156.9)

149.28 105.79 1975

4 La, C6H3iPr2-2,6

(10a) 2.68 1.92 47.69

(156.7) 145.61 100.31 c

5 La, C6F5

(11a) 3.08 1.88 55.77

(162.5) 165.24 108.76 c

6 CH3CCPh - - 24.9 79.9 85.9 2251 7b CH2CCHPh - - 78.9 209.9 94.1 1940

a NMR measurements in C6D6 at 25 °C under a nitrogen atmosphere. The chemical shifts are reported in ppm and coupling constants in Hz. IR measurements under a nitrogen atmosphere. NMR measurements in CDCl3 at 25 °C for entries 6 and 7. b Data taken from Ref. 67. c Compounds not isolated.


25

structure (Table 2-3). The observation that the 13C NMR chemical shifts are intermediate of those observed for η1-CH2CCPh and η1-C(Ph)=C=CH2 complexes is in accord with this view (Entries 8 and 9).

Infrared spectroscopy has been widely used as a diagnostic tool in determining the relative contribution of allenylic and propargylic structures.2,59 In the 2250-1750 cm-1 region of the infrared spectrum, strong absorptions at 1944 and 1920 cm-1 are observed for Cp*2LaCH2CCPh (7a) and Cp*2YCH2CCPh (7b), respectively (Figure 2-8). Comparison with reported data indicates that these values are intermediate of those typically observed for σ-propargyl (2100-2215 cm-1) and σ-allenyl metal complexes (1800-1920 cm-1),60 thereby providing additional support to the view that the present rare-earth metallocene complexes adopt an η3-propargyl/allenyl structure.

Remarkably, comparison with IR data of reported η3-propargyl/allenyl complexes is difficult, because IR data of these compounds are rare in literature.3d-f One of the few literature examples involves the crystallographically characterized Cp2ZrMe(η3-CH2CCPh), which is reported to display two medium absorptions at 2176 and 1924 cm-1.4b,61 These vibrations were not assigned and their understanding is not clear-cut, because a static η3-bonding would be expected to lead to a single absorption in this region. The observation of two absorptions in the 1800-1920 and 2100-2215 cm-1 range has, in fact, typically been interpreted in terms of an equilibrium between η1-allenylic and η1-propargylic organometallic species.2 However, no fluxional behavior was observed in solution Cp2ZrMe(η3-CH2CCPh) down to -40 °C and the authors did not comment on this ambiguity. The titanocene derivative Cp*2Ti(CH2CCPh) is reported to exhibit only one strong absorption at 1902 cm-1 in this region and was assigned the η3-hapticity by comparison with Cp2ZrMe(η3-CH2CCPh).62 Following this line of reasoning, it seems that both NMR and IR spectroscopy are consistent with an η3-allenyl/propargyl structure for the present rare-earth metal complexes.

Yttrium has a nucleus with I = ½ in a natural abundance of 100% and a coupling interaction in Cp*2YCH2CCPh (7b) is observed with the terminal carbons of the CH2CC ligand (Table 2-2). These couplings are in agreement with the nature of metal CH2CC bonding. According to several molecular orbital studies, the CH2CCR ligand binds mainly through its terminal carbons, while the π-bond of the carbon-carbon triple bond does not interact significantly with the metal fragment.63 The observed yttrium-carbon coupling of the propargyl carbon (5.9 Hz) is notably smaller than that of other yttrocene-bonded methylene carbons, such as Cp*2Y(CH2C6H3Me2-3,5)64 (1JYC = 23.2 Hz) and Cp*2Y[η3-CH(Ph)CH2Ph]56 (1JYC = 18.7 Hz), but is comparable to that of the terminal allyl carbon in Cp*2Y(η3-CH2CH=CH2)40 (1JYC = 3.8 Hz) and the smallest coupling

Table 2-3. Selected NMR data for allenyl and propargyl complexes.a Entry Complex CH2 C-1

(1JCH) C-2 C-3 Ref.

1 [(PPh3)2Pt(η3-CH2CCPh)]+ 2.74 48.3 (170) 97.3 102.1 [4f,e] 2 [(PPh3)2Pd(η3-CH2CCPh)]+ 3.19 50.9 94.2 105.4 [4g] 3 Cp2ZrMe(η3-CH2CCPh) 3.37 55.5 (167) 120.5 114.1 [4b] 4 CpMo(η3-CH2CCPh)(η2-CH3CCPh) 3.90 41.6

(158, 165) 142.3 124.8 [63b]

5 Cp2Zr(CH2CCPh)2 2.80 30.7 (151) 128.4 103.5 [4b] 6 Cp*(TBM)Zr(η3-CH2CCMe) 2.51 51.0 99.9 91.7 [4h] 7 [Cp*2Zr(η3-CH2CCMe)]+ 2.93 61.7 (158) 128.4 103.5 [88] 8 [Pt(CO)(PPh3)2(η1-CH2CCPh)]+ 1.98 7.1 92.2 86.1 [4e] 9 [Pt(CO)(PPh3)2(η1-CPhCCH2)]+ 3.65 72.9 203.1 101.5 [4e]

a Chemical shifts in ppm and coupling constants in Hz.

Scheme 2-15. Reported first-order yttrium-carbon coupling constants in [Cp2Y(µ-η1:η1-C,N-HC=NCtBu)]2 and [(Cp*2Y)2(µ-O,C-OCH=CHC≡ C)].

O

YCp*2

Cp*2YCp2Y YCp2

N

N

35 Hz

5 Hz

12 Hz

53 Hz

Chapter 2

26

constant in the asymmetrically bridging formimidoyl carbon in [Cp2Y(µ-η1:η1-N,C-HC=NCtBu)]2 (1JYC = 5 Hz, 1JY’C = 35 Hz) (Scheme 2-15).65 The yttrium-carbon coupling constant of the terminal carbon (11.9 Hz) in 7b is similar to that observed for the terminal acetylide carbon with the distant Cp*2Y moiety in [(Cp*2Y)2(µ-O,C-OCH=CHC≡ C)] (1JYC = 12 Hz, 1JY’C = 53 Hz) (Scheme 2-15).66 These findings reveal that the observed yttrium-carbon coupling constants are consistent with the proposed delocalized metal η3-CH2CCPh bonding.

2.4.3. Ligand and metal ion size effects

The observed NMR spectral parameters of the propargyl derivatives Cp*2LnCH2CCAr clearly indicate that the chemical shift of the propargylic CH2 protons varies with the nature of the aromatic group (δ 3.08-2.71 ppm), while that of the Cp* ligand changes only little (δ 1.93-1.88 ppm) (Table 2-2). In fact, the CH2 proton resonances move upfield with increasing ortho-substitution in the Cp*2LaCH2CCC6H3R2-2,6 (R = H 7a, Me 9a, iPr 10a) series. In view of the relatively high field proton resonance of the allenylic CH2 group in phenylallene67 (δ 4.85 ppm) and η1-allenyl metal complexes (e.g. entry 8, Table 2-3) as compared to the the relatively low field proton resonance of the propargylic CH2 group in 1-phenyl-1-propyne (δ 1.65 ppm) and η1-propargyl metal complexes (e.g. entry 9, Table 2-3), this finding suggests a shift to either a more propargyl-like η3-propargyl structure or a shift towards the formation of the η1-propargyl in an equilibrium mixture between a η1-propargyl and a η1-allenyl derivative.3c

Replacing the phenyl group by a pentafluorophenyl group seems to have the opposite effect, favoring either a more allenyl-like η3-propargyl structure or the formation of the η1-allenyl in the equilibrium mixture.68 It seems reasonable to ascribe this change in character to the α-carbanion-stabilizing property of the pentafluorophenyl group.43 Electronic substituent effects in propargyl-allenyl equilibria of organoindium derivatives have been reported.69 For these compounds, both experimental and theoretical data revealed that the electron-donating nature of a methyl group at the terminal carbon destabilizes the sp2 carbanion in the allenylic structure relative to the propargylic structure, thereby favoring the formation of the η1-propargyl species.

The 13C NMR spectral data within the Cp*2LaCH2CCAr series are in agreement with the trend observed with 1H NMR spectroscopy (Table 2-2). When the steric bulk at the ortho-positions of Cp*2LaCH2CCAr is increased, the CH2 and CH2CC resonances shift upfield and CH2CC resonances shift downfield, consistent with the view that an increase of steric bulk at the ortho-positions increases the propargylic character of Cp*2LaCH2CCAr. When the phenyl group in Cp*2LaCH2CCPh is replaced by a pentafluorophenyl group, the opposite change in character is observed with 13C NMR spectroscopy (i.e. the CH2 and CH2CC resonances shift downfield and the CH2CC shift upfield) which is in agreement with the proposed increase in allenylic character.

20

40

60

80

100

120

140

160

180

200

130.5 131 131.5 132 132.5 133 133.51JCH (Hz)

Car

bon

chem

ical

shi

ft (p

pm) C-1 C-2 C-3

Figure 2-1. Plot of the carbon chemical shifts of the propargylic (C-1), internal (C-2) and terminal (C-3) carbon atoms of the CH2CC group in 7a, 9a, 10a and 11a (Table 2.2) versus the first-order carbon-hydrogen coupling constant (1JCH) of the propargylic methyl group of the neutral ligand CH3CCAr (Table 2.1).


27

Another interesting feature of the NMR spectral data concerns the first-order carbon-hydrogen couplings of the propargylic carbon (C-1) in the Cp*2LaCH2CCAr series. It can be seen that these 1JCH values (in Hz) decrease in the same order as the order of increasing propargylic character based on the carbon chemical shifts, i.e. C6F5 (162.5) > C6H5 (158.3) > C6H3Me2-2,6 (156.9) > C6H3

iPr2-2,6 (156.7). Although 1JCH couplings are well-known to correlate with the hybridization of the carbon atom in hydrocarbons, polar substituents have a much greater effect on 1JCH couplings and their presence renders these couplings unsuitable for estimating the s character of the carbon bonding orbital in some cases.58 Assuming that the polar effects of the metal center are constant in the series Cp*2LaCCAr, the decrease in 1JCH for the propargylic carbon arguably reflects a change in hybridization which can be interpreted as a change from a sp2-type carbon in H and I to a more sp3-type carbon in G (Scheme 2-16).70

Additional evidence for the proposed changes in the bonding of the present Cp*2LaCH2CCAr complexes as a function of the steric and electronic properties of the ligand is supplied by comparing the spectral properties of the complexes with those of the corresponding neutral ligand. Based on both NMR spectroscopy, the relative σ-electron-withdrawing ability of the ligand CH3CCAr was found to increase in the following order: 3 (C6H3Me2-2,6, 131.0) ≈ 4 (C6H3

iPr2-2,6, 131.0) < 1 (Ph, 131.4) < 5 (C6F5, 133.0) (Table 2-1). It can be seen that several structurally diagnostic spectral parameters of the Cp*2LaCH2CCAr complexes, such as the proton chemical shift of the CH2 group (Figure 2-1), the carbon chemical shifts of the C3 ligand (Figure 2-2) and the first-order carbon-hydrogen coupling constant of the CH2 group (Figure 2-3), correlate in a similar manner with the σ-electron withdrawing ability of the ligand. These spectral parameters indicate also that the propargylic character of Cp*2LaCH2CCC6H3Me2-2,6 is somewhat higher than that of Cp*2LaCH2CCC6H3

iPr2-2,6, despite the fact that the σ-electron withdrawing ability of their aromatic substituents is similar. This minor discrepancy may plausibly be ascribed to the increased steric bulk of the 2,6-diisopropylphenyl group relative to the 2,6-dimethylphenyl group. In summary, these findings strongly suggest that the bonding of Cp*2LaCH2CCAr becomes increasingly more propargylic in character upon increasing the steric bulk of the aromatic substituent

2.4

2.6

2.8

3

3.2

3.4

130.5 131 131.5 132 132.5 133 133.51JCH (Hz)

Pro

ton

chem

ical

shi

ft (p

pm)

Figure 2-2. Plot of the proton chemical shift of the CH2 group of 7a, 9a, 10a and 11a (Table 2.2) versus the first-order carbon-hydrogen coupling constant (1JCH) of the propargylic methyl group of the neutral ligand CH3CCAr (Table 2.1)

Scheme 2-16. The hybridization of the terminal carbon atoms in the limiting structures of the prop-2-ynyl/allenyl anion.70

CH2 C CH CH2 C CH CH2 C CH- -

sp3 sp sp2 sp sp2 sp2

G H I

C C C HHH

C C CH

HH

C C C H

HH

Chapter 2

28

and more allenylic in character upon an increase in the σ-electron-withdrawing ability of the aromatic substituent.

When the lanthanum metal center is substituted by the smaller yttrium,71 the relative position of the η3-allenyl/propargyl ligand in either an η3-allenyl/propargyl continuum or an η1-allenyl/η1-propargyl equilibrium mixture is difficult to ascertain spectroscopically. A clear-cut interpretation of the observed spectral parameters is complicated by a difference in inductive/field effects of the metals.72 Also, the solid-state molecular structures of 7a and 7b did not provide additional insight into the influence of the metal ion size on the nature of bonding of the η3-allenyl/propargyl ligand in 7a and 7b (Section 2.4.4).

2.4.4. The molecular structures of Cp*2LnCH2CCPh (Ln = La, Y).

In order to shed more light on the nature of the bonding in the present propargyl derivatives Cp*2LnCH2CCAr, X-ray crystallographic studies were performed on Cp*2LaCH2CCPh (7a) and Cp*2YCH2CCPh (7b). In both cases, single crystals were grown from toluene solutions at low temperature. Complexes 7a and 7b crystallize in the orthorhombic space groups Pna21 with Z = 4 and Pca2a1 with Z = 4, respectively. The crystal structures of 7a and 7b are shown in Figures 2-3 and 2-4, respectively, and selected bond distances and angles are given in Table 2-4. The bent metallocene in 7a is similar to that in other lanthanum73 and lanthanide metallocenes.74 The 137.1° Ct1-La-Ct2 angle and the average 2.54 La-Ct1 and 2.51 Å La-Ct2 distances are normal. The C5Me5 are in the usual staggered conformation with a twist angle of 28.9° compared to 36° for a perfectly staggered arrangement.75

The phenyl-substituted propargyl/allenyl ligand is clearly bound in an η3-mode. The La-C(propargyl) distances for La-C21, La-C22 and La-C23 are 2.811(6), 2.695(7) and 2.740(7) Å, respectively. At first glance, the main interaction of the metal center with the C3 ligand center would seem to take place on C22. However, theoretical studies have shown that the metal bonding with an η3-propargyl/allenyl ligand occurs through the terminal carbon atoms.63 In agreement with delocalized η3-bonding, the observed La-C21 and La-C22 distances are longer than that of reported La-C σ-bond lengths, such as 2.537(5) and 2.588(4) Å in Cp*La[CH(SiMe3)2]2 and 2.651(8) and 2.627(10) Å in Cp*La[CH(SiMe3)2]2(THF).76

The backbone of the η3-propargyl/allenyl ligand is virtually coplanar with La (atom displacements from the least squares plane: La 0.000, C21 0.001, C22 0.003, C23 0.002 Å). Such coplanar rearrangements have also been reported for other η3-propargyl/allenyl complexes and this structural feature is believed to be characteristic of η3-propargyl/allenyl complexes.3d-f The atoms La, C21, C22 and C23 all lie in the mirror plane that reflects the two Cp* ligands of 7a. This symmetry, obviously, results in the observed magnetic equivalence of the two hydrogen atoms bonded to C21 and the Cp* ligands in the 1H and 13C NMR spectrum of 7a (Section 2.4.2). The crystallographic C21-C22 and C22-C23 bond distances of 1.36(1) and 1.23(1) Å, respectively, are

150

155

160

165

170

130.5 131 131.5 132 132.5 133 133.51J CH (Hz)

1 JC

H (H

z)

Figure 2-3. Plot of the first-order carbon-hydrogen coupling constant (1JCH) of the propargylic CH2 group in 7a, 9a, 10a and 11a (Table 2.1) versus the first-order carbon-hydrogen coupling constant (1JCH) of the propargylic methyl group of the neutral ligand CH3CCAr (Table 2.2)


29

Figure 2-4. The molecular structure of 7a with thermal ellipsoids drawn at the 50% probability level. The hydrogen atoms are omitted for clarity.

Figure 2-5. The molecular structure of 7b with thermal ellipsoids drawn at the 50% probability level. The hydrogen atoms are omitted for clarity. intermediate between those generally accepted for C-C single (1.45 Å) and double (1.31 Å) and double and triple (1.20 Å) bonds, respectively, and the C3 ligand backbone is thus consistent with resonance between propargyl and allenyl structures. The relatively large difference of 0.13(1) Å between the two C-C bond lengths suggests that the propargylic contribution to the bonding description of 7a is relatively more important than the allenylic contribution. Although both of the resonance forms of the η3-propargyl/allenyl ligand would be expected to lead to a linear geometry at the central carbon atom, the bond angle C21-C22-C23 of159.3(7)° indicates a considerable deviation from linearity. This structural feature appears to be another characteristic of η3-

propargyl/allenyl complexes, as angles of 145-155° have been reported for other (phenyl-substituted) η3-propargyl/allenyl complexes (Table 2-4).3d-f

The overall geometry of Cp*2YCH2CCPh (7b) is similar to that of its lanthanum congener 7a. The Y-C bond distances are smaller and reflect the differences in Y3+ (1.019 Å) and La3+ (1.160 Å) ionic radii for eight-coordination.71 The metrical parameters corresponding to the Cp*2Y fragment of 7b are not exceptional, as

Chapter 2

30

values for the Ct1-Y-Ct2 angle (139.3°) and the average Y-Ct distances (2.36 Å) are normal.77 The C5Me5 ligands are in a staggered conformation with a twist angle of 24.1°. The Y-C(propargyl) bond lengths of 2.653(3), 2.529(3) and 2.560(3) Å are longer than that of reported Y-C σ-bond lengths, such as 2.44(2) Å in Cp*2YMe(THF)78a, 2.468(7) Å in Cp*2YCH(SiMe3)2.78b It is interesting to note that the smaller bond distance of Y-C23 (2.560(3) Å) relative to Y-C21 (2.653(3) Å) is reflected by the smaller yttrium-carbon coupling constant of the propargylic carbon (1JCH = 5.2 Hz) relative to that of the terminal carbon (1JCH = 11.9 Hz).

Similar to Cp*2LaCH2CCPh (7a), Cp*2YCH2CCPh (7b) exhibits a nearly coplanar rearrangement of the C3 propargyl carbons and the metal center, albeit slightly more distorted (atom displacements from the least squares plane: Y 0.001, C21 0.005, C22 0.009, C23 0.005 Å). Most of the observed differences corresponding to the propargyl/allenyl ligand of 7a and 7b are within experimental error, but there are two exceptions. Firstly, the C21-C22-C23 angle of 155.9(3)° indicates that the C3 ligand is more bent the yttrium derivative. It seems natural to ascribe this to the smaller size of yttrium. Secondly, the phenyl group is almost coplanar with the plane defined by the C3 ligand and the metal in 7b, whereas the phenyl group is virtually perpendicular to the same plane in 7a. This difference may also be the result of the smaller size of the metal center, as the more proximate Cp* ligands are likely to force the phenyl group more into the equatorial girdle, but other effects resulting from

Table 2-5. Selected bond lengths (Å) and angles (°) in Cp*2LnCH2CCPh (Ln = La, Y) complexes.a

Ln = La (7a) Ln = Y (7b) Bond lengths

C21-C22 1.362(10) C21-C22 1.366(4) C22-C23 1.234(10) C22-C23 1.268(4) C23-C24 1.465(9) C23-C24 1.462(4) La-C21 2.811(6) Y-C21 2.653(3) La-C22 2.695(7) Y-C22 2.529(3) La-C23 2.740(7) Y-C23 2.560(3) av. La-C(Cp*) 2.806(16) av. Y-C(Cp*) 2.654(4) 2.780(16) 2.652(4) La-Ct1 2.535(3) Y-Ct1 2.362 La-Ct2 2.509(4) Y-Ct2 2.362

Bond angles Ct1-La-Ct2 137.1(1) Ct1-Y-Ct2 139.3 C21-C22-C23 159.3(7) C21-C22-C23 155.9(3) C22-C23-C24 141.2(7) C22-C23-C24 141.5(3) La-C21-C22 71.0(4) Y-C21-C22 70.2(2) La-C23-C22 74.9(4) Y-C23-C22 74.7(2)

Torsion angles C22-C23-C24-C29 77.9(14) C22-C23-C24-C25 174.6(4) C21-C22-C23-C24 177.0(16) C21-C22-C23-C24 172.9(6) a Cp*1 = C1-C5, Cp*2 = C11-C15, Ct = centroid.

Table 2-4. Selected bond distances (Å) and angles (°) for 7a and 7b and related compounds CpZr(Me)(η3-CH2CCPh)4b (D), Cp*Zr(TBM)(η3-CH2CCMe)4h (E), CpMo(η2-MeCCPh)(η3-CH2CCPh)63b (F) and [(PPh3)2Pt(η3-CH2CCPh)]OTf 4f (G).

Complex 7a 7b D E F G C1-C2 1.362(10) 1.366(4) 1.344(5) 1.376(7) 1.404(9) 1.395(14) C2-C3 1.234(10) 1.268(4) 1.259(4) 1.218(7) 1.281(8) 1.227(13) M-C1 2.811(6) 2.653(3) 2.658(4) 2.498(3) 2.278(6) 2.186(11) M-C2 2.695(7) 2.529(3) 2.438(3) 2.408(4) 2.164(5) 2.150(8) M-C3 2.740(7) 2.560(3) 2.361(3) 2.444(5) 2.105(5) 2.273(9) α 159.3(7) 155.9(3) 155.4(3) - 146.1(6) 152.2(9)

RHH

M

12

3α


31

conjugation and solid-state packing cannot be excluded. When comparing the structural data of 7a and 7b with that of other (phenyl-substituted)

propargyl/allenyl complexes reported in literature (Table 2-4), it can be seen that they have similar structures. In all cases, the metal propargylic carbon (C1) distance is larger than the metal internal carbon (C2) distance. The M-C1 distance is larger than the M-C3 distance for CpZr(Me)(η3-CH2CCPh), Cp*Zr(TBM)(η3-CH2CCMe), CpMo(η2-MeCCPh)(η3-CH2CCPh) and smaller for [(PPh3)2Pt(η3-CH2CCPh)]OTf. This observation may be related to the relative contribution of the allenyl or propargyl resonance structure in the η3-propargyl/allenyl complexes, but classification of the present η3-propargyl/allenyl rare-earth metal complexes in either of these two groups is hampered by the observed experimental error. The carbon-carbon distances in the η3-propargyl/allenyl ligand are similar for all complexes, the C1-C2 distance being larger than the C2-C3 distance in all complexes, and the angle α varies from 145 to 160°, seemingly both as a function of metal radius and the coordination environment.

2.5. Reactivity of propargyl/allenyls

2.5.1. Introduction

Spectroscopic and structural analysis of the lanthanidocene propargyls has provided evidence for static η3-propargyl/allenyl structures. Further insight into the properties of this largely unexplored class of rare-earth metallocenes was obtained by exploring their reactivity towards a variety of substrates. The documented reactivity of η3-propargyl/allenyl transition-metal complexes is dominated by the addition of nucleophiles to cationic complexes and the reported reactions may be classified as nucleophilic additions to the metal or the nucleophilic, regiospecific additions to the central carbon of the η3-propargyl/allenyl ligand (Scheme 2-17).3d-f When the nucleophile adds to the metal, η1-propargyl or η1-allenyl complexes are formed, depending on the nature of the nucleophile. These η1-propargyl and η1-allenyl complexes may display further reactivity, such as insertion of unsaturated reagents into the M-CH2 bond and the addition of electrophiles to the ligand.3a-c

If the η1-tautomers are kinetically accessible, the reactive chemistry of present η3-propargyl/allenyl

complexes will most likely involve reactions of η1-propargyl and η1-allenyl derivatives, depending on their relative importance. It seems reasonable to expect that the η1-propargylic and η1-allenylic tautomer exhibit each distinct C-H activation and insertion reaction sequences, in analogy to rare-earth metalllocene η1-alkyl and η1-hydride derivatives. Examples of rare-earth metal complexes containing ligands undergoing a conversion from ηn- to η1-bonding include allyl Cp*2Ln(η3-CH2CH=CHR) and tris(pentamethylcyclo-pentadienyl) derivatives Cp*3Ln. Permethyllanthanidocene allyls undergo insertion reactions with ethylene, CO and CO2 and the bonding of the allyl ligand is fluxional in solution.79,80 Although these findings indicate that access to an η1 form is facile in solution, the crystallographic studies obtained so far have shown that the allyl ligands adopt η3-structures in the solid state. Similarly, the observed reactivity of Cp*3Sm with ethylene, H2, CO, PhCN and PhCNO is in agreement with that of an η1-alkyl, but no spectroscopic or structural evidence for a Cp*2Sm(η1-C5Me5) species has been observed in solution or in the solid state.81

Scheme 2-17. Typical reactions of η3-propargyl/allenyl metal complexes.

Y[M]

[M]X

+

YH

X

[M]

X

X

[M]

X

[M].

+

-

Chapter 2

32

2.5.2. The reactivity towards protic acids

Methanol When Cp*2YCH2CCPh (7b), Cp*2LaCH2CCPh (7a) and Cp*2LaCH2CCC6H3Me2-2,6 (8a) were

allowed to reacted with a an excess of methanol-d4 (5-10-fold molar excess) in benzene-d6, the orange solutions turned yellow instantaneously. On the basis of NMR and GC-MS analysis, the organic reaction products were identified as the corresponding acetylenic ArCCCH2D and allenylic ArCD=C=CH2 deuterolysis products (Ar = Ph, C6H3Me2-2,6) (Scheme 2-18). Because the relative amount of allenylic and acetylenic quenchings products is expected to reflect the relative importance of the η1-propargylic and η1-allenylic tautomers in the present η3-allenyl/propargyl complexes, analogous reactions with methanol were conducted for Cp*2LnCH2CCAr. The ratios of acetylenic and allenylic products were determined by in situ 1H NMR spectroscopy (using appropriate long pulse delays to avoid signal saturation under the present anaerobic conditions and long experiment times to allow for reliable signal-to-noise ratios) and are shown in Table 2-6.

NMR spectroscopy indicated the following order of increasing propargylic character within the lanthanum series: Cp*2LaCH2CCC6F5 (10a) < Cp*2LaCH2CCPh (7a) < Cp*2LaCH2CCC6H3Me2-2,6 (8a) < Cp*2LaCH2-CCC6H3

iPr2-2,6 (9a) (Section 2.4.3). Surprisingly, the relative amount of acetylenic quenching product did not increase with the degree of propargylic bonding in the above η3-allenyl/propargyl complexes Cp*2LnCH2CCAr. Instead, the relative amount of acetylenic quenching product was found to decrease with the steric size of the substituent of the η3-propargyl/alleny ligand. Moreover, the higher relative amount of acetylenic quenching product for 7a as compared to 7b suggests that the formation of acetylenic quenching products increases with the coordination space around the metal center under the assumption that the bonding in Cp*2LaCH2CCPh (7a) and Cp*2YCH2CCPh (7b) have a similar degree of propargylic character (Section 2.4.3).

These results can plausibly be rationalized as proceeding via intial Lewis base coordination of methanol (Scheme 2-19). Nucleophilic attack at the metal center may take place at the substituted side or at the unsubstituted side of the η3-propargyl/allenyl ligand. Obviously, the latter mode of nucleophilic attack is sterically favored. The formation of the Lewis-base adducts is followed by the electrophilic attack of the

Scheme 2-18. The reactions of the lanthanidocene η3-propargyl/allenyl complexes with methanol-d4.

CD3ODAr

D+

D

Ar

Ln = La, Ar = Ph (7a), C6H3Me2-2,6 (8a)Ln = Y, Ar = Ph (7b)

Cp*2Ln

Ar

Table 2-6. Allene/alkyne ratios for the reaction of Cp*2LnCH2CCAr with methanol.a

Entry Ln, Ar alkyne:allene 1 Y, C6H5 (7b) 1.00:1.00 2 La, C6H5 (7a) 1.11:1.00 3 La, C6H5Me2-2,6 (8a) 1.06:1.00 4 La, C6H5

iPr2-2,6 (9a) 0.58:1.00 5 La, C6F5 (10a) 1.17:1.00 a Reactions in benzene-d6 at room temperature. The ratios were determined by in situ 1H NMR spectroscopy.

Cp*2Ln

Ar

CH3OHAr + .

Ar


33

polarized proton on the nearest carbon of the η3-propargyl/allenyl ligand. As a result, the η1-propargyl-like Lewis base adduct J, formed from nucleophilic base attack at the unsubstituted side of the η3-propargyl/allenyl ligand, will give rise to the acetylenic quenching product upon protonolysis and the η1-allenyl-like Lewis base adduct K, formed from nucleophilic base attack at the substituted side of the η3-propargyl/allenyl ligand, will give rise to the allenylic quenching product. The formation of Lewis bases is well-documented for transition-metal η3-propargyl/allenyl complexes,3d-f while initial Lewis base coordination has previously been proposed for the reactions of rare-earth metallocene alkyl derivatives Cp*2LnR (Ln = Sc102a, R = Me; Ln = Y82a, La82b, Ce82b, R= CH(SiMe3)2) with alcohols ROH.

(2.2) 7a

18a 19a

Cp*2La

LaCp*2

Ph

Ph

.

Ph Ph

Cp*2La

Ph

Ph

Ph

Ph

Ph

Cp*2La + + + +

+ PhC CH

Phenylacetylene

The above results revealed that Lewis base coordination of methanol at the unsubstituted side is only moderately favored over Lewis base coordination at the phenyl-substituted side of the η3-propargyl/allenyl ligand in Cp*2LaCH2CCPh (7a) and Cp*2YCH2CCPh (7b). This finding suggests that electronic effects are more important than steric effects in determining the side of nucleophilic attack at the metal center in the propargyl complexes Cp*2LnCH2CCPh. In this light, it was decided to study the effect of another Brønsted acid having different Lewis basic properties. The choice of phenylacetylene was based on the relatively high acidity of the acetylenic proton and the soft Lewis basic character of both the carbon-carbon triple bond and the phenyl group.83 Methanol, in contrast, is typically classified as a hard Lewis base, according to Pearson’s hard-soft-acid-base (HSAB) principle.84

Reactions of Cp*2La(η3-CH2CCPh) (7a) with phenylacetylene were performed in benzene-d6 at room temperature and their progress was monitored by in situ 1H NMR spectroscopy, using hexamethyldisiloxane (HMDSO) as an internal standard. When a stoichiometric amount of phenylacetylene was added to a benzene-d6 solution of 7a at room temperature, the light-orange solution turned immediately dark red and only 57% of the propargyl was converted upon complete consumption of phenylacetylene. The propargyl derivative (7a) was

Scheme 2-19. The proposed mechanism for the reactions of the Cp*2Ln(η3-CH2CCAr) complexes with methanol-d4.

J

K

CD3ODLn

Ar

Cp*2Ln ODC

CD3

Ar.

ArD

.D

ArCp*2Ln

ArO DD3C

Cp*2Ln

Ar

OCD3

DCp*2Ln C

DO

HH

Ar

CD3

+

+

Cp*2LnOCD3

Cp*2LnOCD3

Chapter 2

34

converted into two new organometallic products that were identified as the butatrienediyl 18a (24%) and the but-1-en-3-yn-1-yl derivative 19a (9%), on the basis of NMR spectroscopy (Chapter 4). The major organic products in the reaction mixture were identified as the protonated η3-propargyl/allenyl ligand (phenylallene and 1-phenyl-1-propyne in a 1.00:0.20 ratio, respectively) and trans-1,4-diphenylbut-1-en-3-yne (eq. 2.2).

Upon standing at room temperature the amount of phenylallene and 19a decreased under the formation of two unidentified lanthanocene species having Cp* 1H NMR resonances at δ 1.97 and 1.92 ppm, while the amount of 18a and 7a did not change significantly. When 7a was allowed to react with a two- and ten-fold molar excess of phenylacetylene, 80% and 88% of 7a was converted, respectively, upon complete consumption of phenylacetylene. In both cases, phenylacetylene was converted into phenylallene, 1-phenyl-1-propyne, (E)-1,4-diphenylbut-1-en-3-yne and traces of 2,4-diphenylbut-1-en-3-yne, according to NMR and GC-MS analysis.

The formation of phenylacetylene dimers and the observation of lanthanocene derivatives, previously observed in the lanthanocene-catalyzed oligomerization reaction of phenylacetylene, strongly suggest that the reaction of 7a with phenylacetylene gives rise to the monomeric, alkynyl derivative Cp*2LaCCPh which has been implicated as the active catalyst in the permethyllanthanocene-catalyzed oligomerization reaction of phenylacetylene (Chapter 4). Monomeric, permethyllanthanidocene alkynyl derivatives Cp*2LnCCR are unstable and dimerize into dinuclear, µ-alkynyl derivatives [Cp*2Ln(µ-CCR)]2. In some cases, these dimeric, alkynyl derivatives undergo subsequent C-C coupling to form butatrienediyl derivatives [(Cp*2Ln)2(µ-RC=C=C=CR)].85 Examples of the latter two types of dinuclear compounds have been isolated and characterized, but no spectroscopic or structural evidence for monomeric, lanthanidocene alkynyl derivatives have been reported in literature. The present results reveal that Cp*2LaCCPh reacts faster with phenylacetylene than Cp*2La(η3-CH2CCPh) (7a), thereby accounting for the incomplete conversion of 7a in the reaction with excess phenylacetylene.

Another feature of the reaction of 7a with phenylacetylene concerns the reactivity of Cp*2LaC(Ph)=C(H)CCPh (19a) with phenylallene. The reactivity of allenes and alkenyl metal derivatives is relatively unexplored in organo rare-earth metal chemistry.86 Even so, both C-H activation and insertion reactions are plausible processes, in analogy to the well-documented reactive chemistry of rare-earth metallocene η1-alkyl and η1-hydride derivatives.8 Reactions of 19a with phenylallene, proceeding via insertion into the La-C bond, yield organic compounds C25H20 after protonolysis. GC-MS analysis did not supply evidence for the presence of such organic compounds after quenching reaction mixtures with H2O, thereby excluding the possibility that 19a reacts with phenylallene via insertion reactions.87

Nonregioselective C-H activation of phenylallene by Cp*2LaC(Ph)=C(H)CCPh (19a) yields, in principle, two new allenyl lanthanocene species and Cp*2La(η3-CH2CCPh) (Scheme 2-21). This scenario accounts for the observation of two new lanthanocene species, having Cp* 1H NMR resonances at δ 1.97 and 1.92 ppm (vide supra). Unfortunately, the complexity of the reaction mixtures thwarted attempts to identify these

Scheme 2-20. The proposed reaction sequences for the reaction of 7a with phenylacetylene at room temperature.

Ph

Ph

.Ph

Ph

PhPh

Cp*2La

Ph

PhPh

Cp*2La

Ph

+

Cp*2La LaCp*2

Ph

Ph

Cp*2LaC CPh

Ph

Ph

Ph

Ph

LaCp*2Cp*2La

Ph

7a

19a

20a

18a


35

proposed allenyl derivatives unequivocally and quenching experiments with H2O and D2O did provide experimental evidence to support this hypothesis. For the sake of convenience, these allenylic derivatives will be referred to as Cp*2LaX (δ 1.92 ppm) and Cp*2LaY (δ 1.97 ppm) in the proceeding discussion. Nnonselective reactivity of allenylic substrates towards alkyl d0 metal complexes has also been observed for isolectronic group 4 metallocene cations.88 In an effort to obtain additional evidence for the above proposed reaction sequences, reactions of Cp*2La(η3-CH2CCPh) (7a) with phenylacetylene were performed in toluene-d8 at low temperatures and monitored by normalized, in situ 1H NMR spectroscopy. Phenylacetylene (1.1 equiv.) was condensed onto a toluene-d8 solution of 7a at -196 °C and allowed to warm up to -60 °C in the probe of the spectrometer. 1H NMR spectroscopy indicated quantitative conversion of 7a into the dimeric, bridged alkynyl derivative [Cp*2La(µ-CCPh)]2 (20a) (one singlet for Cp* at δ 2.21 ppm, see Figure 2-6) and Cp*2LaC(Ph)=C(H)CCPh (19a) (one singlet for Cp* at δ 2.01 ppm), accompanied by the formation of phenylallene and 1-phenyl-1-propyne (in a 1.00:0.08 ratio, respectively).

Slowly warming the reaction mixture to room temperature resulted in the conversion of the dimeric alkynyl derivative [Cp*2La(µ-CCPh)]2 (20a) into the butatrienediyl species [(Cp*2La)2(µ-η3:η3-PhC=C=C=CPh)] (18a) (one singlet for Cp* at δ 2.04 ppm which overlaps at low temperatures with the solvent, see Figure 2-6)

Scheme 2-21. The possible C-H activation reaction sequences of Cp*2LaC(Ph)=C(H)CCPh with phenylallene.

Cp*2La

Ph

Ph

Cp*2La

Ph

.Ph

H+ +

Ph

Ph

Cp*2La

Ph

Ph

Cp*2La

Ph

.Ph

H+ +

Ph

Ph

Ph

Cp*2La

Ph

.Ph H

+ +

Ph

Ph

Cp*2La Ph

2.20 2.10 2.00 1.90 ppm

(b)(a)

(c)(d)(e)

(h)(g)(f)

(i)(j)

Figure 2-6. Variable-temperature 500 MHz 1H NMR spectra of the Cp* region during the reaction of 7a and phenylacetylene at low temperature in C7D8 under the following conditions: (a, upper line) 7a at -60 °C, (b) addition of phenylacetylene at -60 °C, (c) -50 °C, (d) -40 °C, (e) -30°C , (f) -20 °C, (g) -10° C, (h) 0 °C, (i) 10 °C, (j, lower line) after 14 h at 25 °C. See text for details.

Chapter 2

36

and the reformation of 7a (one singlet for Cp* at δ 1.90 ppm). The C-C coupling reaction of 20a to yield 17a has been studied in more detail for reactions of Cp*2LaCH(SiMe3)2 and [Cp*2La(µ-H)]2 with phenylacetylene (Chapter 4). At higher temperature, the complete conversion of phenylallene was observed, accompanied by the conversion of 19a (one singlet for Cp* at δ 1.97 ppm) into Cp*2LaX (δ 1.92 ppm) and Cp*2LaY (δ 1.97 ppm). Finally, the slow formation of a new lanthanocene derivative having a Cp* 1H NMR resonance at δ 2.01 ppm was also observed. This derivative was also formed as a minor product in the low-temperature reactions of Cp*2LaCH(SiMe3)2 and [Cp*2La(µ-H)]2 with phenylacetylene and was tentatively assigned to an alkynyl derivative of unknown structure (Chapter 4). The rapid and quantitative conversion of Cp*2La(η3-CH2CCPh) (7a) at low temperature followed by its reformation at higher temperature was unexpected and studied in more detail by a quantitative analysis.

0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10 11

Con

cent

ratio

n (m

M)

phenylallene 7a

20a 19a

18a Cp*2LaY

Cp*2LaX

-60 °C -50 °C -40 °C -30 °C -20 °C -10 °C 0 °C 10 °C 25 °C

Figure 2-7. Concentration profile of the reaction of 7a and phenylacetylene at low temperature in C7D8, followed in time during slowly warming to room temperature. Reaction conditions: (1) 7a at -60 °C, 10 min; (2) 10 min after addition of phenylacetylene at -60 °C; (3) after 10 min at -50 °C; (4) after 10 min at -40 °C; (5) after 10 min at -30 °C; (6) after 10 min at -20 °C; (7) after 10 min at -10° C, (8) after 10 min at 0 °C, (9) after 10 min at 10 °C, (10) after 10 min at 25 °C; (11) after 14 h at 25 °C. The lines connecting the data points are drawn as guides for the eye. All species were monitored by normalized, 500 MHz 1H NMR spectroscopy.

Scheme 2-22. The reaction sequences proposed to account for the observed reactivity of phenylallene during the reaction of 7a with phenylacetylene at low temperature and subsequent warming to room temperature.

Ph

.Ph

Ph

Cp*2La

Ph

Cp*2LaC CPh+Cp*2La LaCp*2

Ph

Ph

Cp*2La

Ph

+ +

Cp*2LaC CPh +

Ph

Cp*2La

Ph

.Ph

+

Ph

Cp*2La

Ph

+ Cp*2LaX + Cp*2LaY +

Ph

Ph

20a 7a

7a

19a

19a


37

Kinetic profiles of the reaction products were obtained by monitoring their concentration in time by means of normalized, in situ 1H NMR spectroscopy using an internal reference (hexamethyldisiloxane) and appropriate long pulse delays (Figure 2-7). The rapid and quantitative protonolysis of Cp*2La(η3-CH2CCPh) (7a) by phenylacetylene affords Cp*2LaCCPh which dimerizes into 20a. The experimental data indicate that the formed dimeric bridged alkynyl derivative 20a reacts via two distinct processes at -30 °C. The reaction of 20a with phenylallene competes with intramolecular C-C coupling of 20a to yield 17a. The kinetic profiles indicate also that the reaction of 20a with phenylallene affords 7a, selectively. The concomitant increase in concentration of 19a undubitately originates from the reaction of Cp*2LaCCPh and phenylacetylene, both formed from the reaction of of 20a with phenylallene (Scheme 2-22).

In accord with previous results, the present results also indicated that Cp*2LaC(Ph)=C(H)CCPh (19a) reacts with phenylallene at 0 °C, forming mainly Cp*2La(η3-CH2CCPh) (7a), but at higher temperature also Cp*2LaX (δ 1.92 ppm). At room temperature, both 19a and phenylallene were completely consumed within several hours, to yield among others trans-1,4-diphenylbut-1-en-yne and Cp*2LaY (δ 1.97 ppm).

Concluding remarks The reactions of Cp*2La(η3-CH2CCPh) (7a) with phenylacetylene and methanol revealed that the relative amounts of propargylic and allenylic quenching products depend on the nature of the Lewis basic, Brønsted acid (i.e. methanol gave an allene-to-alkyne ratio of 1.00:1.11, while phenylacetylene resulted in a ratio of 1.00:0.20). Under the assumption that the reaction of 7a with phenylacetylene proceeds via initial alkyne coordination, the present data provide evidence that the tendency for Lewis base coordination at the sterically most hindered side of the η3-propargyl/allenyl ligand in 7a increases with the softness of the Lewis base. Circumstantial evidence for initial Lewis base coordination in the reactions of 7a with phenylacetylene is supplied by the observation of the Lewis base adduct Cp*2LaCCPh(PhCCH) in the permethyllanthanocene-catalyzed oligomerization of phenylacetylene (Chapter 4), coordination equilibria of alkynes and divalent lanthanidocenes Cp*2Ln (Ln = Sm, Eu, Yb) in benzene-d6 solution89 and alkene coordination prior to insertion into Ln-alkyl bonds.90 Based on the linear, rod-shape structure of phenylacetylene, it seems reasonable to assume that the steric requirements for Lewis base coordination of phenylacetylene are either similar or larger than that of methanol in 7a. In this light, the higher relative amount of allenylic quenching product for the reaction of 7a with phenylacetylene as compared to the reaction of 7a with methanol suggests that the side of nucleophilic attack is mainly determined by electronic factors rather than steric factors in the reaction of 7a with nucleophiles. The above results indicate that the relative amount of propargylic and allenylic quenching products is influenced by the reaction temperature as well (i.e. at room temperature the reaction of 7a with phenylacetylene produced an allene-to-alkyne ratio of 1.00:0.20, but at -60 °C a ratio of 1.00:0.08 was found). If alkyne coordination takes place prior to protonolysis, it seems that the formation of the sterically most hindered Lewis base adduct, the η1-allenyl-like derivative of Cp*2La(η3-CH2CCPh) (7a), is favored by relatively low reaction temperatures. The η1-allenyl-like derivative of 7a may therefore represent the kinetic product, while the η1-propargyl-like derivative of 7a represents the thermodynamic product of the reaction of 7a with phenylacetylene.

2.5.3. Thermolysis

Prior to further reactivity studies, it was considered insightful to investigate the thermal stability of the present Cp*2LnCH2CCAr complexes. Thus, a toluene-d8 solution of Cp*2LaCH2CCPh (7a) was heated to 120 °C. After two days, small signals appeared in the Cp* (δ 2.05, 2.01, 1.99, 1.83 ppm) and aliphatic region (δ 0.59, 0.45 ppm) which increased in intensity upon further heating. Also, a color change from light yellow to bright orange was observed after heating several days at 120 °C. The propargyl NMR signals disappeared almost completely after 10 days. Attempts to identify the thermolysis products were unsuccesfull due to the high reactivity and instability of the products. The reaction mixture was quenched with methanol-d4 after complete conversion of 7a. Subsequent GC/GC-MS analysis indicated the presence of Cp*H-dn and two oligodeuterated isomers of C18H16, corresponding to the dimers of the phenyl-substituted propargyl ligand.

The above results revealed that 7a is thermally quite robust. It seems natural to ascribe the observed thermal stability to the delocalized η3-bonding of the propargyl/allenyl ligand, providing the electrophilic metal center with both steric and electronic saturation. Several studies of the thermal stability of Cp*2LaCH(SiMe3)2 (6a) have been reported in literature. The decomposition of 6a in cyclohexane-d12 at 120 °C was found to be relatively rapid (quantitative within 2 h) at 120 °C via intramolecular methyl C-H activation, yielding highly

Chapter 2

38

reactive and unstable products which defied chracterization.91 The decomposition of 6a in toluene-d8 at 112 °C produced the benzyl derivative Cp*2LaCD2C6D5 and CH2(SiMe3)2 within several hours (t1/2 = 3 h), both selectively and quantitatively.92 Evidence was presented that solvent metalation is preceded by the intramolecular C-H activation of the Cp* methyl group, affording the fulvene derivative Cp*(C5Me4CH2-η5:η2)La. The reversible nature of inter- and intramolecular C-H activation was put forward to account for the observed H/D exchange between solvent and Cp* ligand.

Accordingly, thermal decomposition of 7a can be envisioned to occur by intramolecular metalation, forming the fulvene derivative Cp*(C5Me4CH2-η5:η2)La and the protonated η3-propargyl/allenyl ligand, possibly both 1-phenyl-1-propyne and phenylallene (Scheme 2-23). Subsequent, reversible solvent metalation accounts for the observed deuteration of the Cp* ligand. The formation of the observed dimers of the η3-propargyl/allenyl ligand can be explained by insertion of 1-phenyl-1-propyne and/or phenylallene into the La-C bond of 7a. The observation of only two isomers points to the regiorandom insertion of 1-phenyl-1-propyne into the La-CH2 bond of the η3-propargyl/allenyl derivative, however. Evidence for this view is supplied by the observed tendency of phenylallene to undergo C-H activation rather than insertion reactions with Cp*2LnR complexes (Section 2.5.2) and catalytic reactions of 7a with excess 1-phenyl-1-propyne. A detailed study of the latter process demonstrated that insertion of 1-phenyl-1-propyne into the La-CH2 bond of 7a represents the major reactive pathway under the present reaction conditions (Chapter 3).93 The fact that the dimers of the η3-propargyl/allenyl ligand are oligodeuterated implies that the protonated η3-propargyl/allenyl ligand undergoes reversible transmetalation with the benzyl derivative Cp*2LaCD2C6D5 before insertion. Due to the complexity of the 1H NMR spectrum, only Cp*2La(η3-CD2C6D5) was identified spectroscopically, based on its reported Cp* 1H NMR resonance at δ 1.82 ppm.92a,b The observation of 1H NMR resonances at δ 0.59 and 0.45 ppm are, furthermore, consistent with the presence of α-metalated methyl groups.94

Heating mixtures of 7a and 7b for several days to 120 °C in benzene-d6 gave similar organic products as observed for 7a in toluene (i.e. only the presence of oligodeuterated Cp*H and isomeric dimers of the protonated η3-propargyl ligand was indicated by GC-MS analysis after treating the reaction mixture with methanol). Quantitative 1H NMR analysis of the decrease of the propargylic CH2 proton resonance normalized against hexamethyldisiloxane (4.5 mM) in benzene-d6 solution indicated that the thermolysis of 7a was first-order in 7a for at least 4 half-lives, as was evident from a linear plot of ln[7a]0/[7a]t versus time (R2 = 0.9889, t1/2

= 47.1 h).

Scheme 2-23. Proposed mechanism for the thermolysis of 7a.

La

Ph

7a

La CH2C CPh

C HH2

La C

C H

Ph

C CH2

H2

La + Ph

.Ph

+La

120 °C

C7D8-n+La LaD

D

d5

dn dn

dn dn


39

2.5.4. The reactivity towards Lewis bases

Tetrahydrofuran The reactions of Lewis base with η3-propargyl/allenyl transition-metal complexes are known to give the Lewis base adduct of η1-propargyl or η1-allenyl derivatives, depending on the nature of the Lewis base.3d-f To investigate whether the present Cp*2LnCH2CCAr complexes exhibit an analogous reactivity, the reaction of Cp*2LaCH2CCPh (7a) with an equimolar amount of tetrahydrofuran (THF) was conducted in benzene-d6 at room temperature. No color change was observed and 1H NMR spectroscopy revealed a rapid and clean reaction. The upfield shifts of the THF 1H and 13C NMR resonances (e.g. ∆δ -0.55 and -0.04 ppm for the α- and β-carbon, respectively, in the 13C NMR spectrum) are consistent with the coordination to the electrophilic metal center. The interpretation of the shifted propargyl NMR resonances is somewhat ambiguous, however. An increase in allenylic character is suggested by the downfield shift of the CH2 proton resonance (∆δ +0.13 ppm), whereas the upfield shift of its CH2 carbon resonance (∆δ -1.37 ppm) points to an increase in propargylic character (Table 2-7). In spite of this contradiction, an η3-propargyl/allenyl structure is presently favored, based on the similarity of the NMR spectral parameters with the crystallographically characterized [Cp*2La(η3-CH2CCPh)(C5H5N)] (7a·py) (Section 2.5.5). The complex 7a·THF is stable in benzene-d6 solution at room temperature, as no changes were observed in the 1H NMR spectrum after standing for two days. When the reaction mixture was heated to 50 °C, the intensity of the NMR signals of the base adduct 7a·THF slowly decreased, giving rise to small vinylic 1H NMR resonances and several unidentified proton resonances in the Cp* region. After 24 h, no resonances attributable to 7a·THF were observed and the reaction mixture was quenched with methanol-d4. Subsequent GC-MS analysis indicated the presence of Cp*D, THF and one unidentified dimer C18H16 of the protonated phenyl-substituted propargyl ligand (identical to C18H16 found in the thermolysis reaction of 7a, Section 2.5.3). The thermal lability of 7a·THF relative to the parent compound 7a is striking. The exact reason for this difference in stability is unknown at present. One possibility is that the additional ligand provides extra steric and electronic saturation to the metal center, thereby stabilizing the formation of an η1-propargyl (or η1-allenyl) structure in 7a. Addition of THF has, in fact, been found to facilitate the rapid η3-C3H5 to η1-C3H5 interconversion in Cp*2Nd(C3H5) and Cp*2Sm(C3H5).40 If a transient η1-propargyl or η1-allenyl species is formed, it may undergo intramolecular C-H activation with the C-H bond α to the oxygen atom, with or without the intermediacy of Cp*FvLa(THF), forming Cp*2La(η1-2-C4H4O) and CH3CCPh. The former process has been observed for Cp*2YMe(THF)57 and several ether adducts of lanthanidocene hydrides, Cp*2LnH(ROR’) (Ln = Lu95, Y96, Ce96, La96), while the reaction of the samarium derivative Cp*2SmMe(THF) with benzene-d6 and toluene-d8 is reported to proceed via intramolecular Cp* C-H activation.97 Although a temperature effect cannot be ruled out, this rationalization also accounts for the observation of only one of the two previously observed dimers C18H16 of the η3-propargyl/allenyl ligand in the thermolysis reaction of 7a (Section 2.5.3).98 Apparently, the formation of one tautomer is kinetically much facile. The observed mixture of products is probably the result of several types of C-O activation processes, such as reported for Cp*2YH(THF)96 and (C5H4R)2YCH2SiMe3(THF) (R = H, Me).99 Nonetheless, no spectroscopic evidence for typical C-O cleavage products such as Cp*2LaOnBu or [Cp*2La(µ-OCH=CH2)]n was obtained by comparison with reported NMR data.96

Scheme 2-24. The reactions of 7a and 7b with Lewis bases.

+ O

Cp*2La

PhO

Cp*2La

Ph

+ Cp*2Ln

Ph Ph

Cp*2LnN

N

Ln = La (7a), Y (7b)

7a

Chapter 2

40

Pyridine A clean and quantitative reaction was also observed for the reaction of Cp*2LaCH2CCPh (7a) with 1 equivalent of pyridine. The formation of the corresponding adduct (7a·py) is indicated by the upfield shifts of the pyridine proton (∆δ -0.12, -0.17 and -0.19 ppm for α-, β-, and γ-CH, respectively) and carbon resonances (∆δ -0.55, -0.30 and +1.77 ppm for α-, β-, and γ-CH, respectively) upon metal coordination. The changes of the propargyl 1H and 13C NMR resonances are similar as observed previously for 7a·THF, albeit slightly larger (Table 2-7). Again, the downfield shift of the CH2 proton resonance (∆δ +0.34 ppm) suggests an increase in allenylic character, while the upfield shift of its carbon resonance (∆δ -2.93 ppm) and the increase in 1JCH imply, conversely, an increase in propargylic character. An increase in propargylic character is also indicated by IR spectroscopy, revealing a shift of the symmetrical C≡ C stretch from 1944 cm-1 to 1973 cm-1 (Figure 2-8). A single-crystal X-ray analysis study of 7a·py established unambiguously that the bonding of the propargyl/allenyl ligand in 7a·py is still tri-hapto, despite pyridine coordination (Section 2.5.5). The Lewis base adduct Cp*2LaCH2CCPh(C5H5N) (7a·py) was obtained on a preparative scale by the reaction of Cp*2LaCH2CCPh (7a) with a small excess of pyridine (1.1 equiv) in a pentane solution at room temperature. Cooling a pentane solution of 7a·py afforded yellow crystals in a high isolated yield (83%). The complex 7a·py was found to be indefinately stable in the solid state at room temperature, while 1H NMR spectroscopy indicated that benzene-d6 solutions of 7a·py remained unchanged at room temperature for at least seven days, but slowly underwent decomposition at 50 °C.

Thermal decomposition of 7a·py in benzene-d6 solution at 50 °C was accompanied by a color change from light-yellow to brown. The intensity of the pyridine and Cp* proton resonances decreased significantly within seven days at 50 °C, but only one new signal at δ 3.20 ppm formed concomitantly. Subsequent heating to 80 °C led to the formation of small vinylic signals and the complete disappearance of NMR signals due to 7a·py within two days. Addition of methanol-d4, followed by GC/GC-MS analysis indicated the presence of 2,2’-bipyridine as the major organic product. Compounds with m/z 197-198 (a mixture of C14H15N and C14H14ND), 234 (two isomers of C18H14D2) and 333 were formed as minor products.

The above results indicate that thermal decomposition of 7a·py is complex and that the nature of the quenched decomposition products differ from that observed for the analogous reactions of 7a and 7a·THF. The masses of the observed quenching products provide evidence for the occurence of variety of C-C coupling processes. The broad 1H NMR resonance at δ 3.20 ppm and the presence of 2,2’-bipyridine point to the formation of the paramagnetic species Cp*2La(η2-2,2’-bipyridyl) which is reported to be formed from Cp*2La(η1-2-C5H4N)(C5H4N) upon heating at 50 °C.100 However, the complexes Cp*2La(η1-2-C5H4N) or Cp*2La(η1-2-C5H4N)(C5H4N) were not observed spectroscopically by comparison with reported 1H NMR data. To investigate the effect of metal size on the bonding mode of the η3- propargyl/allenyl ligand upon Lewis base coordination, the reaction of Cp*2YCH2CCPh (7b) with pyridine in benzene-d6 was also studied. NMR spectroscopy indicated that the addition of 1 equiv. of pyridine to 7b led to the rapid and clean formation of the corresponding Lewis base adduct 7b·py. The upfield shifts of the CH2CC carbon resonances are larger than previously observed for 7a·py (Table 2-7). The observed yttrium-carbon coupling of the propargyl and terminal carbon resonances indicate that the propargyl/allenyl ligand is still bound in an η3-fashion. Remarkably,

Table 2-7. IR and NMR spectral parameters of Lewis base adducts.a

7a 7a·THF 7a·py 7b 7b·py

1H NMR

CH2 2.82 2.95 3.16 2.82 2.69 Cp* 1.91 1.97 1.94 1.93 1.93 13C NMR

CH2CC 54.00 (158.3)

52.63

51.07 (159.5)

49.33 (159.1, 5.2)

47.22 (157.2, 7.7)

CH2CC 152.19 152.90 152.61 155.09 151.58 CH2CC 112.75 b b 106.79

(11.9) 104.88 (9.7)

IR

νC≡ C 1944 c 1973 1923 2148 a NMR spectra measured in C6D6 at 25 °C. Chemical shifts in ppm and 1JCH and 1JYC coupling constants in Hz. IR spectra measured as Nujol mulls. b Not observed. c Complex not isolated.


41

the CH2 1H NMR resonance moved upfield (∆δ -0.19 ppm) rather than downfield as observed for 7a upon coordination of THF and pyridine. IR spectroscopy indicates a substantial increase of propargylic character, as evidenced by a shift of the C≡ C stretch from 1923 cm-1 to 2148 cm-1 (Figure 2-8). The Lewis base adduct Cp*2YCH2CCPh(C5H5N) (7b·py) was prepared analogously to Cp*2LaCH2CCPh(C5H5N) and isolated in reasonable yields (63%) as an off-white thermolabile solid. Attempts to obtain single-crystals of 7b·py suitable for X-ray analysis failed. Freshly prepared 7b·py turned dark red within hours upon standing at room temperature. NMR and GC/GC-MS analysis indicated the formation of a multitude of decomposition products that defied characterization. In a closed atmosphere, 7b·py was found to be stable in benzene-d6 solution at room temperature for 7 days. Heating to 50 °C, however, resulted in slow decomposition in a manner similar to that observed for 7a·py. The same organic compounds were found with GC-MS analysis after quenching the reaction mixture with methanol-d4, while the formation a broad signal in the 1H NMR spectrum at δ 4.20 ppm, accompanied by slowly disappearing signals due to 7a·py, are consistent with the formation of the paramagnetic derivative Cp*2Y(η2-2,2’-bipyridyl).101

Many examples are reported in literature involving the reaction of rare-earth metallocene derivatives Cp*2LnR (e.g. Ln = Lu95, R = H, Me; Ln = Sc102a, R = Me, CH2C6H5, C6H5; Ln = Y102b, R = CH(SiMe3)2; Ln = Y66a, R = η1-2-C4H3O, η1-2-C4H3S) with pyridine. These reactions proceed via the initial formation of the corresponding Lewis base adduct, followed by transmetalation, forming Cp*2Ln(η1-2-C5H4N) and RH. However, no spectroscopic evidence for the (transient) formation of Cp*2Y(η1-2-C5H4N) or Cp*2Y(η1-2-C5H4N)(C5H4N)

5001000150020002500300035000

20

40

60

80

100

Tran

smitt

ance

(%)

ν (c m -1)

7 a 7 a ·p y

5 0 01 0 0 01 5 0 02 0 0 02 5 0 03 0 0 03 5 0 00

2 0

4 0

6 0

8 0

1 0 0

Tran

smitt

ance

(%)

ν ( c m -1 )

7 b 7 b ·p y

Figure 2-8. Infrared spectra of 7a (upper spectrum, black line), 7a·py (upper, grey), 7b (lower, black) and 7b·py (lower, grey) as nujol mulls.

Chapter 2

42

was found by comparison with reported 1H NMR data.103 The formation of unidentified material, accompanied by gradual coloration of the reaction mixture to deep red, has previously been noted for rare-earth metal pyridyl and phenyl complexes.104

2.5.5. The molecular structure of Cp*2La(η3-CH2CCPh)(py)

Complex 7a·py crystallized in the triclinic space group P1,2 with Z = 4. The solid-state molecular structure is depicted in Figure 2-9 and selected bond distances and angles are given in Table 2-8. The Cp*2La moiety of 7a·py is similar to that of reported bent permethyllanthanocenes Cp*2La(X)(Y) complexes, having a Ct1-La-Ct2 angle (134.2°) and average La-Ct1 (2.57 Å) and La-Ct2 (2.58 Å) distances.73 Crystallographically characterized formally nine-coordinated bent lanthanidocenes (C5R5)2LnL3 are relatively scarce in literature and the only other example is -to the best of our knowledge- represented by [Cp*2La(NCCH3)(DME)][BPh4].105 Although the geometry of 7a·py may seem reminiscent of a bent metallocene with three equatorial ligands or a highly distorted trigonal bipyramid, neither description is entirely accurate. In fact, irregular geometries are common in the structural chemistry of lanthanide complexes, as ligand field effects are practically absent in lanthanide metals.1

The Cp* ligands are in an unusual eclipsed conformation with a twist angle of 2.85 °. It has been suggested that the presence of eclipsed Cp* ligands is the result of steric crowding,106 but examples of structures with eclipsed rings in which steric crowding is not so obvious have been reported as well.107 The steric crowding in 7a·py is evidenced by the La-N distance of 2.743(2) Å which is considerably longer than those reported for [Cp*2La(NCCH3)2(DME][BPh4]105 (2.60(2) Å), Cp*2LaNHMe(NH2Me)108a (2.70(1) Å) and [1,2,4-(Me3Si)3C5H2]2LaI(C5H5N)108b (2.643(1) Å). Steric crowding in [Cp*2La(NCCH3)2(DME)][BPh4] was indicated by nonequivalent La-N distances. The observed La-N distance of in 7a·py is not exceptional, however, as it falls in the 2.70-2.80 Å range, reported for nonmetallocene lanthanum complexes.109

Comparing the crystal structures of 7a and 7a·py reveals several changes upon pyridine coordination. Firstly, a distortion of the coplanar rearrangement of the metal center with the η3-propargyl/allenyl carbon backbone is observed (atom displacements from the least squares plane relative to 7a·py: La ∆ +0.002, C26 ∆ +0.010, C27 ∆ +0.018, C28 ∆ +0.010 Å). An apparent shift towards a more propargyl-like structure is also observed, as evidenced by the increased distances between lanthanum and the terminal carbon (∆ 0.08(1) Å). This change is also reflected by the upfield shift of the CH2CC 13C NMR resonance (∆δ -2.93 ppm), but is in contradiction with the observed downfield shift of the CH2CC 1H NMR resonance (∆δ +0.34 ppm). These findings suggest that the propargylic carbon resonance is a more reliable structural indicator for the relative contribution of propargyl- and allenyl structures than the corresponding proton resonance. In order to accommodate pyridine at the metal center, the average La-Ct1 (∆ 0.04(2) Å) and La-Ct2 distances (∆ 0.07(2) Å)

Table 2-8. Selected bond lengths (Å) and angles (°) in Cp*2LaCH2CCPh (7a) and Cp*2LaCH2CCPh(C5H5N) (7a·py).

7a 7a·py Bond lengths

C21-C22 1.362(10) C26-C27 1.356(4) C22-C23 1.234(10) C27-C28 1.248(4) C23-C24 1.465(9) C28-C29 1.454(4) La-C21 2.811(6) La-C26 2.822(3) La-C22 2.695(7) La-C27 2.726(3) La-C23 2.740(7) La-C28 2.823(3) av. La-C(Cp*) 2.806(16) av. La-C(Cp*) 2.841(7) av. La-C(Cp*) 2.780(16) av. La-C(Cp*) 2.852(7) La-Ct1 2.535(3) La-Ct1 2.571 La-Ct2 2.509(4) La-Ct2 2.584 La-N 2.743(2)

Bond angles Ct1-La-Ct2 137.1(1) Ct1-La-Ct2 134.2 C21-C22-C23 159.3(7) C26-C27-C28 161.0(3) C22-C23-C24 141.2(7) C27-C28-C29 142.8(3) La-C21-C22 71.0(4) La-C26-C27 71.9(2) La-C23-C22 74.9(4) La-C28-C27 72.7(2)


43

have increased relative to 7a, together with the Ct1-La-Ct2 angle (∆ 2.9(1)°), as is expected for an increase in coordination number and concomitant steric crowding.

Another interesting feature of the crystallographic structure of 7a·py is the position of the pyridine ligand which is cis relative to the phenyl group and approximately coplanar with lanthanum and the carbons of the C3 backbone (atom displacements from the least squares plane: La -0.044, N -0.032, C26 -0.066, C27 0.054 Å). Such a coplanar and cis rearrangement seems sterically unfavorable and may point to an electronic origin.

In principle, quantum chemical investigations lend themselves as a valuable complement to experiments in order to understand the structure and reactivity of metal complexes.110 However, the presence of the large number of electrons in s, p, and f shells renders the quantum chemical calculations of lanthanide metal complexes less straightforward. Even so, several theoretical studies have shown the atomic 4f shell of the lanthanide atom is strongly stabilized and does not contribute significantly to the chemical bonding.111 The [Cp2Ln]+ frontier orbitals may therefore assumed to involve mostly 5d metal orbitals.

The electronic structure of Cp2MLn complexes has been thoroughly analyzed in terms of interaction of the bent Cp2M moiety with the n ligands L.112 In C2v symmetry, the Cp2M fragment has three low-lying orbitals 1a1, b2 and 2a1 and two high-lying orbitals b1 and a2. The b1 and a2 orbitals are destabilized by strong interaction with the π orbitals of the Cp rings. Only the three low-lying orbitals are capable of bonding with additional ligands L. The a1 orbital is mainly s, z and z2 in character, 1b2 mainly yz and y and 2a1 mainly y2-z2, but the exact contribution of the atomic orbitals to the Cp2M molecular orbitals varies with the metal and the Cp(centroid)-M-Cp(centroid) angle. Interestingly, Lauher and Hoffmann predicted coordination of carbon monoxide along the line perpendicular to the Cp(centroid)-M-Cp(centroid) plane for d0 Cp2MR2 complexes,

Figure 2-9. The molecular structure of 7a·py with thermal ellipsoids drawn at the 50% probability level. The hydrogen atoms are omitted for clarity.

Scheme 2-25. A schematic presentation of the frontier orbitals of the bent metallocene Cp2Ln moiety and the LUMO of d0 Cp2MR2.

Ln z

y

x2a1

1b2

1a1

CpR

RCp

LUMO

Chapter 2

44

because this lateral approach was considered to provide good overlap between the lowest unoccupied molecular orbital (LUMO), approximately of a z2 type, and the carbon lone pair of CO (Scheme 2-25).112a Nucleophilic attack from lateral positions in the carbonylation reaction of zirconocenes was later corroborated experimentally.113

To determine the possible coordination site(s) of the Cp*2Ln(η3-CH2CCPh) complexes, a molecular orbital calculation was performed on the model complex Cp2Y(η3-CH2CCPh) in a geometry modeled after the crystal structure of Cp*2Y(η3-CH2CCPh) (7b). Computational simplifications involving the replacement of lanthanide metals with yttrium and permethylated Cp* ligands with unsubstituted cyclopentadienyl ligands are well-established.114,112 The former simplification is justified by a large body of experimental work that demonstrates that yttrium behaves very similarly to most lanthanides.1 The electronic properties of the ligand, on the other hand, are believed to be only of importance in reactions involving a change of oxidation state.115 The calculations were carried out with the Turbomole program at the bp86/RIDFT level using the Turbomole SV(P) basisset on all atoms (small-core pseudopotential on Y).116 Electron donating reactants are expected to interact with the unoccupied molecular orbitals of 7b. The calculated lowest and second lowest unoccupied molecular orbital (SLUMO) are depicted in Figure 2-10.

The position of the coordinated pyridine in the solid-state molecular structure of 7a·py corresponds well with the spatial extension of the calculated LUMO and reveals that Lewis bases may coordinate in a trans (to the phenyl group) and coplanar rearrangement as well. The latter coordination mode seems actually less sterically hindered. These results provide additional support for the formation of η3-propargyl-like and η3-allenyl-like Lewis bases, the proposed intermediates in the reaction of 7a with methanol and phenylacetylene (Section 2.5.2). It can be anticipated that Lewis base coordination of these protic acids at the position cis to the phenyl group results in the formation of the propargylic quenching product, while coordination trans to the phenyl group leads to the allenylic quenching product (Scheme 2-19). Following this line of reasoning, the regioselectivity of the protonolysis reaction of 7a appears to be related to the preferential nucleophilic attack at either the cis or trans lobe of the LUMO, but more experimental data is needed to support this hypothesis. Interestingly, a similar picture was also put forward to rationalize the reactions of [(PPh3)2Pt(η3-CH2CCPh)]+ with nucleophiles.63c In this case, a theoretical study indicated that the nucleophilic addition reactions of simple donor nucleophiles to the platinum metal center proceed via a frontier-orbital controlled nucleophilic attack that utilizes a LUMO which is largely composed of the Pt 6pz atomic orbital.

2.5.6. The reactivity towards unsaturated substrates

Ethylene Attempted ethylene polymerization reactions with Cp*2La(η3-CH2CCPh) (7a) and Cp*2Y(η3-CH2-

CCPh) (7b) in toluene (1.6 mM) indicated that these complexes did not exhibit any reactivity towards ethylene (7.5 atm) after 0.2 h at 80 °C. The reaction mixtures remained solutions without precipitation of polymer and only the expected quenching products (i.e. Cp*H, phenylallene and 1-phenyl-1-propyne) were observed with GC-MS after addition of methanol.

Figure 2-10. The calculated LUMO (left) and SLUMO (right) of 7b.


45

1-Hexene In accord with above results involving ethylene, no reaction was observed with 1H NMR spectroscopy after heating a reaction mixture of 7a and 7b (20 mM) and excess 1-hexene (100-150 equiv.) for 4 days at 100 °C in benzene-d6. When the reaction temperature was increased to 120 °C, slow substrate conversion ws observed for 7a, but not for 7b. The reaction mixture of 7a and 1-hexene (145 equiv.) was monitored with 1H NMR spectroscopy for 74 days at which point 64% of the substrate was converted and quenched with methanol. GC-MS analysis indicated the presence of cis- and trans-hex-2-ene, hexane and derivatives of 1-hexene dimers (e.g. methyl-substituted undecenes and undecanes), trimers (m/z 250) and tetramers (m/z 334). Unidentified organic compounds having masses consistent with derivatives from the cross-coupled product between 1-phenyl-1-propyne and 1-hexene were also found.

The absence of substituted derivatives of the pentamethylcyclopentadiene ligand argues against the involvement of fulvene derivatives, while the formation of 1-hexene isomers, hexane and both saturated and unsaturated 1-hexene oligomers strongly suggest that a lanthanum hydride species is formed. The monomeric hydride derivative Cp*2LaH is a well-known and highly active catalyst for alkene hydrogenation.53b,d Because these reactions are kinetically very facile, the generation of such a hydride derivative in the present reaction mixture must be very slow. A plausible route towards Cp*2LaH involves β-H elimination after 1,2- or 2,1-insertion of 1-hexene into the metal-carbon bond of the η3-allenyl/propargyl derivative. Unfortunately, the present experimental data do not allow differentiation between insertion into the propargylic La-CH2 bond or the allenylic La-CPh bond of 7a. Because the insertion of 1-phenyl-1-propyne into the propargylic La-CH2 bond represents the major pathway in the reaction of 7a with 1-phenyl-1-propyne (vide infra), the former insertion reaction is presently favored (Scheme 2-26). In view of the well-documented high reactivity of Cp*2LaH and the observed slow consumption of 1-hexene, either insertion of 1-hexene or β-H elimination must be rate-limiting. As NMR resonances of 7a were still present after several days and no major organometallic intermediates were observed with NMR spectroscopy, slow insertion is presently favored. In spite of the higher degree of propargylic bonding in Cp*2La(η3-CH2CCC6H3Me2-2,6) (9a) as compared to Cp*2LaCH2CCPh (7a) (Section 2.4.3), the behavior of 9a towards 1-hexene was similar to that of 7a. Slow conversion of 1-hexene into analogous compounds was observed after heating a benzene-d6 solution of 9a in the presence of an excess of 1-hexene for several days at 100 °C. 1-Phenyl-1-propyne When a reaction mixture of Cp*2La(η3-CH2CCPh) (7a) was heated to 80 °C in the presence of an excess of 1-phenyl-1-propyne (10-25 equiv) slow consumption of 1-phenyl-1-propyne was observed. NMR analysis of the reaction mixture pointed to the catalytic cyclodimerization reaction of 1-phenyl-1-propyne into mainly (E)-3-benzylidene-2-methyl-1-phenylcyclobutene (eq. 2.3). Catalytic cyclodimerization of alkyl- and silyl-substituted methylacetylenes was previously observed for Cp*2LnCH(SiMe3)2 (Ln = La, Ce), while a catalytic cycle based on a kinetic and mechanistic study is given in Chapter 3. The reaction mixtures obtained with 7a were identical to those observed for the reaction of Cp*2LaCH(SiMe3)2 (6a) and [Cp*2La(µ-H)]2 (12a) with excess 1-phenyl-1-propyne (Section 2.3.2). Interestingly, the major products were found to originate from

Scheme 2-26. The proposed route towards the formation of Cp*2LaH as observed in the reaction of 7a with 1-hexene.

7a Ph

Cp*2La120°C

Cp*2La

Ph

Cp*2La

Ph

Ph

Ph

+Cp*2LaH

+Cp*2LaH

Chapter 2

46

substrate insertion into the propargylic La-CH2 bond of 7a, while the minor products originated from substrate insertion into the allenylic La-CH(Ph) bond.

(2.3) Ph + +

Cp*2LaR5 mol%

C6D6100 °C

Ph

Ph

Ph

Ph

Ph

Ph

R = CH(SiMe3)2, CH2CCPh, H

Aspects concerning the influence of the reaction temperature, the lanthanide metal and the catalyst

structure on the rate and selectivity of the catalytic cyclodimerization reaction are discussed in Chapter 3. It should be noted that no catalytic cyclodimerization of 1-phenyl-1-propyne was observed with Cp*2YCH2CCPh (7b).

(2.4)

+ +

Cp*2LaCH2CCPh5 mol%

C6D6100 °C

unidentifiedproducts

1-(2-Methylphenyl)-1-propyne Catalytic cyclodimerization was also observed for 1-(2-methylphenyl)-1-propyne, when a mixture of Cp*2LaCH2CCPh (7a) was heated to 100 °C in the presence of an excess of 1-(2-methylphenyl)-1-propyne (10-25 equiv) (eq. 2.4). The rate of substrate consumption was considerably lower and the reaction products were also formed less selectively as observed for the reactions of 7a with 1-phenyl-1-propyne, however. Further details are discussed in Chapter 3.

(2.5)

9a7a 3

Cp*2La + Cp*2La +

Ph

Ph

1-(2,6-dimethylphenyl)-1-propyne In contrast to the reaction of Cp*2La(η3-CH2CCC6H5) (7a) with 1-(2-methylphenyl)-1-propyne, NMR spectroscopy did not indicate the formation of oligomers of 1-(2,6-dimethylphenyl)-1-propyne (3) upon heating a benzene-d6 solution of 7a to 100 °C in the presence of 3 (20 equiv.). Instead, transmetalation of 7a with 3 took place, as evidenced by the formation of Cp*2La(η3-CH2CCC6H3Me2-2,6) (9a) and 1-phenyl-1-propyne (Eq. 2.5). Upon further heating, both 1-phenyl-1-propyne and 7a were completely converted into (E)-3-benzylidene-2-phenyl-1-methylcyclobutene and 9a, respectively. The reaction mixture was quenched after 10 days with methanol-d4 and GC-MS analysis indicated the presence of trace amounts of cross-coupling products of 3 and 1-


47

phenyl-1-propyne (m/z 260) and coupling products of 3 (m/z 288), besides the expected organic compounds formed upon deuterolysis (i.e. Cp*D, 1-2,6-dimethylphenyl-1-propyne-dn, 2,6-dimethylphenyl-propadiene-d1 and isomers of (E)-3-benzylidene-2-phenyl-1-methylcyclobutene).

The formation of (E)-3-benzylidene-2-phenyl-1-methylcyclobutene reveals that insertion of 1-phenyl-1-propyne into the propargylic La-CH2 bond of 7a took place, forming the cyclodimer after subsequent intramolecular alkyne insertion and protonolysis by 1-phenyl-1-propyne or 1-(2,6-dimethylphenyl)-1-propyne (3), as previously proposed for the lanthanocene-catalyzed cyclodimerization reaction of 1-phenyl-1-propyne (Chapter 3). Because the electronic properties of 1-phenyl-1-propyne and 3 are comparable (Section 2.3.2), the above results suggest that the insertion of 3 into 7a is inhibited for sterical reasons. The large amount of (E)-3-benzylidene-2-phenyl-1-methylcyclobutene relative to the cross-coupling product of 1-phenyl-1-propyne and 3 reveals, in addition, that the insertion of 1-phenyl-1-propyne into 7a is rapid relative to the insertion of 1-phenyl-1-propyne into 9a. This observation may be attributed to slow transmetalation of 7a with 3, producing only a small amount of 9a relative to 7a, and/or slow insertion of 1-phenyl-1-propyne into 9a. To discriminate between both possibilities, analogous reactions with 9a were conducted. NMR spectroscopy provided evidence that the bonding in Cp*2LaCH2CC(C6H3Me2-2,6) (9a) displayed a higher degree of propargylic character than that in Cp*2LaCH2CCPh (7a). To determine whether an increased propargylic character results in a higher tendency to undergo insertion reactions for the present aryl-substituted η3-propargyl/allenyl complexes Cp*2LaCH2CCAr, the reactivity of 9a was investigated towards 1-phenyl-1-propyne and 1-(2,6-dimethylphenyl)-1-propyne (3). When a benzene-d6 solution of 9a was allowed to react in the presence of a 20-fold molar excess of 3, no reactivity was observed after heating several days at 100 °C by 1H NMR spectroscopy. The addition of 1-phenyl-1-propyne (1 equiv.), followed by subsequent heating to 100 °C, led to its slow conversion into (E)-3-benzylidene-2-phenyl-1-methylcyclobutene (Scheme 2-27). Upon complete conversion of 1-phenyl-1-propyne, the reaction mixture was quenched with methanol-d4. Subsequent GC-MS analysis indicated among others the presence of trace amounts of homo- and cross-coupling products of 3 (m/z 288 and 260). These findings indicate that that insertion of both 1-phenyl-1-propyne and 1-(2,6-dimethylphenyl)-1-propyne (3) into Cp*2LaCH2CC(C6H3Me2-2,6) (9a) does not take place. Apparently, the steric constraints posed by the additional 2,6-dimethyl groups in the phenyl-substituted propargyl impede insertion. Similarly, 2,6-dimethyl substitution at the phenyl group of the substrate seems to hinder substrate insertion into 7a as well. Thus, catalytic cyclodimerization of 1-arylalk-2-ynes is found to be quite sensitive towards ortho substitution of the simple 1-phenyl-1-propyne system. No insertion chemistry was observed upon substituting the phenyl group of either the η3-propargyl/allenyl ligand or the substrate with two ortho-methyl groups. 1-Pentafluorophenyl-1-propyne When Cp*2La(η3-CH2CCPh) 7a was heated to 80 °C in benzene-d6 in the presence of a 20-fold molar excess of 1-pentafluorophenyl-1-propyne (5) and monitored with 1H NMR spectroscopy, no significant consumption of 5 was observed after 20 days. Instead, slow conversion of Cp*2La(η3-CH2CCPh) into Cp*2La(η3-CH2CCC6F5) (11a) took place. This conversion was complete within 48 h at 80 °C during which new Cp* proton resonances appeared, accompanied by the formation of several vinylic and aliphatic proton resonances. Further heating led to the decomposition of 11a, as observed previously for the reaction of Cp*2LaCH(SiMe)2 (6a) and 5 (Section 2.3.2). Quenching the mixture with methanol-d4, followed by GC-MS analysis, indicated the presence of 1-pentafluorophenyl-1-propyne-dn, pentafluorophenylpropadiene-d1, Cp*D, 1-

Scheme 2-27. The reaction of 9a with 3 and 1.

9a 3

Ph

+Cp*2La

9a 3 1

+Cp*2La

9a

+Cp*2La+Ph

Ph

Chapter 2

48

phenyl-1-propyne and small amounts of several organic compounds of which the major products were identified as C18H5DF10 (m/z 413), consistent with the mass of a dimer of 5, and four isomers of C27H13DF10 (m/z 529), consistent with a trimer composed of two 5 units and one 1 unit. Heating a similar mixture to 120 °C resulted in a more rapid decomposition of 11a, while C18H5DF10 was the only coupling product observed with GC-MS. These results can be rationalized by the reaction sequences depicted in Scheme 2-28. Rather than insertion into the La-C bond of 7a as observed for 1-phenyl-1-propyne (1), 1-pentafluorophenyl-1-propyne (5) undergoes transmetalation with 7a. The four observed isomers of C27H13DF10 can be explained by regiorandom insertion of 5 and 1 into 11a. The observation of only one isomer of C18H5DF10 suggests that insertion of 5 into 11a is more rapid than insertion of 1-phenyl-1-propyne into 11a. This observation implies as well that one of the two regioisomeric products formed upon insertion of 5 into 11a is more reactive towards 1-phenyl-1-propyne than the other and that the more reactive regioisomeric product is completely converted by 1-phenyl-1-propyne. Because the structure of the products could not be determined unequivocally by NMR spectroscopy, it is not known which of the two regioisomeric insertion products of 5 into 11a is the least reactive towards 1-phenyl-1-propyne and produces C18H5DF10 upon deuterolysis.

The exact reason for the reluctance of 5 to undergo insertion reactions with 7a as compared to 1 is unknown at present. The similar steric requirements of the pentafluorophenyl group as compared to the phenyl group argue for electronic reasons, however.117 Also, the absence of intramolecular alkyne insertion of the insertion product of 11a and 5 is not understood presently, mainly because the precise effect of the increased electron-withdrawing nature of the alkyne substituent (C6F5 versus C6H5) on the stability of the alk-1-ene-4-yn-1-yl species is difficult to assess a priori.118 Diphenylethyne As 1-phenyl-1-propyne was found to undergo insertion into the metal-carbon bond of Cp*2LaCH2CCPh (7a), it was considered interesting to explore the reactivity of 7a towards diphenylethyne as well. Because the steric size of the methyl group is smaller than that of the phenyl group,49 the steric requirements for insertion of diphenylethyne into 7a are somewhat higher than that of 1-phenyl-1-propyne, though.

Scheme 2-28. The proposed reaction sequences to account for the products observed for the reaction of 7a with 5.

511a C6F5

Cp*2La

C6F5

Cp*2La

Ph

C6F5

C6F5

CD3OD

7a 5

+ C6F5 +

11a 1

Ph

C6F5

Cp*2La

C6F5

+

Cp*2La

Ph

C6F5

C6F5

Cp*2La

Ph

C6F5

C6F5

Cp*2La

Ph

C6F5

C6F5

11

D

Ph

C6F5

C6F5

D

Ph

C6F5

C6F5

D

Ph

C6F5

C6F5

D

Ph

C6F5

C6F5CD3ODCD3ODCD3OD

+ C6F5Cp*2La

C6F5

Cp*2La

Ph

Cp*2La

C6F5


49

A benzene-d6 solution of 7a and diphenylethyne (1.2 equiv) was heated for several days to 100 °C and the progress of reaction was monitored with 1H NMR spectroscopy. As the resonances of 7a slowly disappeared, resonances previously observed in the thermolysis of 7a in benzene-d6 solution appeared, while the total amount of proton intensities in the aliphatic and vinylic region decreased as well. Analogous to the thermolysis reaction, the light-yellow solution changed into red-orange during heating. The reaction mixture was quenched with methanol after 20 days and GC/GC-MS analysis indicated the presence of small amounts of 1-phenylprop-1-ene-dn and phenylallene-dn, pentamethylcyclopentadiene-dn and several isomeric and oligodeuterated compounds (e.g. m/z 232, 261 and 429) of which trans-diphenylethene and an unknown compound (m/z 429) were identified as the major products. Although the details of the formation of the observed products are not known presently, the masses of the observed organic products are inconsistent with products originating from insertion of diphenylethyne into 7a. It is believed that the observed products are the result of reactions involving a fulvene Cp*(C5Me4CH2-η5:η2)La or phenyl derivative Cp*2LaC6D5 both formed upon thermolysis of 7a (Section 2.5.3). Evidence for this view comes from the multitudinous incorporation of deuterium atoms in the observed quenching products, indicating extensive H/D scrambling. To investigate the possibility of a cross-coupling reaction between 1-phenyl-1-propyne and diphenylethyne, a reaction mixture of 7a with a 20-fold molar excess of both diphenylethyne and 1-phenyl-1-propyne was heated for several days at 100 °C. NMR spectroscopy indicated that diphenylethyne was not consumed and that products from catalytic cyclodimerization of 1-phenyl-1-propyne formed exclusively. Carbon monoxide Many examples of facile carbon monoxide insertion into the metal-carbon bond of rare-earth metallocene η1-alkyl derivatives exist in literature.119 The fact that CO insertion was also observed for ηn-bound (n > 1) rare-earth metal complexes, such as allyl Cp*2Ln(η3-CH2CHCH2),79 ortho-pyridyl Cp*2Y(η2-C,N-C5H4N)120 and tris(pentamethyl-cyclopentadienyl) derivatives Cp*3Ln,81 encouraged us to explore the reactivity of the present η3-propargyl/allenyl derivatives Cp*2Ln(η3-CH2CCAr) towards carbon monoxide as well.

When an excess of carbon monoxide (1 atm.) was applied to a benzene-d6 solution of Cp*2La(η3-CH2CCPh) (7a), no reactivity was observed with 1H NMR spectroscopy after 5 days at room temperature. A multitude of Cp* 1H NMR resonances formed slowly upon heating to 100 °C, accompanied by a color change from light-yellow to red-orange. After 7 days at 100 ºC, only ~5% of 7a was converted. No evidence for CO insertion into 7a was obtained by means of NMR spectroscopy and GC/GC-MS after quenching the mixture with methanol, however. Similar results were also obtained for analogous reactions of Cp*2Y(η3-CH2CCPh) (7b). It appears that the η3-propargyl/allenyl bonding of both 7a and 7b is too strong to allow insertion of CO into the metal-carbon bond at room temperature. Nonselective reactivity was observed upon increasing the reaction temperature, but the details of these reactions are unknown at present. As 7a was found to react only slowly with excess CO, even at high temperatures, the possibility of trapping an intermediate of the catalytic cyclodimerization reaction of 1-phenyl-1-propyne was investigated. The intermediates proposed are alkenyl derivatives (Chapter 3) whose (reported) reactivity is relatively unexplored in organolanthanide chemistry.121 The application of CO (1 atm) to a reaction mixture containing 7a and 1-phenyl-1-propyne (20 equiv.) led after heating to 100 °C to the decomposition of 7a into several unidentified species, as evidenced by the decrease of the 7a proton resonances and the concomitant increase of numerous proton

Scheme 2-29. The observed reactivity of 7a towards CO and diphenylethyne, in the presence and absence of 1-phenyl-1-propyne.

7a Ph

Cp*2La

PhPh

PhPh

Ph

Ph

Ph

CO

COPh

catalyst decomposition

no reactionno reaction

Chapter 2

50

resonances in the Cp* region. Complete conversion of 7a was observed within 3 days at 100 ºC. No evidence for CO insertion was found with 1H/13C NMR and GC/GC-MS after quenching the mixture with methanol.

2.6. Conclusions

Novel rare-earth metallocene η3-propargyl/allenyl derivatives have been prepared via the reactions of the alkyl derivatives Cp*2LnCH(SiMe3)2 with the corresponding 1-aryl-1-propynes CH3CCAr. The synthetic utility of this route towards the Cp*2LnCH2CCAr complexes is limited by the slow rate of propargylic metalation. The reactions of the hydride derivatives [Cp*2Ln(µ-H)]2 derivatives with 1-aryl-1-propynes CH3CCAr also afforded the desired Cp*2LnCH2CCAr complexes in most cases. However, these reactions were less selectively and the high solubility of Cp*2LnCH2CCAr complexes hindered their isolation from the reaction mixtures in some cases.

Spectral and structural analysis provided evidence for a static η3-bonding description of the propargylic/allenylic ligand in Cp*2LnCH2CCAr (Ln = La, Y), but a shift towards more η1-propargylic and more η1-allenylic bonding was observed for Cp*2LaCH2CCAr complexes, depending on the aromatic substituent of the η3-propargyl/allenyl ligand. The former shift was favored by more sterically hindered substituents, while the latter shift was promoted by more σ-electron-withdrawing substituents.

The reactivity of the present η3-propargyl/allenyl complexes Cp*2LnCH2CCAr was found to involve the formation of Lewis base adducts, C-H activation reactions with Brønsted acids and insertion reactions of unsaturated substrates. The reactions of Cp*2LnCH2CCPh (Ln = La, Y) with Lewis bases, such as THF and pyridine, yielded the corresponding Lewis base adduct, both rapidly and selectively, in which the propargyl/allenyl ligand was still bound in a η3-fashion. Reactions of the Cp*2LnCH2CCAr complexes with Brønsted acids furnished both acetylenic (CH3CCAr) and allenylic (CH2=C=CHAr) protonolysis products. The insertion reactivity of Cp*2LnCH2CCPh towards unsaturated substrates was severely limited by the strong η3-bonding of the propargylic/allenylic ligand. Even so, insertion of 1-phenyl-1-propyne into Cp*2LaCH2CCAr was implicated by its catalytic conversion into cyclodimers, as observed for the reaction of Cp*2LaCH2CCAr (or its catalyst precursors) with 1-phenyl-1-propyne. Catalytic cyclodimerization seems to be confined to reactions of Cp*2LnR (R = CH(SiMe3)2, H, η3-CH2CCPh) complexes having relatively large metal centers (Ln = La, Ce) in combination with substrates having sterically relatively unhindered carbon-carbon triple bonds (Ar = Ph, C6H4Me-2).

2.7. Experimental Section

General considerations. All reactions and manipulations of air and moisture sensitive compounds were performed under a nitrogen atmosphere using standard Schlenk, vacuum line and glovebox techniques. Deuterated solvents were dried over Na/K alloy prior to use. Other solvents were dried by percolation over columns of aluminum oxide, BASF R3-11 supported Cu oxygen scavenger, and molecular sieves (4Å) (pentane), or by distillation from Na/K alloy (THF, cyclohexane). The compounds Cp*2LnCH(SiMe3)2 (Ln = La122, Y123), [(Cp*2La(µ-H)]2

122, 2,6-dimethyliodobenzene13

, n-butyl nitrite124, copper bronze125, copper(I) iodide126, propynyllithium127 and Pd(PPh3)2Cl2

128 were prepared according to literature procedures. Reagents were purchased from Aldrich, Acros Chimica, Strem, Merck or Fluka and were used as received unless stated otherwise. 1-Phenyl-1-propyne (Aldrich) was dried over CaH2 before use (vide infra). Diphenylethyne (Aldrich) was sublimed before use. Dimethyl sulfoxide was dried at least three times on freshly activated 4-Å molecular sieves.129

Physical and analytical measurements. NMR spectra were recorded on a Varian VXR-300 (FT, 300 MHz, 1H; 75 MHz, 13C), Varian XL-400 (FT, 400 MHz, 1H; 100 MHz, 13C) or a Varian Inova 500 (FT, 500 MHz, 1H; 125.7 MHz, 13C) spectrometers. NMR experiments on air-sensitive samples were conducted in flame-sealable tubes or tubes equipped with a Teflon valve (Young). The 1H NMR spectra were referenced to resonances of residual protons in deuterated solvents and reported in ppm relative to tetramethylsilane (δ 0.00 ppm). The 13C NMR spectra were referenced to carbon resonances of deuterated solvents. 19F NMR spectra were referenced externally to hexafluorobenzene in CDCl3 (δ -163.0 ppm). IR spectra of pure compounds, KBr pellets or nujol solutions of the samples were recorded on a Mattson 4020 Galaxy FT-IR spectrophotometer. The


51

elemental analyses were performed by H. Kolbe, Mikroanalytisches Laboratorium, Mülheim an der Ruhr, and the Microanalytical Department of the University of Groningen. GC analyses were performed on a HP 6890 instrument with a HP-1 dimethylpolysiloxane column (19095 Z-123). GC-MS spectra were recorded at 70 eV using a HP 5973 mass-selective detector attached to a HP 6890 GC as described above.

General drying procedure for 1-methylalk-2-ynes. The liquids obtained after synthesis or received after purchase were brought in a flask containing freshly ground CaH2 and allowed to react for at least one day at 50 °C under nitrogen. Vacuum transfer afforded colorless oils which were stored at -30 °C and under nitrogen. 2-Propynyltoluene (2). A 1-L, three-neck, round-bottom flask, equipped with stir bar and gas-inlet adaptor, was charged with THF (200 mL) and degassed by freeze-thaw-pump cycles. A 3-L flask was connected to a lecture bottle of propyne and filled with an appropriate amount of propyne gas (~122 mmol) by adjusting the pressure. From this flask, the gas was allowed to condense into the reaction vessel which was cooled to -100°C with a liquid nitrogen-ethanol bath. The 1-L flask was brought under nitrogen, connected to a cooler and drop funnel, while still maintaining the temperature of the bath at ca. -100°C. Then n-BuLi (42.0 mL, 105 mmol, 2.5 M in hexanes) was added dropwise under stirring over a period of 1 h after which the temperature of the bath was allowed to warm up to –78° C. A solution of ZnBr2 (22.6 g, 100 mmol, flame-dried in vacuo) in 100 ml of THF was added dropwise to the reaction mixture at -78° C during 30 min while strirring. After addition the cooling bath was removed and the reaction mixture was allowed to warm up to room temperature. Then, 2-iodotoluene (8.6 ml, 67 mmol) and Pd(PPh3)Cl2 (2.3 g, 3.3 mmol) were added and the suspension was stirred for 14 h at room temperature. The reaction mixture was quenched with 200 mL of brine and filtered. The organic layer was extracted with petroleum ether (40-60 °C) and dried over MgSO4. Rotatory evaporation afforded an orange oil which was purified by means of column chromatography (silica, 230-400 mesh, 60 Å) with petroleum ether. The volatiles were removed by means of rotatory evaporation and vacuum distillation using a 20-cm Vigreux column. Drying over CaH2 and subsequent vacuum transfer afforded a light yellow liquid. Yield: 7.34 g (85%).

1H NMR (300 MHz, CDCl3, 25 °C): δ 2.15 (s, CCCH3, 3 H), 2.43 (s, CH3, 6 H), 6.9-7.2 (m, CH, 3 H). 13C NMR (75 MHz, CDCl3, 25 °C): δ 4.46 (CCCH3), 21.03 (CCH3), 77.24 (CH3C), 94.03 (ArC), 123.74 (i-C, Ph), 126.48 (m-C, Ph), 126.82 (p-C, Ph), 140.04 (o-C, Ph). GC-MS, m/z (relative intensity): 131 (11), 130 (M+, 100), 129 (M+ - H, 61), 128 (62), 127 (28), 116 (5), 115 (M+ - CH3, 78), 102 (8), 89 (6), 77 (8), 75 (4), 74 (5), 64 (8), 63 (11), 62 (4), 51 (11), 50 (5), 39 (5). IR (neat, [cm-1]): 3066 (m), 3022 (m) 2917 (m), 2853 (m), 2734 (w), 2249 (w), 1600 (w), 1487 (m), 1456 (m), 1377 (m), 116 (m), 1045 (m), 765 (s), 493 (s). Anal. Calcd. for C10H10 (130.19): C, 92.26%; H, 7.74%. Found: C, 92.12%; H, 7.63%.

1,3-Dimethyl-2-(prop-1-ynyl)benzene (3). A 250-mL Schlenk flask was charged with 13.0 mL (32.5 mmol) of n-BuLi (2.5 M in hexanes) and 30 mL of THF. A 3-L flask was connected to a lecture bottle of propyne and filled with an appropriate amount of propyne gas (40 mmol) by adjusting the pressure. From this flask, propyne was allowed to condense into the Schlenk flask which was cooled to -100°C with a liquid nitrogen-ethanol bath. After 15 min of stirring a solution of 7.70 g (34.2 mmol) of anhydrous ZnBr2 in 20 mL of THF was added. The mixture was stirred for 10 min at –78 °C. Then 5.0 g (22 mmol) of 2,6-dimethyliodobenzene13 and a solution of 0.78 g (1.1 mmol) of Pd(PPh3)Cl2 and 0.90 mL (2.2 mmol) of n-BuLi in 10 mL of THF are added and the mixture is allowed to warm up to room temperature while stirring. The reaction mixture was stirred at room temperature for 10 days and quenched with aqueous solutions of ammonium chloride and ammonium carbonate. Addition of ether, extraction of the organic layer, drying over MgSO4 and rotatory evaporation afforded a yellow oil which was purified by means of column chromatography (silica, 230-400 mesh, 60 Å) with hexanes. Yield: 2.41 g (78%)

1H NMR (300 MHz, CDCl3, 25 °C): δ 2.15 (s, CCCH3, 3 H), 2.43 (s, CCH3, 6 H), 6.9-7.2 (m, CH, 3 H). 13C NMR (125.7 MHz, CDCl3, 25 °C): δ 4.50 (q, 1JCH = 131.5 Hz, CCCH3), 21.28 (q, 1JCH = 126.8 Hz, CH3), 77.20 (m, CCCH3), 94.20 (q, 3JCH = 10.6 Hz, CCCH3), 123.72 (m, i-C), 126.47 (dqd, , 1JCH = 158.5 Hz, 2JCH = 5.2 Hz, 3JCH = 8.6 Hz, m-CH), 126.83 (d, 1JCH = 159.2 Hz , p-CH), 140.36 (q, 2JCH = 6.4 Hz, 2JCH = 6.4 Hz, o-C). IR (neat, [cm-1]): 3067 (m), 3022 (m), 2917 (s), 2852 (m), 2733 (m), 2242 (m), 2047 (w), 1928 (m), 1579 (m), 1467 (s), 1378 (m), 1263 (m), 1164 (m), 1093 (m), 1033 (m), 9676 (m), 920 (m), 769 (s) 483 (s). GC-MS, m/z (relative intensity): 144 (M+, 90), 143 (M+ - H, 13), 142 (3), 141 (10), 130 (1), 129 (M+ - CH3, 90), 128 (100), 127 (31), 115 (22), 102 (6), 89 (4), 77 (8), 75 (4), 65 (4), 63 (9), 39 (7). Anal. Calcd. for C11H12 (144.22): C, 91.61%; H, 8.39%. Found: C, 91.85%; H, 8.36%.

2,6-Diisopropylbenzenediazonium tetrafluoroborate. According to a general procedure for dediazotization of amines.14 Aqueous ethanol (30 mL, 96%) and HBF4 (14.4 mL, 110 mmol, 48 wt. % solution) were slowly added to 2,6-diisopropylaminobenzene (3.60 g, 20.3 mmol) in an Erlenmeyer and stirred at 0 °C. After addition of an excess of n-butylnitrite124 (5.85 g, 56.7 mmol), the mixture was stirred for 30 min at 0 °C.

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Addition of cold diethyl ether and storage at -30 °C led to the precipitation of a white powder. The powder was subsequently washed on a glass filter with cold diethyl ether and light petroleum ether and kept moist prior to use. The explosive nature upon drying impeded the determination of the (crude) yield.

1H NMR (300 MHz, CDCl3, 25 °C): δ 1.23 (d, 3JHH = 6.8 Hz, CH3, 12 H), 3.23 (sept, 3JHH = 6.8 Hz, 2 H), 7.0-7.1 (m, CH, 3 H). 13C-{1H} NMR (75 MHz, CDCl3, 25 °C): δ 22.75 (CH3), 27.06 (d, J = 3.7 Hz, CH), 117.12 (m, i-C), 123.98 (m, m-CH), 135.02 (d, J = 15.87 Hz, p-CH), 158.30 (d, J = 244.2 Hz, CCHMe2).

2,6-Diisopropyliodobenzene. A 500-mL, three-neck, round-bottomed flask, equipped with a gas-inlet adaptor, drop funnel and a Teflon-coated stir bar, was charged with 2 g of freshly prepared copper bronze125 and placed in a nitrogen atmosphere. 16.52 g (99.5 mmol) of KI, 8.30 g (32.7 mmol) of iodine and 50 mL of dry DMSO were added and the resulting mixture was stirred and heated to 60 °C. After addition of 20 g of freshly prepared n-butylnitrite124 a solution of 12.1 g (68.2 mmol) of 2,6-diisopropylaniline in 30 mL of DMSO was added dropwise. The reaction mixture was stirred for 2 h at 60 °C. The reaction was stopped by adding aqueous solutions of sodium chloride and sodium bisulfite. Filtration, separation, drying over MgSO4 and rotatory evaporation gave a yellow oil which was purified with column chromatography (silica, hexanes). Yield: 15.2 g (78 %) of a light-yellow viscous liquid. 1H NMR (300 MHz, CDCl3, 25 °C): δ 7.20 (m, CH, 1 H), 7.04 (m, CH, 2 H), 3.37 (septet, 3JHH = 6.8 Hz, 2 H), 1.20 (d, 3JHH = 6.8 Hz, 6 H). 13C NMR (125.7 MHz, CDCl3, 25 °C): δ 23.37 (q, 1JCH = 126.8 Hz, CH3), 39.41 (d, 1JCH = 128.2 Hz, CH), 109.14 (m, CI), 123.79 (d, 1JCH = 156.4 Hz, CH), 128.30 (d, 1JCH = 160.2 Hz, CH), 151.12 (m, C). IR (neat, [cm-1]): 2950 (m), 2915 (m), 2850(m), 2230 (w), 1907 (w), 1665 (w), 1490 (m), 1463 (m), 1365 (m), 1105 (w), 1020 (m), 910 (m), 872 (m), 835 (s), 790 (m), 728 (s). GC-MS, m/z (relative intensity): 290 (1), 289 (9), 288 (M+, 70), 274 (13), 273 (M+ - CH3, 100), 146 (10), 145 (8), 133 (9), 131 (33), 128 (14), 117 (21), 115 (23), 105 (10), 91 (28). Anal. calcd. for C12H17I (288.17): C, 50.02%; H, 5.95%. Found: C, 50.15%; H, 6.07%.

Propynyl copper. According to a modification of reported procedures for the preparation of alkynyl copper compounds.36 25.0 g (100 mmol) of CuSO4·5H2O was dissolved in 100 mL of concentrated aqueous ammonia in a 500-mL Schlenk flask, equipped with a Teflon-coated stir bar. After stirring at 0 °C for 15 min 200 ml of water was added, followed by the slow addition of 13.9 g (200 mmol) of HONH2·HCl. Subsequently, propyne gas was bubbled through the solution for 24 h during which a yellow solid precipitated. The suspension was filtered under vacuum and the yellow powder was washed with water, ethanol and ether yellow powder. CAUTION: The yellow powder is explosive when dry and was kept moist with ether. Crude (wet) yield: 8.6 g (84%).

1,3-Diisopropyl-2-(prop-1-ynyl)benzene (4). 8.6 g (84 mmol) of freshly prepared copper(I) propynyl is brought in a 400-mL Schlenk flask, equipped with a Teflon-coated stir bar, and evacuated. After addition of 240 mL of dry pyridine and 3.0 g (10 mmol) of 2,6-diisopropyliodobenzene the reaction mixture was heated on reflux at 120 °C for 10 days. The mixture was quenched with water. Addition of petroleum ether (40-60), filtration, extraction of the organic layer with petroleum ether (40-60), drying over MgSO4 and rotatory evaporation afforded a yellow oil which was purified by means of column chromatography (neutral alumina) with petroleum ether. Yield: 1.58 g (76%) of a colorless oil.

1H NMR (500 MHz, CDCl3, 25 °C): δ 1.24 (d, 3JHH = 6.9 Hz, CH(CH3)2, 6 H), 2.13 (s, ArCCH3, 3 H), 3.52 (septet, 3JHH = 6.9 Hz, CHMe2, 2 H), 7.18 (d, 3JHH = 7.8 Hz, m-CH, 2 H), 7.30 (t, 3JHH = 7.8 Hz, p-CH, 1 H). 13C NMR (125.7 MHz, CDCl3, 25 °C): δ 4.55 (q, 1JCH = 131.4 Hz, CCCH3), 23.61 (qdq, 1JCH = 125.8 Hz, 2JCH = 5.1 Hz, 3JCH = 5.1 Hz, CH3), 31.92 (dsept, 1JCH = 128.7 Hz, 2JCH = 4.0 Hz, CHMe2), 76.73 (q, 2JCH = 4.6 Hz, CH3C), 93.76 (q, 3JCH = 10.7 Hz, ArC), 121.64 (m, i-C), 122.32 (ddd, , 1JCH = 158.5 Hz, 2JCH = 4.8 Hz, 3JCH = 7.9 Hz, m-CH), 127.55 (d, 1JCH = 159.9 Hz, p-CH), 150.52 (m, o-C). IR (neat, [cm-1]): 2962 (s), 2870 (m), 2243 (w), 1575 (w), 1463 (m), 1362 (m), 1253 (w), 1179 (w), 1107 (w), 1060 (w), 935 (w), 801 (m), 754 (m), 483 (s). GC-MS, m/z (relative intensity): 201 (8), 200 (M+, 51), 185 (M+ - CH3, 33), 158 (12), 157 (67), 156 (11), 155 (15), 154 (9), 153 (19), 152 (16), 151 (4), 144 (14), 143 (100), 141 (37), 129 (38), 128 (70), 115 (36), 91 (9). HR-MS: C15H20, calc.: 200.15650, found: 200.15734. Pentafluoro(prop-1-ynyl)benzene (5). The following procedure represents a modified version of previously published ones.36 Propyne gas (ca. 60 mmol) was condensed in vacuo in THF (80 mL) which was cooled at -196 °C in a single-bulbed, 250-mL Schlenk flask, equipped with stir bar. The flask was brought under a nitrogen atmosphere and allowed to warm up to -80 °C. Dropwise addition of n-butyl lithium (21.0 mL, 52.5 mol, 2.5 M in hexanes) under stirring after which the solution was allowed to warm up to -30 °C, forming a white suspension. The mixture was cooled to -60 °C and a solution of hexafluorobenzene (8.74 g, 47.0 mmol) in THF (50 mL) was slowly added to the reaction mixture under vigorous stirring. After addition the reaction mixture was allowed to slowly warm up to room temperature and stirred overnight. Addition of water (100 mL),


53

extraction with diethyl ether, drying over MgSO4 and rotatory evaporation (50 °C, 100 mbar) afforded a whitish oil. Sublimation at 80 °C under vacuum (~ 1 mmHg) provided a white crystalline solid. Yield: 2.32 g (24%).

1H NMR (400 MHz, CDCl3, 25 °C): δ 2.13 (s, CH3, 3 H). 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 147.60 (m, 1JCF = 253 Hz, C5F5), 140.97 (d, 1JCF = 259 Hz, C5F5), 137.55 (d, 1JCF = 251 Hz, C5F5), 100.59 (t, 2JCF = 18 Hz, C5F5C), 99.53 (m, C≡ C), 63.81 (m, C≡ C), 4.821 (s, CH3). 19F NMR (376.5 MHz, CDCl3, 25 °C): δ -139.34 (dd, 3JFF = 21.2 Hz, 4JFF = 7.0 Hz, o-F, 2 F), -156.27 (t, 3JFF = 20.5 Hz, p-F, 1 F), -164.39 (ddd, 3JFF = 21.2 Hz, 3JFF = 21.6 Hz, 4JFF = 7.5 Hz, m-F, 2 F). IR (nujol, [cm-1]): 2926 (s), 2855 (s), 2721 (w), 2254 (w), 2213 (w), 1463 (m), 1377 (m), 1261 (w), 1103 (w), 1020 (w), 974 (w), 835 (m), 489 (s). GC-MS, m/z (relative intensity): 207 (10), 206 (M+, 100), 205 (M+ - H, 73), 188 (7), 187 (69), 186 (5), 180 (6), 179 (9), 167 (7), 161 (8), 156 (28), 155 (5), 117 (7), 105 (6), 104 (4), 103 (4), 93 (8). Anal. Calcd. for C9H3F5: C, 52.45%; H, 1.47%. Found: C, 52.25%; H, 1.38%.

Cp*2YCH2CCPh (7b). Hydrogen at a pressure of 1 bar was supplied to a solution of Cp*2YCH(SiMe3)2 (0.58 g, 1.12 mmol) in hexanes (5 mL). Stirring for 3 h at room temperature produced a light-yellow suspension. The hydrogen atmosphere was replaced by nitrogen and 1-phenyl-1-propyne (0.2 mL, 1.60 mmol) was added. Immediately, a red suspension formed. The suspension was stirred for 30 min at room temperature during which it was degassed several times. The solid was separated by decantation and filtration. Subsequent in vacuo removal of the solvent yielded a red crystalline material (0.49 g, 92% yield). Crystals suitable for X-ray analysis were obtained by recrystallization in toluene at low temperature. 1H NMR (500 MHz, C6D6, 25 °C): δ 1.93 (s, Cp*, 30 H), 2.82 (s, CH2, 2 H), 6.98 (t, 3JHH = 7.5 Hz, p-Ph, 1 H), 7.09 (t, 3JHH = 7.5 Hz, m-Ph, 2 H), 7.19 (d, 3JHH = 7.8 Hz, o-Ph, 2 H). 13C NMR (125.7 MHz, C6D6, 25 °C): δ 11.15 (q, 1JCH = 125.6 Hz, C5(CH3)5,), 49.33 (dt, 1JCH = 159.1 Hz, 1JYC = 5.2 Hz, CH2C≡ C), 106.79 (d, 1JYC = 11.9 Hz, CH2C≡ C), 117.78 (m, C5Me5), 126.69 (dt, 1JCH = 160.8, 3JCH = 7.7 Hz, p-CH), 128.60 (dd, 1JCH = 159.4 Hz, 3JCH = 7.6 Hz, m-CH), 132.40 (dtd, 1JCH = 159.1 Hz, 3JCH = 7.1 Hz, 4JCH = 1.4 Hz, Hz, o-CH), 132.77 (tdd, 2JCH = 3.7 Hz, 3JCH = 8.1 Hz, 4JCH = 2.0 Hz, i-C), 155.09 (t, 2JCH = 2.5 Hz, CH2C≡ C). IR (nujol, [cm-1]): 2924 (m), 2856 (m), 2727 (m), 1923 (m), 1592 (m), 1544 (m), 1455 (m), 1378 (m), 1269 (m), 1154 (m), 1065 (m), 1021 (m), 916 (w), 846 (w), 768 (m), 698 (m), 627 (m), 482 (s). Anal. Calcd. for C29H37Y (474.52): C, 73.41; H, 7.86. Found: C, 73.24; H, 7.89.

Cp*2LaCH2CCPh (7a). Hydrogen at a pressure of 1 bar was supplied to a solution of Cp*2LaCH(SiMe3)2 (0.82 g, 1.42 mmol) in pentane (5 mL). Stirring for 2 h at room temperature produced a yellow suspension. The hydrogen atmosphere was replaced by nitrogen and 1-phenyl-1-propyne (0.4 mL, 3.20 mmol) was added. Immediately, a deep red suspension forms. The suspension is stirred for 1 h at room temperature during which it was degassed several times. The solid was separated by decantation and filtration. Subsequent in vacuo removal of the solvent yielded a red crystalline material (0.68 g, 91% yield). Crystals suitable for X-ray analysis were obtained by recrystallization in toluene at low temperature. 1H NMR (300 MHz, C6D6, 25 °C): δ 1.92 (s, Cp*, 30H), 2.82 (s, CH2, 2H), 6.99 (t, 3JHH = 7.3 Hz, p-H, 1 H), 7.12 (dd, 3JHH = 7.2 Hz, 3JHH = 7.3 Hz, m-H, 2 H), 7.24 (d, 3JHH = 7.2 Hz, o-H, 2 H). 13C NMR (125.7 MHz, C6D6, 25 °C): δ 10.61 (q, 1JCH = 124.5 Hz, C5(CH3)5), 54.02 (t, 1JCH = 158.3 Hz, CH2C≡ C), 112.76 (m, CH2C≡ C), 119.46 (m, C5Me5), 126.19 (dt, 1JCH = 161.8 Hz, 3JCH = 7.4 Hz, p-CH), 128.71 (dd, 1JCH = 153.0 Hz, 3JCH = 8.0, m-CH), 130.84 (dtd, 1JCH = 159.9 Hz, 3JCH = 7.0 Hz, 2JCH = 1.4 Hz, o-CH), 132.39 (t, 3JCH = 7.1 Hz, i-C), 152.19 (s, CH2C≡ C). IR (nujol, [cm-1]): 2920 (m), 2850 (m), 1944 (s), 1587 (m), 1550 (w), 1455 (s), 1380 (m), 1260 (w), 1090 (w), 1065 (w), 1020 (m), 900 (w), 760 (s), 690 (m), 600 (s), 450 (m). Anal. Calcd for C29H37La (524.52): C, 66.41; H, 7.11. Found: C, 66.24; H, 7.00.

Cp*2LaCH2CCC6H3Me2-2,6 (9a). Hydrogen (1 bar) was applied to a solution of Cp*2LaCH(SiMe3)2 (0.56 g, 0.98 mmol) in hexanes (5 mL). The yellow suspension is stirred for 2 h at room temperature. The hydrogen atmosphere was replaced by nitrogen and 1-(2,6-dimethylphenyl)-1-propyne (0.21 g, 1.46 mmol) was added. The suspension was stirred overnight at room temperature and degassed several times. After overnight stirring, the suspension turned deep red. The reaction mixture was extracted with hexanes and evaporated to dryness forming a deep red oil. Crystallization from hexanes at low temperature yielded red crystalline material (0.12 g, 22% yield). Crystals suitable for X-ray analysis were obtained by recrystallization in toluene and cooling the solution. 1H NMR (300 MHz, C6D6, 25 °C): δ 1.92 (s, Cp*, 30 H), 2.18 (s, CH3, 6 H), 2.71 (s, CH2, 2 H), 6.94 (s, CH, 3 H). 13C NMR (125.7 MHz, C6D6, 25 °C): δ 11.10 (q, 3JCH = 124.8 Hz, C5(CH3)5), 22.36 (q, 3JCH = 125.5 Hz, CH3), 49.12 (t, 1JCH = 156.9 Hz, CH2C≡ C), 105.81 (t, 3JCH = 7.0 Hz, CH2C≡ C), 119.04 (m, C5Me5), 127.14 (d, 1JCH = 159.0, p-CH), 128.31 (d, 1JCH = 156.9, m-CH), 130.96 (m, i-CH), 140.74 (m, o-CH), 149.28 (t, 2JCH = 2.6 Hz, CH2C≡ C). IR (nujol, [cm-1]): 2925 (s), 2853 (s), 2725 (w), 1975 (m), 1786 (w), 1566 (w), 1465 (s),

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1377 (m), 1260 (w), 1168 (w), 1093 (w), 1020 (m), 779 (m), 720 (m), 500 (s). Anal. Calcd for C31H41La (552.57): C, 67.38%; H, 7.48%. Found: C, 67.29%; H, 7.54%. Reaction of Cp*2LaCH(SiMe3)2 with CH3CCPh. NMR Scale. Cp*2LaCH(SiMe3)2 (12.3 mg, 21.4 µmol) was dissolved in C6D6 and CH3CCPh (3.0 µL, 24 µmol) was added with a microsyringe in the glovebox. No changes were observed with 1H NMR spectroscopy after 24 h at room temperature. Also, when the sample is heated to 50 °C for 12 h, no changes were observed. After heating the sample to 80 °C for 3 h, the formation of Cp*2LaCH2CCPh was observed and the yellow solution turned slowly deep red upon further heating. After 17 days at 80 °C the reaction mixture was quenched with CD3OD. The presence of CH2(SiMe3)2, Cp*D, 1-phenyl-1-propyne-d1 and phenylallene-d1 was indicated by 1H NMR and GC-MS.

1-Phenyl-1-propyne-d1: 1H NMR (C6D6, 25 °C, 300 MHz): δ 1.65 (s, CH2D, 2H).130 GC-MS, m/z (relative intensity): 118 (6), 117 (72), 116 (100), 115 (15), 90 (8), 89 (6), 64 (6), 63 (8), 51 (7).

Phenylpropa-1,2-diene-d1: 1H NMR (C6D6, 25 °C, 300 MHz): δ 4.85 (s, CH2, 2 H).131 GC-MS, m/z (relative intensity): 118 (7), 117 (76), 116 (100), 90 (6), 63 (9).

Analogous reactions of Cp*2LaCH(SiMe3)2 with CH3CCC6H4Me-2, CH3CCC6H3Me2-2,6 and CH3-

CCC6H3iPr2-2,6 and Cp*2YCH(SiMe3)2 with CH3CCPh in C6D6 gave similar results.

Reaction of Cp*2LaCH(SiMe3)2 with CH3CCC6F5. NMR Scale. Cp*2LaCH(SiMe3)2 (15 mg, 26 µmol) was dissolved in C6D6 and CH3CCC6F5 (5.5 mg, 27 µmol) was added in the glovebox. No changes were observed with 1H NMR spectroscopy after 24 h at 50 °C. After heating the sample to 80 °C for 3 h, the formation of Cp*2LaCH2CCPh and CH2(SiMe3)2 was observed. Complete conversion of the alkyl into the propargyl was observed after 13 days at 80 °C. The reaction mixture was subsequently quenched with CD3OD, producing a mixture of CH2(SiMe3)2, Cp*D, 1-pentafluorophenyl-1-propyne-d1 and pentafluorophenylpropadiene-d1 as indicated by 1H and 19F NMR and GC-MS.

Cp*2LaCH2CCC6F5: 1H NMR (C6D6, 25 °C, 400 MHz): δ 3.08 (s, CH2, 2 H), 1.88 (s, Cp*, 30 H). 19F NMR (C6D6, 25 °C, 376 MHz): δ -140.32 (dd, J = 7.0, 21.2, o-F, 2 F), -156.98 (t, J = 20.5, p-F, 1 F), -164.94 (ddd, J = 8.1, 21.5, 21.6, m-F, 2 F). 13C{1H} NMR (C6D6, 25 °C, 125.7 MHz): δ 10.41 (s, C5Me5), 55.77 (s, CH2), 108.76 (m, CH2CC), 119.70 (s, C5Me5), 137.86 (d, 1JCF = 249.5, m/o-CF), 141.21 (m, i-C), 146.97 (d, 1JCF = 249.5, m/o-CF), 147.70 (d, 1JCF = 249.5, p-CF), 165.24 (m, CH2CC).

1-Pentafluorophenyl-1-propyne-d1: 1H NMR (C6D6, 25 °C, 400 MHz): δ 1.47 (s, CH2D, 2H). 19F NMR (C6D6, 25 °C, 376 MHz): δ -147.46 (dd, J = 7.6, 21.4, o-F, 2 F), -157.00 (t, J = 20.5, p-F, 1 F), -164.96 (ddd, J = 8.1, 21.5, 21.6, m-F, 2 F). GC-MS, m/z (relative intensity): 118 (6), 117 (72), 116 (100), 115 (15), 90 (8), 89 (6), 64 (6), 63 (8), 51 (7).

Pentafluorophenylpropadiene-d1: 1H NMR (C6D6, 25 °C, 400 MHz): δ 4.72 (s, CH2, 2H). 19F NMR (C6D6, 25 °C, 376 MHz): δ -145.00 (dd, J = 7.6, 21.4, o-F, 2 F), -159.73 (t, J = 21.4, p-F, 1 F), -165.69 (m, m-F, 2 F). GC-MS, m/z (relative intensity): 208 (2), 207 (M+, 26), 206 (M+ - H, 100), 205 (M+ - 2H or M+ - D, 67), 188 (M+ - F, 21), 187 (M+ - F - H, 75), 186 (M+ - F - 2H or M+ - F - D, 6), 157 (10), 156 (37), 155 (8), 117 (14), 105 (11). GC-MS, m/z (calc., found): 209 (0.4, 0.5), 208 (9.6, 8.7), 207 (100, 100). Reaction of [Cp*2La(µ-H)]2 with CH3CCPh. NMR Scale. [Cp*2La(µ-H)]2 (5.2 mg, 6.3 µmol) was dissolved in C6D6 and CH3CCPh (1.6 µL, 13 µmol) was added with a microsyringe in the glovebox. The light-yellow solution turned immediately deep red. A complex mixture is observed with 1H NMR spectroscopy. Within 5 min at room temperature signals attributable to the hydride and substrate have disappeared and three major Cp* signals (δ 1.92, 1.90, 1.88 ppm) are observed of which the propargyl (δ 1.92 ppm) consititutes only 30% of the total Cp* 1H NMR resonance intensities as determined by line-shape analysis. Besides signals corresponding to cis-1-phenylpropene, unidentified 1H NMR resonances in the vinylic region (δ 6.5 - 2.5 ppm) were observed. After 1 day at room temperature the reaction mixture was quenched with CD3OD forming Cp*D, cis-1-phenylprop-1-ene, 1-phenyl-1-propyne-d1 and phenylallene-d1 as indicated by 1H NMR and GC-MS. Cis-1-phenylprop-1-ene: 1H NMR (C6D6, 25 °C, 500 MHz): δ 6.41 (dq, 4JHH = 1.7 Hz, 3JHH = 11.5 Hz, =CH, 1 H), 5.63 (dq, 3JHH = 11.5 Hz, 3JHH = 7.3 Hz, =CH, 1 H), 1.69 (dd, 4JHH = 1.7 Hz, CH3, 3 H).132 GC-MS, m/z (relative intensity): 119 (9), 118 (75), 117 (100), 116 (10), 115 (43), 91 (29).

The analogous reaction of [Cp*2La(µ-H)]2 with CH3CCPh in C7D8 gave similar results. Reaction of [Cp*2La(µ-H)]2 with CH3CCPh at low temperature. NMR Scale. A Teflon-capped NMR tube was charged with [Cp*2La(µ-H)]2 (6.8 mg, 8.3 µmol). On the vacuum line, a solution of CH3CCPh (2.1 µL, 17 µmol) in 0.45 mL of toluene-d8 was condensed onto the evacuated NMR tube at -196 °C. The reaction was monitored with 1H NMR spectroscopy as the temperature was increased gradually. Five Cp* 1H resonances (i.e. δ 2.05, 2.02, 1.98, 1.91, 1.84) were observed after 15 min at -60 °C of which the hydride (δ 2.05, 13%) and the propargyl (δ 1.91, 12%) represented only a small amount, as determined by line-shape analysis. In addition, several other unidentified vinylic signals were observed. After reaching room temperature nine Cp* 1H


55

resonances were observed (i.e. δ 2.04, 2.01, 1.99, 1.97, 1.96, 1.91, 1.88, 1.87, 1.84 ppm) of which the propargyl (δ 1.91 ppm, 13%) constituted only a small amount. No changes were observed with 1H NMR spectroscopy after 24 h at room temperature and the reaction mixture was quenched with CD3OD producing a mixture containing Cp*D, cis-1-phenylprop-1-ene-d1, 1-phenyl-1-propyne-d1 and phenylallene-d1, as indicated by 1H NMR and GC-MS. Cis-1-phenylprop-1-ene-d1: 1H NMR (C6D6, 25 °C, 500 MHz): δ 6.41 (dq, 4JHH = 1.7 Hz, 3JHH = 11.5 Hz, =CH, 1 H), 5.63 (dq, 3JHH = 11.5 Hz, 3JHH = 7.3 Hz, =CH, 1 H), 1.69 (dd, 4JHH = 1.7 Hz, CH3, 3 H). Reaction of [Cp*2La(µ-H)]2 with PhCCPh and CH3CCPh. NMR Scale. [Cp*2La(µ-H)]2 (5.0 mg, 6.1 µmol) was dissolved in C6D6 and PhCCPh (2.4 mg, 14 µmol) was added in the glovebox. The light-yellow solution turned immediately orange. A clean reaction forming the alkenyl derivative Cp*2LaC(Ph)=CH(Ph) was observed (vide infra). No changes in the 1H NMR spectrum were observed after standing for 5 days at room temperature. Upon addition of 1-phenyl-1-propyne (3.5 mg, 30 µmol) the alkenyl derivative was completely converted into the propargyl derivative Cp*2LaCH2CCPh within 24 h at room temperature, giving rise to cis-diphenylethene. The reaction mixture was quenched with CD3OD. GC/GC-MS and 1H NMR analysis indicated the presence of Cp*D, 1-phenyl-1-propyne-d1, 1-phenyl-1-propyne, phenylallene-d1 and cis-diphenylethene.

Cp*2LaC(Ph)=CH(Ph) (15a): 1H NMR (C6D6, 25 °C, 500 MHz): δ 7.69 (s, CH, 1 H), 7.29 (m, CH, 2 H), 7.12 (m, CH, 2 H), 7.04 (m, CH, 1 H), 6.99 (m, CH, 5 H), 1.84 (s, Cp*, 30 H). 13C NMR (125.7 MHz, C6D6, 25 °C): δ 10.95 (q, 1JCH = 125.2 Hz, C5Me5), 120.74 (m, C5Me5), 123.44 (dt, 1JCH = 152.6 Hz, 3JCH = 7.1 Hz, o-CH), 124.70 (dtd, 1JCH = 159.4 Hz, 2JCH = 1.2 Hz, 3JCH = 7.6 Hz, m-CH), 125.57 (dm, 1JCH = 154.1 Hz, o-CH), 126.59 (dt, 1JCH = 163.2 Hz, 3JCH = 7.1 Hz, m-CH), 129.89 (dtd, 1JCH = 153.7 Hz, 1JCH = 1.4 Hz, 1JCH = 7.9 Hz, p-CH), 130.83 (dd, 1JCH = 152.6 Hz, 1JCH = 152.6 Hz, p-CH), 134.28 (dt, 1JCH = 147.2 Hz, 3JCH = 4.0 Hz, =C(Ph)H), 146.25 (s, i-C), 151.08 (s, i-C). The signal corresponding to LaC was not observed.

Cis-diphenylethene: 1H NMR (C6D6, 25 °C, 500 MHz): δ 6.46 (s, =CH, 2 H). The aromatic proton signals overlapped with others. GC-MS, m/z (relative intensity): 180 (99), 179 (100), 178 (66), 177 (9), 176 (12), 166 (9), 165 (51), 152 (15), 151 (8), 102 (9), 89 (19), 76 (14), 63 (7), 51 (9). GC-MS, m/z (calc., found): 182 (1.1, 1.7), 181 (18.4, 15.3), 180 (100.0, 100.0). Cp*2LaC(Ph)=C(Ph)H (15a). To a suspension of [Cp*2La(µ-H)]2 (35.0 mg, 42.6 µmol) in cyclohexane (5 mL) was added diphenylacetylene (15.2 mg, 85.3 µmol). The light-yellow suspension turned into an orange solution immediately. Removal of the solvent in vacuo gave a yellow solid in quantitative yield. Isolated yield: 49.2 mg (98%). 1H NMR (500 MHz, C7D14, 25 °C): δ 7.40 (s, D), 7.22 (t, 3JHH = 7.7 Hz, G), 7.20 (t, 3JHH = 7.7 Hz, A), 7.07 (tq, 3JHH = 7.1 Hz, 4JHH = 7.4 Hz, F), 6.95 (tq, 3JHH = 7.4 Hz, , 4JHH = 1.1 Hz, B), 6.91 (d, 3JHH = 7.1 Hz, E), 6.73 (dq, 3JHH = 7.1 Hz, 4JHH = 7.1 Hz, C), 1.77 (s, Cp*, 30 H). 13C NMR (125.7 MHz, C7D14, 25 °C): δ 10.51 (q, 1JCH = 125.4 Hz, C5Me5), 120.60 (s, C5Me5), 122.97 (dt, 1JCH = 148.6 Hz, 3JCH = 7.1 Hz, 3), 124.25 (d, overlap hampered determination of coupling constants, 2), 125.53 (ddd, 1JCH = 153.5 Hz, 3JCH = 6.2 Hz, 3JCH = 6.2 Hz, 8), 126.18 (dt, 1JCH = 162.2 Hz, 3JCH = 7.3 Hz, 9), 129.58 (dd, 1JCH = 156.1 Hz, 3JCH = 7.9 Hz, 1), 130.83 (dd, 1JCH = 158.6 Hz, 3JCH = 7.9 Hz, 10), 133.72 (dt, 1JCH = 147.5 Hz, 3JCH = 4.1 Hz, 6), 146.25 (q, 3JCH = 6.6, 7), 150.83 (q, 3JCH = 7.5 Hz, 4), 217.47 (s, 5). 1H-13C gHSQC (500.0 MHz, 125.7 MHz, C7D14, 25 °C): A1, B2, C3, D6, E8, F8, G10. 1H-13C gHMBC (500.1 MHz, 125.7 MHz, C7D14, 25 °C): A2-4, B1,3, C1-5, D3-8, E6-9, F8,10, G7-9. Anal. Calcd. for C34H41La (588.61): C, 69.38%; H, 7.02%. Found: C, 69.51%; H, 6.90%. Reaction of Cp*2LaCH2CCPh with excess of H2. NMR Scale. Hydrogen gas was applied (1 atm.) onto a solution of Cp*2LaCH2CCPh (9.6 mg, 18.2 µmol) in 0.50 mL of benzene-d6. Within several minutes the yellow solution turned red and became darker in color upon standing. 1H NMR spectroscopy indicated the instantaneous formation of [(Cp*2La)(µ-H)]2. Two other unidentified lanthanocene derivatives formed concomitantly, based on the presence of new Cp* 1H NMR resonances at δ 1.95 amnd 1.71 ppm. The reaction mixture was allowed to stand at room temperature for 1 day during which the progress of reaction was monitored

Scheme 2-30. Numbering scheme of Cp*2LaC(Ph)=CH(Ph) (15a).

HCp*2La

A

B

C

D

E

F

G1

2

3 4

5 6

7 8

9

10

Chapter 2

56

periodically with 1H NMR spectroscopy. Upon standing, more [(Cp*2La)(µ-H)]2 formed at the expense of Cp*2LaCH2CCPh. After 12 h, Cp*2LaCH2CCPh was completely consumed, while the intensities of the Cp* 1H NMR resonances at δ 1.95 and 1.71 ppm started to decrease. After 18 h, Cp*2LaCH2CCPh and n-propylbenzene were the only compounds present in the product mixture, as indicated by NMR spectroscopy and GC/GC-MS analysis upon quenching with methanol. Reaction of [Cp*2La(µ-H)]2 with CH3CCC6H3Me2-2,6. NMR Scale. [Cp*2La(µ-H)]2 (7.8 mg, 9.5 µmol) was dissolved in C6D6 (0.50 mL) and CH3CCC6H3Me2-2,6 (3 mg, 20.8 µmol) was added with a microsyringe in the glovebox. The light-yellow solution turned darker and became light-orange within several minutes at room temperature. Within 10 min 1H NMR resonances of the hydride and substrate disappeared and Cp*2LaCH2CCC6H3Me2-2,6 and cis-1-(2,6-dimethylphenyl)prop-1-ene were observed. After 1 day at room temperature the reaction mixture did not change and was quenched with CD3OD forming Cp*D, cis-1-(2,6-dimethylphenyl)prop-1-ene, 1-(2,6-dimethylphenyl)-1-propyne-d1 and (2,6-dimethylphenyl)allene-d1 as indicated by 1H NMR and GC-MS. Cis-1-(2,6-dimethylphenyl)prop-1-ene: 1H NMR (C6D6, 25 °C, 300 MHz): δ 6.21 (dm, 3JHH = 11.4 Hz, =CH, 1 H), 5.64 (dq, 3JHH = 11.4 Hz, 3JHH = 6.9 Hz, =CH, 1 H), 2.15 (s, CH3, 6 H), 1.32 (dd, 3JHH = 11.4 Hz, 4JHH = 1.7 Hz, CH3, 3 H). Minor contamination with its monodeuterated derivative led to unreliable intensities in the mass spectrum.

1-(2,6-Dimethylphenyl)-1-propyne-d1: GC-MS, m/z (relative intensity): 147 (1), 146 (11), 145 (M+, 98), 131(7), 130 (M+ - CH3, 65), 129 (M+ - CH3 - H, 100), 128 (M+ - CH3 - 2 H or M+ - CH3 - D, 62), 116 (14), 115 (10), 89 (3), 77 (C6H6

+, 7), 51 (C4H3+, 8). GC-MS, m/z (calc., found): 147 (0.5, 0.7), 146 (12.0, 11.8), 145

(100, 100). (2,6-Dimethylphenyl)propa-1,2-diene-d1: GC-MS, m/z (relative intensity): 146 (4), 145 (M+, 32), 144

(M+ - H, 14), 143 (M+ - 2 H or M+ - D, 4), 142 (M+ - 3 H or M+ - H - D, 8), 131(12), 130 (M+ - CH3, 65), 129 (M+ - CH3 - H, 100), 128 (M+ - CH3 - 2 H or M+ - CH3 - D, 62), 115 (8), 103 (5), 102 (3), 77 (C6H6

+, 7), 63 (7), 51 (C4H3

+, 9). GC-MS, m/z (calc., found): 147 (0.5, 0.9), 146 (12.0, 13.0), 145 (100, 100). Reaction of [Cp*2La(µ-H)]2 with CH3CCC6H3

iPr2-2,6. NMR Scale. [Cp*2La(µ-H)]2 (6.6 mg, 8.0 µmol) was dissolved in C6D6 and CH3CCC6H3

iPr2-2,6 (3.5 µL, 17 µmol) was added with a microsyringe in the glovebox. No clear color change was observed. After 1 h at room temperature two major Cp* 1H NMR resonances are observed (δ 2.22, 1.92) of which the propargyl is the major species (58% according to line-shape analysis). Also cis-1-(2,6-diisopropyl-phenyl)-1-propene was observed. After 2 days at room temperature, the relative concentration of the propargyl (74%) had increased. The reaction mixture was subsequently quenched with methanol forming a mixture containing Cp*H, cis-1-(2,6-diisopropylphenyl)prop-1-ene, 1-(2,6-diisopropylphenyl)-1-propyne and (2,6-diisopropylphenyl)propa-1,2-diene as indicated by 1H NMR and GC-MS. Cp*2LaCH2CCC6H3

iPr2-2,6: 1H NMR (500 MHz, C6D6, 25 °C): δ 1.19 (d, 3JHH = 7.0 Hz, CH3, 12 H), 1.92 (s, Cp*, 30 H), 2.68 (s, CH2, 2 H), 3.34 (sept, 3JHH = 11.1 Hz, CHMe2, 2 H), 7.04-7.18 (m, CH). 13C NMR (125.7 MHz, C6D6, 25 °C): δ 11.00 (q, 3JCH = 125.1 Hz, C5(CH3)5), 23.37 (q, 3JCH = 125.5 Hz, CH3), 32.39 (d, CH), 47.69 (t, 1JCH = 156.7 Hz, CH2C≡ C), 100.31 (t, 3JCH = 7.0 Hz, CH2C≡ C), 119.24 (m, C5Me5), 126.87 (d, 1JCH = 159.4, p-CH), 122.87 (d, 1JCH = 155.6, m-CH), 128.70 (m, i-CH), 148.00 (m, o-CH), 145.61 (t, 2JCH = 2.4 Hz, CH2C≡ C). Cis-1-(2,6-diisopropylphenyl)prop-1-ene: 1H NMR (C6D6, 25 °C, 300 MHz): δ 6.37 (dq, 4JHH = 1.7 Hz, 3JHH = 11.1 Hz, =CH, 1 H), 5.71 (dq, 3JHH = 11.1 Hz, 3JHH = 6.8 Hz, =CH, 1 H), 3.19 (sept, 3JHH = 7.0 Hz, CHMe2, 2 H), 1.37 (dd, 4JHH = 1.7 Hz, 3JHH = 6.8 Hz, CH3, 3 H), 1.18 (d, 3JHH = 7.0 Hz, CH(CH3)2, 6 H).

(2,6-Diisopropylphenyl)propa-1,2-diene: 1H NMR (C6D6, 25 °C, 500 MHz): δ 6.16 (t, 4JHH = 7.0 Hz, CH, 1 H), 4.62 (d, 4JHH = 7.0 Hz, CH2, 2 H), 3.39 (sept, 3JHH = 7.0 Hz, CH, 2 H), 1.18 (d, 3JHH = 7.0 Hz, CH3, 12 H). Reaction of [Cp*2La(µ-H)]2 with CH3CCC6F5. NMR Scale. [Cp*2La(µ-H)]2 (10.7 mg, 13.0 µmol) was added to a solution of CH3CCC6F5 (6.2 mg, 3.0 µmol) in C6D6 in the glovebox. Several Cp* 1H NMR resonances are observed with 1H NMR spectroscopy and the reaction mixture changed slowly in time at RT. After 20 min at RT the 1H NMR resonances of the hydride and substrate have completely disappeared and four major Cp* 1H NMR resonances are observed (δ 1.88, 1.84, 1.83 and 1.80) of which the propargyl (δ 1.88) represents only 7% of the total Cp* 1H NMR resonance intensities according to line-shape analysis. An unknown species resonating at δ 1.80 represents the major species (71%). After 3 days more Cp* signals have appeared and the unknown species (δ 1.80, 37%) has been consumed in the favor of those resonating at δ 1.93 (unknown, 12%), 1.88 (propargyl, 14%) and 1.84 (assigned to the alkenyl species Cp*2LaC(C6F5)=C(CH3)H, 12%). 19F NMR analysis points to at least four major different pentafluorophenyl moieties (in a 0.4:1.0:0.3:0.3 ratio). No changes were observed with 1H NMR spectroscopy after 2 days at room temperature and CD3OD is added. NMR


57

and GC/GC-MS analysis pointed to the presence of 1-(pentafluorophenyl)prop-1-ene-d1 and 2,4-bis(pentafluorophenyl)-3-methylhexa-2,4-diene-d1 in a 1.00:0.33 ratio, respectively. Cp*2LaC(C6F5)=C(CH3)H: 1H NMR (C6D6, 25 °C, 400 MHz): δ 7.67 (q, 3JHH = 6.7 Hz, CH, 1 H), 1.84 (s, Cp*, 30 H), 1.51 (d, 3JHH = 6.7 Hz, CH3, 3 H).

1-(Pentafluorophenyl)prop-1-ene-d1: 1H NMR (C6D6, 25 °C, 400 MHz): δ 6.24 (m, CH, 1 H), 1.48 (dm, 3JHH = 6.7 Hz, , 4JHD = 1.8 Hz, CH3, 3 H). GC-MS, m/z (relative intensity): 210 (4), 209 (M+, 45), 208 (M+ - H, 84), 190 (M+ - F, 13), 189 (M+ - H - F, 26), 188 (M+ - F - 2H or M+ - F - D, 24), 187 (43), 181 (100), 169 (27), 161 (21), 158 (24). GC-MS, m/z (calc., found): 211 (0.4, 0.3), 210 (9.6, 8.7), 209 (100, 100).

2,4-Bis(pentafluorophenyl)-3-methylhexa-2,4-diene-d1: 1H NMR (C6D6, 25 °C, 400 MHz): δ 5.45 (m, CH, 1 H), 1.57 (s, CH3, 3 H), 1.48 (d, 3JHH = 6.8 Hz, CH3, 3 H). GC-MS, m/z (relative intensity): 417 (1), 416 (10), 415 (M+, 50), 401 (18), 400 (M+ - CH3, 100), 385 (M+ - 2 CH3, 11), 350 (17), 205 (17), 188 (16), 187 (23), 182 (32), 181 (59). GC-MS, m/z (calc., found): 417 (2.2, 1.8), 416 (19.0, 19.6), 415 (100, 100). Reaction of Cp*2LaCH2CCPh with phenylacetylene. NMR Scale. Phenylacetylene (2.2 µL, 20 µmol) was added with a microsyringe to a solution of Cp*2LaCH2CCPh (9.6 mg, 18.2 µmol) in 0.50 mL of benzene-d6 (containing 4.36 M hexa-methyldisiloxane as an internal standard). 1H NMR spectroscopy indicated the formation of [(Cp*2La)2(µ-η2:η2-PhC4Ph)], Cp*2LaC(Ph)=C(H)-CCPh and trans-1,4-diphenylbut-1-en-3-yne. The reaction mixture was allowed to stand at room temperature for 12 h after which two consecutive portions (4.4 and 21.0 µL) of phenylacetylene were added. After 1 day and 13C NMR analysis, the mixture was quenched with methanol-d4. GC/GC-MS analysis indicated the presence of 1-phenyl-1-propyne, phenylpropadiene, trans-1,4-diphenylbut-1-en-3-yne, 1,3-diphenylbut-1-en-3-yne, a trimer of phenylacetylene (m/z 306) and three unidentified dimers C18H16 (one of which was also found in the thermolysis reaction of Cp*2La(η3-CH2CCPh) and its THF adduct). The spectral parameters corresponding to [(Cp*2La)2(µ-η2:η2-PhC4Ph)], Cp*2LaC(Ph)=C(H)CCPh and the phenylacetylene oligomers were identical to those reported in Chapter 4.

C18H16: GC-MS, m/z (relative intensity): 234 (2), 233 (18), 232 (M+, 79), 217 (M+ - CH3, 21), 216 (20), 215 (33), 202 (M+ - 2CH3, 19), 153 (25), 141 (70), 128 (45), 115 (100), 91 (78). GC-MS, m/z (calc., found): 235 (0.1, 0.3), 234 (1.8, 2.3), 233 (19.6, 22.3), 232 (100.0, 100.0).

C18H16: GC-MS, m/z (relative intensity): 234 (2), 233 (21), 232 (M+, 93), 217 (M+ - CH3, 24), 216 (23), 215 (37), 202 (M+ - 2CH3, 22), 153 (27), 141 (74), 128 (45), 115 (100, 91 (76). GC-MS, m/z (calc., found): 235 (0.1, 0.1), 234 (1.8, 2.3), 233 (19.6, 21.6), 232 (100.0, 100.0).

Thermolysis of Cp*2LaCH2CCPh. NMR Scale. Cp*2LaCH2CCPh (15.0 mg, 28.6 µmol) was dissolved in toluene-d8 and heated in a sealed NMR tube for 12 days at 100 °C and then for 27 days at 120 °C. The progress of the reaction was monitored with 1H NMR spectroscopy. The mixture was quenched with methanol-d4. GC/GC-MS analysis indicated the presence of Cp*H-d5, Cp*H-d6 and two compounds of m/z 233-235 and 232-234, respectively. The latter two have been identified as oligodeuterated isomers of C18H16, based upon the identical retention times with nondeuterated C18H16 isomers found in other reactions. The mixture of deuterated isomers rendered the quantitative evaluation of the fragmentation pattern for each compound unreliable.

Thermolysis of Cp*2YCH2CCPh. NMR Scale. Cp*2YCH2CCPh (15.0 mg, 31.6 µmol) was dissolved in benzene-d6 and heated in a sealed NMR tube for 30 days at 120 °C. The progress of the reaction was monitored with 1H NMR spectroscopy. After completion the mixture was quenched with methanol and GC/GC-MS analysis indicated the presence of Cp*H-dn (n = 4,5) and three deuterated isomers of C18H16.

C18H14D2: GC-MS, m/z (relative intensity): 235 (7), 234 (M+, 36), 220 (19), 219 (100), 205 (17), 204 (53), 157 (14), 141 (23), 128 (22), 115 (47). GC-MS, m/z (calc., found): 237 (0.1, 0.0), 236 (1.8, 1.1), 235 (19.6, 18.0), 234 (100.0, 100.0).

C18H14D2: GC-MS, m/z (relative intensity): 236 (1), 235 (6), 234 (M+, 33), 220 (19), 219 (100), 205 (13), 204 (51), 203 (23), 202 (17), 157 (15), 141 (11), 142 (23), 141 (25), 128 (24), 115 (37), 91 (24), 77 (11). GC-MS, m/z (calc., found): 237 (0.1, 0.0), 236 (1.8, 1.6), 235 (19.6, 19.4), 234 (100.0, 100.0).

C18H15D: GC-MS, m/z (relative intensity): 235 (1), 234 (6), 233 (M+, 33), 232 (100), 218 (24), 217 (86). 216 (31), 215 (49), 203 (202 (32), 155 (19), 154 (10), 153 (14), 152 (14), 141 (10), 139 (10), 128 (12), 116 (10), 115 (27), 108 (17), 101 (15), 91 (10). GC-MS, m/z (calc., found): 236 (0.1, 0.0), 235 (1.8, 2.6), 234 (19.6, 18.7), 233 (100.0, 100.0). Reaction of Cp*2LaCH2CCPh with THF. NMR Scale. THF (2.3 µL, 29 µmol) was added with a microsyringe to a solution of Cp*2LaCH2CCPh (15.0 mg, 29.6 µmol) in benzene-d6 in the glove box. 1H NMR spectroscopy indicated the clean formation of Cp*2La(η3-CH2CCPh)(THF). The reaction mixture was monitored with 1H NMR for 2 days at room temperature, before heating to 50 °C. After 1 day at 50 °C, the mixture was

Chapter 2

58

quenched with methanol-d4 and GC/GC-MS analysis indicated only the presence of an unidentified dimer C18H16, besides the expected quenching products Cp*D and THF. Cp*2La(η3-CH2CCPh)(THF): 1H NMR (300 MHz, C6D6, 25 °C): δ 1.37 (m, CH2, β-CH2, 4 H), 1.97 (s, Cp*, 30 H), 2.95 (s, CH2, 2 H), 3.55 (m, OCH2, α-CH2, 4 H), 6.95 (t, 3JHH = 7.4 Hz, p-H, 1 H), 7.11 (dd, 3JHH = 7.4 Hz, 3JHH = 7.4 Hz, m-H, 2 H), 7.24 (d, 3JHH = 7.4 Hz, 3JHH = 7.4 Hz, o-H, 2 H). 13C NMR (75 MHz, C6D6, 25 °C): δ 11.02 (C5(CH3)5), 52.63(CH2CC), 25.68 (β-CH2), 68.35 (α-CH2), 118.86 (C5Me5), 124.91 (p-CH), 128.84 (i-C), 128.55 (m-CH), 129.05 (o-CH). Signals corresponding to CH2CC and CH2CC not observed.

C18H16: GC-MS, m/z (relative intensity): 234 (2), 233 (17), 232 (M+, 84), 218 (21), 217 (M+ - CH3, 100), 216 (43), 215 (67), 205 (17), 202 (M+ - 2CH3, 60), 189 (23), 115 (55). GC-MS, m/z (calc., found): 235 (0.1, 0.2), 234 (1.8, 1.9), 233 (19.6, 18.6), 232 (100, 100).

Cp*2La(η3-CH2CCPh)(py) (7a·py). Pyridine (8.0 µL, 98 µmol) was added with a microsyringe to a solution of Cp*2LaCH2CCPh (50 mg, 95 mmol) in hexanes (2 mL) in the glove box. Stirring for 2 h at room temperature produced a clear light-yellow solution. Cooling and concentrating the solution in vacuo afforded yellow crystals suitable for X-ray analysis. Isolated yield: 52 mg (90%). 1H NMR (300 MHz, C6D6, 25 °C): δ 1.94 (s, Cp*, 30H), 3.16 (s, CH2, 2H), 6.51 (ddd, 3JHH = 7.7 Hz, 3JHH = 4.5 Hz, 4JHH = 1.5 Hz, β-H, 2 H), 6.81 (tt, 3JHH = 7.6 Hz , 4JHH = 1.9 Hz, γ-H, 1 H), 6.96 (tt, 3JHH = 7.4 Hz, 4JHH = 1.2 Hz, p-H, 1 H), 7.13 (d, 3JHH = 7.4 Hz, m-H, 2 H), 7.24 (d, 3JHH = 7.9 Hz, o-H, 2 H), 8.40 (dd, 3JHH = 4.5 Hz, 3JHH = 1.9 Hz, α-H, 2 H). 13C NMR (125.7 MHz, C6D6, 25 °C): δ 11.55 (q, 1JCH = 125.1 Hz, C5(CH3)5), 51.07 (t, 1JCH = 159.5 Hz, CH2CC), 117.96 (m, C5Me5), 123.72 (d, 1JCH = 153.6 Hz, p-CH), 123.79 (d, 1JCH = 165.6 Hz, β-CH), 128.58 (m- and o-CH), 136.43 (m, i-C), 137.04 (d, 1JCH = 163.3 Hz, α-CH), 152.16 (s, CH2C≡ C). The signal corresponding to CH2CC was not observed. IR (nujol, [cm-1]): 2923 (s), 2853 (s), 2713 (m), 1973 (m), 1741 (w), 1653 (w), 1591 (m), 1459 (s), 1377 (m), 1215 (m), 1150 (m), 1067 (m), 1021 (m), 898 (m), 838 (m), 748 (m), 694 (m), 686 (m), 477 (s). Anal. Calcd. for C34H42LaN (603.62): C, 67.65%; H, 7.01%. Found: C, 67.51%; H, 6.90%. Cp*2Y(η3-CH2CCPh)(py) (7b·py). Pyridine (20.5 µL, 253 µmol) was added with a microsyringe to a stirred solution of Cp*2YCH2CCPh (120 mg, 253 µmol) in hexanes (4 mL) in the glove box. Immediately the orange solution turned pale yellow. Upon standing at room temperature, the yellow solution gradually turned red and a dark red suspension formed after 1 day. NMR and GC/GC-MS analysis suggested decomposition of the initially formed base adduct into several unidentified compounds. Cooling a freshly prepared solution afforded off-white crystals. Isolated yield: 88.3 mg (63%). After isolation the crystals turned red within several hours at room temperature. Cp*2Y(CH2CCPh)(py): 1H NMR (400 MHz, C6D6, 25 °C): δ 1.93 (s, Cp*, 30 H), 2.71 (s, CH2, 2 H), 6.65 (dd, 3JHH = 5.4 Hz, 3JHH = 5.4 Hz, β-CH, 2 H), 6.94 (m , γ-CH, 1 H), 6.97 (dd, 3JHH = 7.6 Hz, 3JHH = 7.6 Hz, p-CH, 1 H), 7.09 (dd, 3JHH = 7.7 Hz, 3JHH = 7.7 Hz, m-CH, 2 H), 7.22 (dd, 3JHH = 7.8 Hz, 3JHH = 7.8 Hz, o-CH, 2 H), 8.53 (m, α-CH, 2 H). 13C NMR (125.7 MHz, C6D6, 25 °C): δ 11.21 (q, 1JCH = 125.8 Hz, C5(CH3)5,), 47.22 (dt, 1JCH = 157.2 Hz, 1JYC = 7.7 Hz, CH2CC), 104.88 (d, 1JYC = 9.7 Hz, CH2CC), 117.65 (m, C5Me5), 123.46 (d, 1JCH = 163.0 Hz, β-CH), 126.39 (dt, 1JCH = 160.9 Hz, 3JCH = 7.4 Hz, p-CH), 128.57 (dd, 1JCH = 168.4 Hz, 3JCH = 7.6 Hz, m-CH), 132.06 (dtd, 1JCH = 159.3 Hz, 3JCH = 6.3 Hz, 4JCH = 1.9 Hz, Hz, o-CH), 132.68 (m, i-C), 135.49 (d, 1JCH = 160.8 Hz, γ-CH), 150.27 (d, 1JCH = 179.4 Hz, α-CH), 151.58 (s, CH2CC). IR (nujol, [cm-1]): 2925 (s), 2854 (m), 2727 (w), 2148 (m), 1920 (w), 1588 (w), 1462 (m), 1377 (m), 1238 (m), 1021 (m), 855 (w), 800 (w), 759 (w), 705 (m), 520 (s). Reaction of Cp*2LaCH2CCPh with 1-phenyl-1-propyne. NMR Scale. Cp*2La(η3-CH2CCPh) (10.0 mg, 19.1 µmol) was dissolved in 0.50 mL of a benzene-d6 solution of hexamethyldisiloxane (1.4 mM). After addition of 1-phenyl-1-propyne (47.7 µL, 381 µmol) with a microsyringe the mixture was heated to 120 °C and monitored with 1H NMR spectroscopy. After 7 day at 120 °C the substrate was completely consumed and the mixture was quenched with methanol-d4. GC/GC-MS analysis indicated the presence of 1-phenylprop-1-ene, Cp*D, phenylpropadiene-d1, 1-phenyl-1-propyne-dn and at least 15 isomers of C18H16. The three major C18H16 isomers were assigned to the three major products which were characterized by 1D and 2D NMR spectroscopy (Chapter 3). Reaction of Cp*2LaCH2CCPh with 1-pentafluorophenyl-1-propyne. NMR Scale. Cp*2LaCH2CCPh (10.0 mg, 19.7 µmol) was added to a solution 1-pentafluorophenyl-1-propyne (78.6 mg, 381 µmol) in benzene-d6 (0.50 mL). 1H NMR spectroscopy indicated the formation of Cp*2La(η3-CH2CCC6F5). The reaction mixture was monitored with 1H and 19F NMR for 20 days at 80 °C, quenching with methanol-d4. GC/GC-MS analysis indicated the presence of 1-pentafluorophenyl-1-propyne-dn, pentafluorophenylpropadiene-d1, Cp*D, 1-phenyl-1-propyne and several organic compounds of which the major products were identified as C18H5DF10 (m/z 413) and four isomers of C27H13DF10 (m/z 529) on the basis of their fragmentation pattern.


59

C18H5DF10: GC-MS, m/z (relative intensity): 413 (M+, 49), 412 (100), 398 (21), 397 (M+ - CH3, 47), 378 (87), 328 (21), 245 (25), 232 (10), 231 (37), 205 (82), 193 (41), 181 (96), 161 (26). GC-MS, m/z (calc., found): 416 (0.1, -), 415 (1.8, 1.5), 414 (19.5, 17.6), 413 (100.0, 100.0).

C27H13DF10: GC-MS, m/z (relative intensity): 531 (1), 530 (11), 529 (M+, 38), 514 (M+ - CH3, 17), 407 (12), 379 (17), 347 (28), 181 (39), 123 (84), 122 (100), 121 (38), 108 (37), 107 (27), 106 (39), 92 (26), 91 (20). GC-MS, m/z (calc., found): 532 (0.4, -), 531 (4.0, 3.5), 530 (29.3, 30.3), 519 (100.0, 100.0).

C27H13DF10: GC-MS, m/z (relative intensity): 531 (2), 530 (12), 529 (M+, 44), 514 (M+ - CH3, 22), 407 (12), 379 (19), 347 (23), 213 (17), 205 (36), 187 (19), 181 (37), 123 (84), 122 (100), 121 (40), 92 (26), 91 (24). GC-MS, m/z (calc., found): 532 (0.4, -), 531 (4.0, 4.6), 530 (29.3, 27.7), 519 (100.0, 100.0). Reaction of Cp*2LaCH2CCPh with an excess of 1-hexene. NMR Scale. A solution of Cp*2LaCH2CCPh (5.0 mg, 9.5 µmol) in 0.50 mL of benzene-d6 containing hexamethyldisiloxane (4.5 mM) was prepared and transferred into a NMR tube. After addition of 1-hexene (173 µL, 1.38 mmol, 145 equiv.) the tube was sealed off and heated to 100 °C. The progress of the reaction was monitored with 1H NMR spectroscopy. After 3.59 days at 100 °C no changes were observed and the reaction mixture was heated to 120 °C for 74 days. The reaction mixture was quenched with methanol-d4. GC and GC-MS analysis indicated the presence of 1-hexene, hexane, cis- and trans-2-hexene, cis- and trans-7-methyl-4-undecene and 3-dodecene. The unidentified organic compounds with m/z 170 (C12H26), 166 (two isomers of C12H22), 167 (C12H23), 195 (C15H15), 197 (C15H17), 200 (cross-coupling product, C15H20), 250 (two isomers of a 1-hexene trimer, C18H34), 281 (three isomers of C21H29) and 334 (1-hexene tetramer, C24H46) were assigned to their elemental composition, based on the assumption that they represented derivatives of homo- or cross-coupling products of 1-hexene and 1-phenyl-1-propyne.

C12H26: GC-MS, m/z (calc., found): 168 (0.8, 1.0), 167 (13.3, 14.3), 166 (100.0, 100.0). Two isomers of C12H22: GC-MS, m/z (calc., found): 168 (0.8, 1.0), 167 (13.3, 14.3), 166 (100.0, 100.0); m/z (calc., found): 168 (0.8, 1.0), 167 (13.3, 14.7), 166 (100.0, 100.0). C15H20: GC-MS, m/z (calc., found): 202 (1.1, 2.0), 201 (16.5, 19.9), 200 (100.0, 100.0). C15H17: GC-MS, m/z (calc., found): 199 (1.1, 2.1), 198 (16.5, 23.0), 197 (100.0, 100.0). C15H15: GC-MS, m/z (calc., found): 197 (1.1, 2.3), 196 (16.5, 17.5), 195 (100.0, 100.0)

Table 2-9: Summary of crystallographic data of the Cp*2La(η3-CH2CCPh) (7a), Cp*2Y(η3-CH2CCPh) (7b) and [Cp*2La(η3-CH2CCPh)(NC5H5)] (7a·py) complexes.

7a 7b 7a·C5H5N Molecular formula C29H37La C29H37Y C34H42LaN FW 524.52 474.52 603.62

T, K 100(1) 100(1) 100(1) Crystal system orthorhombic orthorhombic triclinic Space group Pna21 Pca2a1 P1,2 a, Å 20.064(1) 16.4646(9) 8.7176(4) b, Å 10.051(2) 10.5009(6) 9.6510(4) c, Å 12.233(3) 14.7315(8) 18.4634(9) α(°) - - 78.405(1) β (°) - - 87.337(1) γ(°) - - 72.103(1) V, Å3 2466.9(8) 2547.0(2) 1447.86(11) Z 4 4 2 ρcalc, gcm-1 1.412 1.237 1.385 F(000) 1072 1000 620 µ(Mo Kα), cm-1 17.44 23.02 14.97 θ range (°) 2.27, 27.48 2.30, 28.28 2.26, 28.28 wR(F2) 0.0990 0.0685 0.0666 refined reflections 5124 6224 6986 refined parameters 276 419 493 R(F) for Fo > 4.0 σ(Fo)

0.0463 0.0297 0.0307

GooF 1.025 1.030 1.028 Weighting (a, b) 0.0, 0.0 0.0393, 0.0 0.0316, 0.0

Chapter 2

60

Two isomers of C18H34: GC-MS, m/z (calc., found): 253 (0.1, 0.0), 252 (1.8, 2.8), 251 (20.0, 24.5), 250 (100.0, 100.0); m/z (calc., found): 253 (0.1, 1.4), 252 (1.8, 8.1), 251 (20.0, 20.5), 250 (100.0, 100.0).

Three isomers of C21H29: GC-MS, m/z (calc., found): 284 (0.2, 0.0), 283 (2.6, 5.9), 282 (23.1, 36.1), 281 (100.0, 100.0); m/z (calc., found): 253 (0.1, 1.4), 252 (1.8, 8.1), 251 (20.0, 20.5), 250 (100.0, 100.0). C24H46: GC-MS, m/z (calc., found): 337 (0.2, 0.0), 336 (3.3, 3.1), 335 (26.3, 26.3), 334 (100.0, 100.0).

X-ray crystallography. In the glovebox, the crystals were transferred from the reaction vessels to a Petri dish filled with a small amount of light mineral oil. A suitable crystal was chosen by examination under a microscope. This crystal was attached to glass fiber which was mounted in a cold nitrogen stream on a Bruker133 SMART APEX CCD diffractometer for data collection. Data collection and structure solution were conducted at the University of Groningen. The structures were solved by Patterson methods and extension of the model was accomplished by direct methods applied to difference structure factors using the program DIRDIF.134 Relevant crystal and data collection parameters are given in Table 2-9.

2.8. References and notes 1 For general reviews of organo rare-earth metal chemsitry, see: (a) Marks, T. J.; Ernst, R. D. In

Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982; Chapter 21. (b) Evans, W. J. Adv. Organomet. Chem. 1985, 24, 131-177. (c) Schaverien, C. J. Adv. Organomet. Chem. 1994, 36, 283-362. (d) Edelmann, F. T. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1995; Vol. 4, Chapter 2. (e) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem. Rev. 1995, 95, 865-986. (f) Anwander, R.; Herrmann, W. A. Top. Curr. Chem. 1996, 179, 1. (g) Edelmann, F. T. Top. Curr. Chem. 1996, 179, 247. (h) Molander, G. A. Chemtracts: Org. Chem. 1998, 18, 237-263. (i) Edelmann, F. T. In Metallocenes; Togni, A., Halterman, R. L., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 1, Chapter 2. (j) Chirik, P. J., Bercaw, J. E. In Metallocenes; Togni, A., Halterman, R. L., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 1, Chapter 3. (k) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H, Chem. Rev. 2002, 102, 1851-1896. (l) Arndt, S.; Okuda, J. Chem. Rev. 2002, 102, 1953-1976 (m) Shibasaki, M.; Yoshikawa, N. Chem. Rev. 2002, 102, 2187-2210. (n) Inanaga, J.; Furuno, H.; Hayano, T. Chem. Rev. 2002, 102, 2211-2226.

2 (a) Klein, J. In The Chemistry of the Carbon-Carbon Triple Bond; Patai, S., Ed.; Wiley: New York, 1978; Part 1; Chapter 9, p. 343. (b) Moreau, J. In The Chemistry of Ketenes, Allenes and Related Compounds; Patai, S., Ed.; Wiley: New York, 1980; Part 1; p. 363. (c) Brandsma, L.; Verkruijsse, H. D. Synthesis of Acetylenes, Allenes and Cumulenes. A Laboratory Manual; Elsevier: Amsterdam; 1981. (d) Bielmann, J.-F., Ducep, J.-B. Org. React. 1982, 27, 1. (e) Landor, P. D. In The Chemistry of the Allenes; Landor, R. S., Ed.; Academic Press: London, 1982; p. 19. (f) Epsztein, R. In Comprehensive Carbanion Chemistry; Buncel, E., Durst, T., Eds.; Elsevier: Amsterdam, 1984; Part B; p. 107. (g) Schuster, H. F., Coppola, G. M. Allenes in Organic Chemistry; Wiley: New York, 1984; Chapter 10. (h) Yamamoto, H. In Comprehensive Organic Synthesis; Trost, B. M.; Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 2; p. 81. (i) Tsuji, J.; Mandai, T. Angew. Chem. Int. Ed. Engl. 1995, 34, 2589.

3 For reviews, see: (a) Wojcicki, A.; Shuchart, C. E. Coord. Chem. Rev. 1990, 105, 35. (b) Welker, M. E. Chem. Rev. 1992, 92, 97. (c) Doherty, S.; Corrigan, J. F.; Carty, A. J.; Sappa, E. Adv. Organomet. Chem. 1995, 37, 39. (d) Wojcicki, A. New J. Chem. 1994, 18, 61. (e) Chen, J.-T. Coord. Chem. Rev. 1999, 190-192, 1143. (f) Wojcicki, A. Inorg. Chem. Comm. 2002, 5, 82.

4 For examples, see: (a) Mo: Krivykh, V. V.; Taits, E. S.; Petrovskii, P. V.; Struchkov, Y. T.; Yanovski, A. I. Mendeleev Commun. 1991, 103. (b) Zr: Blosser, P. W.; Gallucci,J. C.; Wojcicki, A. J. Am. Chem. Soc. 1993, 115, 2994. (c) Re: Casey, C. P.; Yi, C. S. J. Am. Chem. Soc. 1992, 114, 6597. (d) Pt: Huang, T.-M.; Chen, J.-T.; Lee, G. H.; Wang, Y. J. Am. Chem. Soc. 1993, 115, 1170. (e) Pt: Blosser, P. W.; Schimpff, D. G.; Gallucci, J. C.; Wojcicki, A. Organometallics 1993, 12, 1993. (f) Pt: Stang, P. J.; Critell, C. M.; Arif, A. M. Organometallics 1993, 12, 4799. (g) Pd: Baize, M. W.; Blosser, P. W.; Plantevin, V.; Schimpff, Gallucci, J. C.; Wojcicki, A. Organometallics 1996, 15, 164. (h) Zr: Rodriguez, G.; Baza, G. C. J. Am. Chem. Soc. 1997, 119, 343.

5 Watson, P. L. In Selective Hydrogen Carbon Activation. Principles and Progress; Davies, J. A.; Watson, P. L.; Greenberg, A.; Liebman, J. F. (Eds.); VCH: Weinheim, 1990, Chapter 4, p.78.


61

6 (a) Heeres, H. J.; Heeres, A.; Teuben, J. H. Organometallics 1990, 9, 1508. (b) Heeres, H. J. Ph. D.

Thesis, University of Groningen, 1990; Chapter 5. 7 Hajela, S.; Schaefer, W. P.; Bercaw, J. E. J. Organomet. Chem. 1997, 532, 45. 8 Ihara, E.; Tanaka, M.; Yasuda, H.; Kanehisa, N.; Maruo, T.; Kai, Y. J. Organomet. Chem. 2000, 613,

26. 9 Ferrence, G. M.; McDonald, R.; Takats, J. Angew. Chem. Int. Ed. 1999, 38, 2233. 10 For examples, see: (a) Tabuchi, T.; Inanag, J.; Yamaguchi, M. Tetrahedron Lett. 1986, 27, 5237. (b)

Tabuchi, T.; Inanag, J.; Yamaguchi, M. Chem. Lett. 1987, 2275. (c) Makioka, Y.; Koyama, K.; Nishiyama, T.; Takaki, K.; Taniguchi, Y.; Fujiwara, Y. Tetrahedron Lett. 1995, 36, 6283. (d) Mikami, K.; Yoshida, A.; Matsumoto, S.; Feng, F.; Matsumoto, Y.; Sugino, A.; Hanamoto, T.; Inanag, J. Tetrahedron Lett. 1995, 36, 907. (e) Makioka, Y.; Saiki, A.; Takaki, K.; Taniguchi, Y.; Kitamura, T.; Fujiwara, Y. Chem. Lett. 1997, 27.

11 (a) Eckenberg, P.; Groth, U.; Kohler, T. Liebigs Ann. Chem. 1994, 673. (b) Ruan, R.; Zhang, J.; Zhou, X.; Cai, R. Chem. Commun. 2002, 538.

12 Negishi et al. published the synthesis of 2-ethynyl-p-xylene and mesitylacetylene via palladium-catalyzed cross-coupling with ethynylzinc bromide, see: (a) Negishi, E.; Kotora, M.; Xu, Caiding J. Org. Chem. 1997, 62, 8957. For a review on palladium-catalyzed alkynylation, see: (b) Negishi, E.; Anastasia, L. Chem. Rev. 2003, 103, 1979.

13 Ohno, A.; Tsutsumi, A.; Yamazaki, N.; Okamura, M.; Mikata, Y.; Fujii, M. Bull. Chem. Soc. Jpn. 1996, 69, 1679.

14 Subsequent attempts to purify the crude product mixture by means of column chromatography and vacuum distillation failed. Variation in reaction parameters, such as solvent, reaction time and temperature, did not produce the high crude yields, as typically found for aqueous dediazotization reactions of arylamines. For general remarks on the iododediazotization reaction, see: (a) March, J. Advanced Organic Chemistry, 4th ed.; Wiley: New York, 1992. (b) Vogel, A. I.; Furniss, B. S. Vogel's Textbook of Practical Organic Chemistry, 5th ed.; Longman: London, 1991.

15 Although the synthetic utility of the iododediazotization reaction has been exploited for a several decades, its mechanism has not yet been satisfactorily elucidated, despite considerable study. For reviews, see: (a) March, J. Advanced Organic Chemistry, 4th ed.; Wiley: New York, 1992. (b) Vogel, A. I.; Furniss, B. S. Vogel's Textbook of Practical Organic Chemistry, 5th ed.; Longman: London, 1991. For examples of mechanistic studies of the iododediazotization reaction, see: (b) Abeywickrema, A. N.; Beckwith, A. L. J. J. Org. Chem. 1987, 52, 2568. (c) Packer, J. E.; Taylor, R. E. R. Aust. J. Chem. 1985, 38, 991. (d) Carey, J. G.; Jones, G.; Millar, I. T. Chem. Ind. 1960, 97. (b) Hodgson, H. H.; Birtwell, S.; Walker, J. J. Chem. Soc. 1941, 770. (e) Waters, W. A. J. Chem. Soc. 1942, 266. (f) Galli, C. J. Chem. Soc., Perkin Trans. 2 1981, 1459. (g) Kumar, R.; Singh, P. R. Tetrahedron Lett. 1972, 613. (h) Beckwith, A. L. J.; Meijs, G. F. J. Chem. Soc., Chem. Commun. 1981, 136.

16 In contrast to bromo- and chlorodediazotization reactions, the use of copper is generally assumed not to be required in iododediazotization reactions.14 The replacement of the diazonium group by bromide or chloride in the presence of the appropriate copper(I) salts represents the most widely used method to prepare arylbromides and -chlorides (the Sandmeyer reaction). The Sandmeyer reaction is believed to occur via the reduction of diazonium salts by copper(I) species to give aryl radicals that subsequently abstract halogen from copper(II) halides. The use of finely divided (i.e. freshly precipitated) copper or copper bronze is known to act catalytically in the decomposition of diazonium salts, but the yields in the preparation of arylhalides (Gatterman reaction) are usually not as high as those obtained by the Sandmeyer reaction.

17 (a) Hunter, C. A.; Jones, P. S.; Tiger, P. M. N.; Tomas, S. Chem. Commun. 2003, 1642. (b) Motti, E.; Rossetti, M.; Bocelli, G.; Catellani, M. J. Organomet. Chem. 2004, 689, 3741. (c) Alcalde, E.; Dinarès, Rodríguez, S.; Garcia de Miguel, C. Eur. J. Org. Chem. 2005, 1637.

18 Li, Z.; Dong, Y.; Mi, B.; Tang, Y.; Häussler, M.; Tong, H.; Dong, Y.; Lam, J. W. Y.; Ren, Y.; Sung, H. H. Y.; Wong, K. S.; Gao, P.; Williams, I. D.; Kwok, H. S.; Tang, B. Z. J. Phys. Chem. 2005, 109, 10061.

19 (a) van Zanten, B.; Nauta, W. Th. Recl. Trav. Chim. Pays-Bas 1960, 79, 1211. 20 Knorr, R.; Ruhdorfer, J.; Böhrer, P.; Bronberger, H.; Räpple, E. Liebigs Ann. Chem. 1994, 433.

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21 (a) Metal-Catalyzed Cross-Coupling Reactions, Diederich, F., Stang, P. J. (Eds.); Wiley: New York,

1997. (b) Tsuji, J. Palladium Reagents and Catalysts, Wiley: New York, 1995. 22 Lithiation or stannylation of 2,6-diisopropylbromobenzene followed by iodinolysis was not explored

in this study as a synthetic route towards the desired iodo compound. For examples of this approach in the preparation of 2,6-dimethyliodo-benzene, see: (a) Sasaki, Y.; Hirabuki, M.; Ambo, A.; Ouchi, H.; Yamamoto, Y. Bioorg. Med. Chem. Lett. 2001, 11, 327. For a review on the synthesis of iodoarenes, see: (b) Merkushev, E. B. Synthesis 1988, 923.

23 (a) Giumanini, A. G.; Verardo, G.; Gorassini, F.; Strazzolini, P. Recl. Trav. Chim. Pays-Bas 1995, 114, 311. (b) Giumanini, A. G.; Verardo, G.; Geatti, P.; Strazzolini, P. Tetrahedron 1996, 52, 7137.

24 Friedman, L.; Chlebowski, J. F. J. Org. Chem. 1968, 33, 1636. 25 For example, reactions of 2,6-diisopropylaniline with n-butyl nitrite and several iodinating agents (i.e.

iodine, potassium and copper iodide, freshly precipitated copper(I) iodide and methyl iodide), under a nitrogen atmosphere and in air, in the presence and absence of a crown ether, in a variety of solvents (i.e. chloroform, dichloromethane, tetrachloromethane, benzene and THF mixtures of the aforementioned solvents) and at different reaction temperatures yielded mixtures containing the desired iodo compound only in low crude yields (2-20%, GC/GC-MS).

26 2,6-Diisopropylbenzenediazonium tetrafluoroborate was prepared successfully by the general method of Roe and Hawkins from n-butyl nitrite which was added to a solution of the corresponding amine in 40% tetrafluoroboric acid and ethanol. This compound could be isolated as a white solid and was found to be stable, when it was kept moist with ether below room temperature, but decomposed spontaneously in the dry state. See: Roe, A.; Hawkins, G. F. J. Am. Chem. Soc. 1947, 69, 2443.

27 It has been reported that the Negishi reaction tolerates sterically congested substrates better than the Sonogashira reaction and other palladium-catalyzed cross-coupling protocols based on organomagnesium and -tin reagents (the Stille reaction).12b

28 Performing cross-coupling reactions with sterically hindered substrates represents, in fact, one of the major challenges in this area of research and efforts aimed at developing catalyst systems that are able to bring about this type of reactions are ongoing. Some progress has recently been achieved, but most examples involve either catalysts that are not readily available or the cross-coupling of two sp2 carbons in the synthesis of hindered biaryl compounds. Only recently, efficient catalysts for the Sonohashira coupling of sterically hindered aryl halides with 1-alkynes have been described. For examples, see: (a) Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fy, G. C. Org. Lett. 2000, 2, 1729. (b) Köllhofer, A.; Pullmann, T.; Plenio, H. Angew. Chem. Int. Ed. 2003, 42, 1056. (c) Feuerstein, M.; Berthiol, F.; Doucet, H.; Santanelli, M. Synthesis 2004, 1281 and references therein.

29 The propynyl derivative could successfully be prepared from propynylmagnesium bromide and triisopropoxyborane, according to a general procedure for the synthesis of alkynyldiiso-propoxyboranes, see: (a) Brown, H. C.; Bhat, N. G.; Srebnik, M. Tetrahedron Lett. 1988, 29, 2631. (b) Brown, H. C.; Cole, T. E. Organometallics 1983, 2, 1317. (c) Pelter, A.; Smith, K.; Brown, H. C. In Borane Reagents; Academic Press: London, 1988; Chapter 2. (d) Chong, J. M.; Shen, L.; Taylor, N. J. J. Am. Chem. Soc. 2000, 122, 1822. (e) Brown, C. D.; Chog, J. M.; Shen, L. Tetrahedron 1999, 55, 14233.

30 For reviews on the Suzuki-Miyaura reaction, see: (a) a) Suzuki, A.; Miyaura, N. Chem. Rev. 1995, 95, 2457. (b) Suzuki, A.; J. Organomet. Chem. 1999, 576, 147. For examples involving the Suzuki-Miyaura cross-coupling of di-ortho-substituted aryl halides, see: (c) Watanabe, T.; Miyaura, N.; Suzuki, A. Synlett 1992, 207. (d) Rocca, P.; Marsais, F.; Godard, A.; Quéguiner, G. Tetrahedron Lett. 1993, 34, 2937. (e) Hoye, T.; Chen, M. J. Org. Chem. 1996, 61, 7940. (f) Feuerstein, M.; Doucet, H.; Santelli, M. Tetrahedron Lett. 2001, 42, 6667. (g) Yin, J.; Rainka, M. P.; Zhang, X.-X.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124, 1162. (h) Bedford, R. B.; Hazelwood, S. L.; Limmert, M. E. Chem. Commun. 2002, 2610. (i) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F. Angew. Chem. Int. Ed. 2003, 42, 3690. (j) Navarro, O.; Kelly, R. A.; Nolan, S. P. J. Am. Chem. Soc. 2003, 125, 16194. (k) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F. J. Am. Chem. Soc. 2004, 126, 15195.

31 For the synthesis of 2,6-diisopropylphenyl triflate, see: (a) Netscher, T.; Bohrer, P. Tetrahedron Lett. 1996, 37, 8359. (b) Stang, P. J.; Hanack, M.; Subramanian, L. R. Synthesis 1982, 85.

32 Zimmerman, H. E.; Dodd, J. R. J. Am. Chem. Soc. 1970, 92, 6507. 33 For examples, see: (a) Stephens, R. D.; Castro, C. E. J. Org. Chem. 1963, 28, 3313. (b) Castro, C. E.;

Stephens, R. D. J. Org. Chem. 1963, 28, 2163. (c) Castro, C. E.; Gaughan, E. J.; Owsley, D. C. J.


63

Org. Chem. 1966, 31, 4071. (d) Owsley, D. C.; Castro, C. E. Org. Synth. Coll. Vol. 1988, 6, 916. For reviews, see: (e) Lindley, J. Tetrahedron 1984, 40, 1433. (f) Normant, J. F. Synthesis 1972, 63. (g) Jukes, A. E. Adv. Organomet. Chem. 1974, 12, 215. (h) Posner, G. H. Org. React. 1975, 22, 253. (i) Sladkov, A. M.; Gol’ding, I. R. Russ. Chem. Rev. 1979, 48, 868.

34 Copper(I) acetylides are dangerous, because they undergo hazardous explosions, when shocked or exposed to heat. Because of their high sensitivity to shock, friction and heat, copper acetylides must be handled with extreme care. They must be kept cool and, if they are to be stored, should be kept wet. See: (a) Bretherick, L. Handbook of Reactive Chemical Hazards, 2nd edition, Butterworths: London, 1979. (b) Sax, N. I. Dangerous Properties of Industrial Materials, 5th edition, Reinhold: New York, 1979. (c) Brameld, V. F.; Clark, M. T.; Seyfang, A. P. J. Soc. Chem. Ind. 1947, 66, 346.

35 According to a general procedure in which copper(I) acetylides are prepared in situ from organolithium or -magnesium reagents in ether or THF, propynylmagnesium bromide was added to a suspension containing copper(I) iodide in ether at 0 °C. However, replacing ether in vacuo by solvents with higher boiling points such as THF, di-n-butyl ether (bp 141 °C), dimethylformamide (bp 153 °C), and pyridine (bp 115 °C) and the subsequent addition of the iodoarene did not result in the formation of the desired substitution product after prolonged reaction times and at refluxing temperatures. Also the use of THF and freshly precipitated copper(I) iodide in the preparation of the copper(I) acetylide did not change this outcome. Analogous reactions using propynyllithium as a starting material (formed in situ from propyne gas and n-butyl lithium in THF according to reported procedures127) failed also to give the desired propynylarene. For general synthetic procedures towards copper(I) acetylide, see: (a) Marino, J. P.; Floyd, D. M. J. Am. Chem. Soc. 1974, 96, 7138. (b) House, H. O.; Umen, M. J. J. Org. Chem. 1973, 38, 3893. (c) Brandsma, L.; Verkruijsse, H. D. Preparative Polar Organometallic Chemistry, Springer: Berlin, Vol. 1., p. 42. (d) Taylor, R. J. K. In Organocopper Reagents: A Practical Approach, Taylor, R. J. K. (ed.); Chapters 1 and 2. For the preparation of propynyllithium, see: (e) Midland, M. J. Org. Chem. 1975, 40, 2250. (f) Taschner, M. J.; Rosen, T.; Heathcock, C. H. Org. Synth. Coll. Vol. 7, 226. (g) Stang, P. J.; Boeshar, M.; Wingert, H.; Kitamura, T. J. Am .Chem. Soc. 1988, 110, 3272. (h) Marshall, J. A.; Wang, X. J. Org. Chem. 1991, 56, 960.

36 For propynylcopper, see: (a) Atkinson, R. E.; Curtis, R. F. J. Chem. Soc. (C) 1967, 578. (b) Bossenbroek, B.; Sanders, D. C.; Curry, H. M.; Shechter, H. J. Am. Chem. Soc. 1969, 91, 371. For general procedures, see: (c) Ref. 33d. (d) Owsley, D. C.; Castro, C. E. Org. Synth. 1972, 52, 128.

37 Remarkably, further additions of freshly prepared propynyl copper did not result in the complete consumption of the iodoarene.

38 (a) Wiles, M. R.; Massey, A. G. Chem. Ind. 1967, 663. (b) Coe, P. L.; Plevey, R. G.; Tatlow, J. C. J. Chem. Soc. (C) 1966, 597. (c) Coe, P. L.; Tatlow, J. C.; Terrell, R. C. J. Chem. Soc. (C) 1967, 2626.

39 For example, attempts to prepare salt- and solvent-free allyls from reaction of Cp*2YCl with 2-methyl-2-propenylmagnesium chloride led instead to the formation of Cp*2Y(η1-CH2CMe=CH2)·[MgCl2·2THF], see: den Haan, K. H.; Wielstra, Y.; Eshuis, J. J. W.; Teuben, J. H. J. Organomet. Chem. 1987, 323, 181.

40 Only recently, allyl derivatives, Cp*2Ln(η3-CH2CH=CH2), have been recognized as a convenient starting material in organo rare-earth metal chemistry, as they exhibit greater stability in terms of storage and handling than alkyls or hydrides, see: Evans, W. J.; Kozimor, S. A.; Brady, J. C.; Davis, B. L.; Nyce, G. W.; Seibel, C. A.; Ziller, J. W.; Doedens, R. J. Organometallics 2005, 24, 2269 and references therein.

41 (a) Becker, J. Y. J. Organomet. Chem. 1977, 127, 1. (b) Ref. 2a. This finding is consistent with the increased observed kinetic acidity of the 1-alkynes upon substituting the carbon-carbon triple bond with σ-electron-withdrawing groups, see for examples: (c) Drenth, W.; Loewenstein, A. Recl. Trav. Chim. Pays-Bas 1962, 81, 635. (d) harman, H. B.; Vinard, D. R.; Kreevoy, M. M. J. Am. Chem. Soc. 1962, 84, 347. (e) Eaborn, C.; Skinner, G. A.; Walton, D. R. M. J. Chem. Soc. (B) 1966, 922. (f) Charton, M. J. Org. Chem. 1972, 37, 3684. (g) Lin, A. C.; Chiang, Y.; Dahlberg, D. B.; Kresge, A. J. J. Am. Chem. Soc. 1983, 105, 5380. (h) Kresge, A. J.; Powell, M. F. J. Org. Chem. 1986, 51, 819.

42 No acidity constants were found in literature for propynylbenzenes. If the transmisstion of electronic effects via a carbon-carbon triple bond is similar to that of a carboxylic group, a large effect can be anticipated upon substituting the phenyl group with the pentafluorophenyl group. For example, substitution of the phenyl group of benzenecarboxylic acid C6H5CO2H (pKa = 4.2042a) by the

Chapter 2

64

pentafluorophenyl group (C6F5CO2H, pKa = 1.5142b) increased the aqueous acidity equilibrium Ka at 25 °C almost 500-fold. For pKa values, see: (a) Streitweiser, A., Jr.; Klein, H. S. J. Am. Chem. Soc. 1969, 85, 2759. (b) Mosely, P. G. N. Electrochim. Acta 1980, 25, 1309.

43 An impressive body of experimental evidence has accumulated that demonstrated significant stabilizing effects by fluoro and polyfluoroaryl substituents on carbanions. The evidence includes marked acceleration of the rates of carbanionic rearrangements and enhanced hydrocarbon acidities, relative to analogous all-hydrogen systems. For examples, see: (a) Filler, R.; Wang, C. S. J. Chem. Soc. (D) 1968, 287. (b) Streitweiser, A., Jr.; Mares, F. J. Am. Chem. Soc. 1968, 90, 644. (c) Streitweiser, A., Jr.; Hudson, J. A.; Mares, F. J. Am. Chem. Soc. 1968, 90, 648. (d) Streitweiser, A., Jr.; Scannon, P. J.; Niemeyer, H. M. J. Am. Chem. Soc. 1972, 94, 7936. (e) Bordwell, F. G.; Branca, J. C.; Bares, J. E.; Filler, R. J. Org. Chem. 1988, 53, 780. (f) Korenaga, T.; Kadowaki, K.; Ema, T.; Sakai, T. J. Org. Chem. 2004, 69, 7340.

44 For reviews on carbon acids, see: (a) Cram, D. J. Fundamentals of Carbanion Chemistry, Academic Press: New York, 1965. (b) Jones, J. R. The Ionisation of Carbon Acids, Academic Press: London, 1973. (c) Reutov, O. A.; Beletskaya, I. P.; Butin, K. P. CH-Acids, Pergamon Press: Oxford, 1978. (d) R. Stewart The Proton: Applications to Organic Chemistry, Academic Press: New York, 1985. For examples, see: (e) Closs, G. L.; Closs, L. E. J. Am. Chem. Soc. 1963, 85, 2022. (f) Closs, G. L.; Larrabee, R. B. Tetrahedron Lett. 1965, 287. (g) Streitweiser, A., Jr.; Caldwell, R. A.; Young, W. R. J. Am. Chem. Soc. 1969, 91, 529. (h) Maksić, Z. B.; Eckert-Maksić, M. Tetrahedron 1969, 25, 5113. (i) Maksić, Z. B.; Randić, M. J. Am. Chem. Soc. 1973, 95, 6522. (j) Luh, T.-Y.; Stock, L. M. J. Am. Chem. Soc. 1974, 96, 3712.

45 In an attempt to determine the relative (kinetic) acidity of 1-phenyl-1-propyne (1) and 1-pentafluorophenyl-1-propyne (5), n-buyllithium was added to a stirred solution of 1 (1.0 equiv.) and 5 (1.0 equiv.) in THF at -80 °C. Instead of the expected formation of allenylic and acetylenic quenchings products after addition of trimethylsilylchloride (TMSCl), a complicated mixture containing a multitude of unidentified compounds was obtained.

46 For example, the reaction of [Cp*2Sm(µ-H)]2 with propene, 1-butene and allylbenzene form the corresponding η3-allyl complexes in hexane, but give Cp*2SmCH2Ph, exclusively, in toluene.54

47 Dissociation of the dimer [Cp*2Y(µ-H)]2 to the monomer Cp*2YH in reactions of [Cp*2Y(µ-H)]2 with alkenes has been studied in detail. See: Casey, C. P.; Tunge, J. A.; Lee, T.-Y.; Carpenetti III, D. W. Organometallics 2002, 21, 389.

48 The reaction of [Cp*2La(µ-H)]2 with pyridine (2 equiv.) in benzene-d6 produces a 1:1 mixture of Cp*2La(η2-2-C5H4N) and Cp*2La(NC5H8) instantaneously. The formed products were rationalized by ortho-metalation of pyridine resulting in the formation of Cp*2La(η2-2-C5H4N) and H2. This reaction is followed by the rapid capture of the formed H2 by Cp*2La(η2-2-C5H4N) to afford Cp*2La(NC5H8). In marked contrast to this efficient hydrogen trapping is the independent reaction of Cp*2La(η1-2-C5H4N)(C5H5N) with excess H2 (4 atm.). In this case, the hydrogenation product Cp*2La(NC5H8)(C5H5N) is formed quantitatively after 1 day at 20 °C. See: (a) Ringelberg, S. N. Ph. D. Thesis, University of Groningen, 2001; Chapter 5. For another example of efficient hydrogen trapping, see (b): Hao, L.; Harrod, J. F.; Lebuis, A.-M.; Mu, Y.; Shu, R.; Samuel, E.; Woo, H.-E. Angew. Chem. Int. Ed. 1998, 37, 3126.

49 Sung et al. proposed the steric substituent constant SA, based on isodesmic reactions and ab initio calculations of substituted adamantane systems. According to the SA scale, the phenyl (SA = -5.10) is larger than the methyl group (SA = -2.02), see: (a) Sung, K.; Chen, F.-L. Org. Lett. 2003, 5, 889. For reviews on steric effects, see: (b) Gallo, R. Prog. Phys. Org. Chem. 1983, 14, 115. (c) Förster, H.; Vögtle, F. Angew. Chem. Int. Ed. Engl. 1977, 16, 429.

50 For example, see: (a) Fu, P.-F.; Brard, L.; Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 7157. (b) Molander, G. A.; Romero, J. A. C.; Corrette, C. P. J. Organomet. Chem. 2002, 647, 225. (c) Molander, G. A.; Schmitt, M. H. J. Org. Chem. 2000, 65, 3767. (d) Molander, G. A.; Knight, E. E. J. Org. Chem. 1998, 63, 7009.

51 Reversible insertion of the carbon-carbon triple bond of diphenylacetylene into the metal-hydride of Cp*2LaH should yield Cp*2La(Ph)=C(Ph)D upon standing in benzene-d6, due to the well-known tendency of Cp*2LaH to undergo H/D exchange with benzene-d6.122 However, the intensity of the vinylic proton in Cp*2LaC(Ph)=C(Ph)H relative to that of the Cp* groups did not change after


65

standing for 14 days at room temperature, thereby providing evidence against the formation of Cp*2La(Ph)=C(Ph)D via reversible insertion of diphenylacetylene into Cp*2LaH.

52 Catalytic hydrogenation of alkynes and alkenes, via alkenyl and alkyl intermediates, is well-established for rare-earth metallocene hydrides.53b,d

53 For examples, see: (a) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203. (b) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8111. (c) den Haan, K. H.; Wielstra, Y.; Teuben, J. H. Organometallics 1987, 6, 2053. (d) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091. (e) Jeske, G.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8103. (f) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51. (g) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. J. Am. Chem. Soc. 1990, 112, 2314. (h) Evans, W. J.; DeCoster, D. M.; Greaves, J. Organometallics 1996, 15, 3210. (i) den Haan, K. H.; Teuben, J. H. J. Chem. Soc., Chem. Commun. 1986, 682. (j) Booij, M.; Deelman, B.-J.; Duchateau, R.; Postma, D. S.; Meetsma, A.; Teuben, J. H. Organometallics 1993, 12, 3531.

54 Interestingly, the samarium allyl complexes Cp*2Sm(η3-CH2CHCHR) (R = H, Me, Ph) are also reported to be extremely soluble in alkane solvents and an decreasing tendency to crystallize was found for the methyl and phenyl derivatives. Cp*2Sm(η3-CH2CHCHPh) could, in fact, only be obtained as a tractable solid after recrystallization in the presence of a coordinating solvent forming the corresponding adduct. See: Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. J. Am. Chem. Soc. 1990, 112, 2314.

55 Heeres et al. also commented on the extreme solubility of Cp*2CeCH2CCMe and attempts to obtain single crystals for Cp*2LnCH2CCR (Ln = Ce, Y; R = Me, Ph) reported to be unsuccessful.6b

56 Kretschmer, W. P., personal communication 57 den Haan, K. H. Ph. D. Thesis, University of Groningen, 1986; Chapter 6. 58 (a) Kalinowski, H.-O.; Berger, S.; Braun, S. Carbon-13 NMR Spectroscopy; Wiley: Chichester, 1991,

Chapter 4. (b) Friebolin, H. Basic One- and Two-Dimensional NMR Spectroscopy; Wiley-VCH: Weinheim, 1998; Chapter 3.3; p. 97.

59 For examples, see: (a) Reich, H. J.; Holladay, J. E. Angew. Chem. Int. Ed. Engl. 1996, 35, 2365. (b) Reich, H. J.; Thompson, J. L. Org. Lett. 2000, 2, 783. (c) Reich, H. J.; Holladay, J. E.; Walker, T. G.; Thompson, J. L. J. Am. Chem. Soc. 1999, 121, 9769. (d) Reich, H. J.; Holladay, J. E.; Mason, J. D.; Sikorsko, W. H. J. Am. Chem. Soc. 1995, 117, 12137. (e) Reich, H. J.; Holladay, J. E. J. Am. Chem. Soc. 1995, 117, 8470. (f) Lambert, C.; von Ragué Schleyer, P.; Würthwein, E.-U. J. Org. Chem. 1993, 58, 6377.

60 Infrared absorptions at 1850-1900 cm-1 are assigned to allenyl structures and those >2000 cm-1 to propargyl structures in organolithium chemistry, see: (a) Jaffe, F. J. Organomet. Chem. 1970, 23, 53-62. (b) Priester, W.; West, R.; Chwang, T. L. J. Am. Chem. Soc. 1976, 98, 8413. (c) West, R.; Jones, P. C. J. Am. Chem. Soc. 1969, 91, 6156. (d) Klein, J.; Becker, J. Y. J. Chem. Soc., Chem. Commun. 1973, 576. For examples of organozinc and -magnesium compounds, see: (e) Ref. 2b. For examples of organotitanium compounds, see: (f) Ishiguro, M.; Ikeda, N.; Yamamoto, H. J. Org. Chem. 1982, 47, 2225. (g) Furuta, K.; Ishiguro, M.; Haruta, R.; Ikeda, N.; Yamamoto, H. Bull. Chem. Soc. Jpn. 1984, 57, 2768.

61 Blosser, P. W.; Gallucci, J. C.; Wojcicki, A. J. Organomet. Chem. 2000, 597, 125. 62 The paramagnetic Cp*2Ti(CH2CCPh) was furthermore characterized by elemental analysis, see:

Ogoshi, S.; Stryker, J. M. J. Am. Chem. Soc. 1998, 120, 3514. 63 (a) Jemmis, E. D.; Chandrasekhar, J.; von Ragué Schleyer, P.; J. Am. Chem. Soc. 1979, 101, 2848. (b)

Carfagna, C.; Deeth, R. J.; Green, M.; Mahon, M. F.; McInnes, J. M.; Pellegrini, S.; Woolhouse, C. B. J. Chem. Soc., Dalton Trans. 1995, 3975. (c) Graham, J. P.; Wojcicki, A.; Bursten, B. E. Organometallics 1999, 18, 837.

64 (a) den Haan, K. H. Ph. D. Thesis, University of Groningen, 1986, Chapter 6. (b) den Haan, K. H.; Wielstra, Y.; Teuben, J. H. Organometallics 1987, 6, 2053.

65 Evans, W. J.; Meadows, J. H.; Hunter, W. E.; Atwood, J. L. Organometallics 1983, 2, 1252. 66 (a) Ringelberg, S. N. Ph. D. Thesis, University of Groningen, 2001; Chapter 2. (b) Ringelberg, S. N.;

Meetsma, A.; Troyanov, S. I.; Hessen, B.; Teuben, J. H. Organometallics 2002, 21, 1759.

Chapter 2

66

67 Reported spectral data of phenylallene: (a) 13C NMR (CDCl3): Maercker., A; Fischenich, J.

Tetrahedron 1995, 51, 10209. (b) 13C NMR (CDCl3): Schaefer, T.; Kroeker, S.; McKinnon, D. M. Can. J. Chem. 1995, 73, 1478. (c) IR: Fleming, I.; Mwaniki, J. M. J. Chem. Soc., Perkin Trans. 1 1998, 1237. (d) IR: Chen, T. R.; Anderson, M. R.; Grossman, S.; Peters, D. G J. Org. Chem. 1987, 52, 1231.

68 The corresponding protons of pentafluorophenyl-1-propyne and 1-pentafluorophenylpropa-1,2-diene resonate at δ 1.47 and 4.72 ppm, respectively, see Experimental Section.

69 Miao, W.; Chung, L. W.; Wu, Y.-D.; Chan, T. H. J. Am. Chem. Soc. 2004, 126, 13326. 70 Bushby, R. J.; Patterson, A. S.; Ferber, G. J.; Duke, A. J.; Whitham, G. H. J. Chem. Soc., Perkin

Trans. II 1978, 807. 71 Ionic radii for eight-coordinate complexes: La3+ (1.160 Å) and Y3+ (1.019 Å), see: Shannon, R. D

Acta Crystallogr., Sect. A 1976, A32, 751. 72 No inductive/field parameters were found in literature for these metals, but the higher inductive/field

effect of Y3+ versus La3+ is apparent from the carbon chemical shift of the metal-bound methine carbon in Cp*2LaCH(SiMe3)2 (δ 44.69 ppm) versus Cp*2YCH(SiMe3)2 (δ 25.19 ppm) in benzene-d6 solution. In support, the electronegativity of Y3+ (X = 1.22) is higher than that of La3+ (X = 1.10), according to the Pauling scale, see: CRC Handbook of Chemistry and Physics, Lide, D. R. (Ed.); CRC Press: Boca Raton; 1999.

73 (a) Heeres, H. J.; Meetsma, A.; Teuben, J. H. Angew. Chem. Int. Ed. 1990, 29, 420. (b) Scholz, A.; Smola, A.; Scholz, J.; Loebel, J.; Schumann, H.; Thiele, K.-H. Angew. Chem. Int. Ed., 1991, 30, 435. (c) Gagné, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 275. (d) Thiele, K.-H.; Bambirra, S.; Sieler, J. Angew. Chem. Int. Ed. 1998, 37, 2886. (e) Evans, W. J.; Davis, B. L.; Nyce, G. W.; Perotti, J. M.; Ziller, J. W. J. Organomet. Chem. 2003, 677, 89.

74 The average Ln-C(C5Me5) distances parallel the ionic radius of the lanathanide metal, but other distances such as Ln-C(alkyl) do not follow this pattern as closely, see: Evans, W. J.; Foster, S. E. J. Organomet. Chem. 1992, 433, 79.

75 The twist angle is defined as the average of the five smallest dihedral angles formed between the ten planes which consist of a ring carbon and the two centroids.

76 van der Heijden, H.; Schaverien, C. J.; Orpen, A. G. Organometallics 1989, 8, 255. 77 Evans, W. J.; Foster, S. E. J.Organomet.Chem. 1992, 433, 79 78 (a) den Haan, K. H.; de Boer, J. L.; Teuben, J. H. J. Organomet. Chem. 1988, 327, 31. (b) den Haan,

K. H.; de Boer, J. L.; Teuben, J. H. Organometallics 1986, 5, 1726. 79 (a) Evans, W. J.; Seibel, C. A.; Ziller, J. W.; Doedens, R. J. Organometallics 1998, 17, 2103. (b)

Evans, W. J.; DeCoster, D. M.; Greaves, J. Organometallics 1996, 15, 3210. (c) Gerald, J.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8103. (d) Ref. 40. (e) Ref. 54.

80 Evidence was reported for two mechanisms for allyl fluxionality in a series of scandocene allyl compounds that occur simultaneously, i.e. an η3-C3H3 to η1-C3H3 coordination change and a 180° rotation of the C3H3 moiety, see: Abrams, M. B.; Yoder, J. C.; Loeber, C.; Day, M. W. Bercaw, J. Organometallics 1999, 18, 1389.

81 (a) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. Angew. Chem., Int. Ed. Engl. 1997, 109, 774. (b) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. J. Am. Chem. Soc. 1995, 117, 12635. (c) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 9273.

82 (a) den Haan, K. H.; Wielstra, Y.; Teuben, J. H. Organometallics 1987, 6, 2053. (b) Heeres, H. J. Ph. D. Thesis, University of Groningen, 1990; Chapter 3.

83 Reported acidity values for phenylacetylene differ by as much as 10 pK units, depending on the solvent and the method used to determine acidity. Benzene is an aprotic, nonpolar solvent. Comparison with acidities in protic, polar solvents, such as reflected by the pKa values which refer to (equilibrium) acidities in water, seems therefore inappropriate. The most extensive acidity scale in a nonprotic solvent is based on dimethylsulfoxide (DMSO).83e According to the pKDMSO values, the (equilibrium) acidities of phenylacetylene (28.7) and methanol (29.0) are similar. According to gas-phase equilibrium acidities83f ∆G°acid (kcal/mol, 298 K) in which solvent effects are absent, methanol (372.6) is more acidic than phenylacetylene (362.6). For reviews on carbon acids, see: (a) Cram, D. J. Fundamentals of Carbanion Chemistry, Academic Press: New York, 1965. (b) Jones, J. R. The Ionisation of Carbon Acids, Academic Press: London, 1973. (c) Reutov, O. A.; Beletskaya, I. P.;


67

Butin, K. P. CH-Acids, Pergamon Press: Oxford, 1978. (d) R. Stewart The Proton: Applications to Organic Chemistry, Academic Press: New York, 1985. For acidity scales, see: (e) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456. (f) Bartmess, J. E.; Scott, J. A.; McIver, R. T., Jr. J. Am. Chem. Soc. 1979, 101, 6046.

84 (a) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533. (b) Hard and Soft Acids and Bases; Pearson, R. G. (ed.); Dowden, Hutchinson and Ross, Stroudsberg, PA; 1973. (b) Fleming, I. Frontier Orbitals and Organic Chemical Reactions, Wiley-Interscience, London; 1976.

85 (a) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1990, 9, 2628. (b) Heeres, H. J.; Nijhoff, J,; Teuben, J. H.; Rogers, J. D. Organometallics 1993, 12, 2609. (c) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1993, 12, 2618 (d) Forsyth, C. M.; Nolan, S. P.; Stern, C. L.; Marks, T. J.; Rheingold, A. L. Organometallics 1993, 12, 3618.

86 The reactions of Cp*2LuMe5 and Cp*2ScH102 with allene are reported to proceed via C-H activation, forming the allenyl Cp*2Lu(η1-CH=C=CH2) and allyl Cp*2Sc(η3-CH2CH=CH2), respectively. The lanthanide alkenyl derivatives Cp*2LnC(R)=C(R’) are believed to be reactive intermediates in numerous catalytic reactions, such as 1-alkyne oligomerization, alkyne hydroamination and alkyne hydrosilylation, and are presumed to undergo insertion reactions with unsaturated substrates and C-H activation reactions with Brønsted acids.1 However, -to best of our knowledge- no (systematic) reactivity studies of well-defined lanthanidocene alkenyl derivatives are reported in literature.

87 The observation of acetylenic and allenylic phenylacetylene trimers C24H20 (for details, see Chapter 4) with GC-MS provides circumstantial evidence that organic compounds C25H20 are stable under the present experimental conditions.

88 The Lewis-base-free complex Cp*2ZrMe[B(4-C6H4F)4] undergoes selective insertion of 1,2-propadiene at -30 °C, but a complicated mixture is formed upon warming with the excess amount of 1,2-propadiene present. The reaction with 3-methyl-1,2-butadiene is reported to afford a complex mixture of unidentified products. See: Horton, A. D. Organometallics 1992, 11, 3271.

89 (a) Burns, C. J.; Andersen, R. A. J. Am. Chem. Soc. 1987, 109, 941. (b) Schultz, M.; Burns, C. J.; Schwartz, D. J.; Andersen, R. A. Organometallics 2001, 20, 5690. (c) Perrin, L.; Maron, L.; Eisenstein, O.; Schwartz, D. J.; Burns, C. J.; Andersen, R. A. Organometallics 2003, 22, 5447.

90 Casey, C. P.; Lee, T.-Y.; Tunge, J. A.; Carpenetti II, D. W. J. Am. Chem. Soc. 2001, 123, 10762. 91 Thermolysis of Cp*2CeCH(SiMe3)2 is also reported to proceed via intramolecular C-H activation,

forming a tetranuclear fulvene species [Cp*Ce(µ-η5:η1:η1-C5Me3(CH2)2CeCp*2]2 which was characterized by X-ray crystallography.92a-b IR spectroscopy of the thermolysis product of Cp*2LaCH(SiMe3)2 indicated a similar compound.

92 (a) Booij, M.; Meetsma, A.; Teuben, J. H. Organometallics 1991, 10, 3246. (b) Booij, M. Ph. D. Thesis, University of Groningen, 1989; Chapter 3. (c) Ryu, J.-S.; Marks, T. J.; McDonald, F. E. J. Org. Chem. 2004, 69, 1038

93 Regiorandom insertion of phenylallene into the La-CH2 of Cp*2La(η3-CH2CCPh) affords, in principle, four different products after protonolysis, whereas 1-phenyl-1-propyne yields only two. Because insertion is believed to proceed regiorandomly at 120° C, internal metalation producing 1-phenyl-1-propyne is presently favored.

94 No NMR spectral data were found in literature for (monomeric) Cp*2LaCH2R complexes, but analogous Cp*2YCH2R derivatives have α-CH proton resonances in the range from δ 0.66 to -0.08 ppm, see: Casey, C. P.; Tunge, J. A.; Lee, T.-Y.; Fagan, M. A. J. Am. Chem. Soc. 2003, 125, 2641.

95 Watson, P. L. J. Chem. Soc., Chem. Commun. 1983, 276. 96 (a) Deelman, B.-J. Ph. D. Thesis, University of Groningen, 1994; Chapter 5. (b) Deelman, B.-J.;

Booij, M.; Meetsma, A.; Teuben, J. H.; Kooijman, H.; Spek, A. L. Organometallics 1995, 14, 2306. 97 Evans, W. J.; Chamberlain, L. R.; Ulibarri, T. A.; Ziller, J. W. Organometallics 1988, 110, 6423. 98 Following the previous line of reasoning93, insertion of 1-phenyl-1-propyne into the La-CH2 bond of

Cp*2La(η3-CH2CCPh) is likely to occur regioselectively at 50 °C, affording only one dimer after protonolysis.

99 Evans, W. J.; Dominguez, R.; Hanusa, T. P. Organometallics 1986, 5, 1291. 100 Ringelberg, S. N. Ph. D. Thesis, University of Groningen, 2001; Chapter 5. 101 Ringelberg, S. N. Ph. D. Thesis, University of Groningen, 2001; Chapter 4.

Chapter 2

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102 (a) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D. ;

Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203. (b) Thompson, M. E.; Bercaw, H. E. Pure Appl. Chem. 1984, 105, 6491.

103 (a) Deelman, B.-J.; Stevels, W. M.; Teuben, J. H.; Lakin, M. T.; Spek, A. L. Oranometallics 1994, 13, 3881. (b) Deelman, B.-J. Ph. D. Thesis, University of Groningen, 1994; Chapter 4.

104 The deep coloration noted for rare-earth metal pyridyl, furyl and phenyl complexes has tentatively been assigned to products formed from radical coupling or ring-opening of (hetero)aromatic units. For examples, see: (a) Ref. 103. (b) Evans, W. J.; Meadows, J. H.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1984, 106, 1291. (c) Fryzuk, M. D.; Jafarpour, L.; Kerton, F. M.; Love, J. B.; Patrick, B. O.; Rettig, S. J. Organometallics 2001, 20, 1387. (d) Arndt, S.; Elvidge, B. R.; Zeimetz, P. M.; Spaniol, T. P.; Okuda, J. Organometallics 2006, 25, 793.

105 Hazin, P. N.; Bruno, J. W.; Schulte, G. K. Organometallics 1990, 9, 416. 106 Watson, P. L.; Whitney, J. F.; Harlow, R. L. Inorg. Chem. 1981, 20, 3271. 107 (a) Evans, W. J.; Drummond, D. K. J. Am. Chem. Soc. 1986, 108, 7440. (b) [(Cp*2Sm)2(µ-η2:η2-

PhC4Ph)]: Evans, W. J.; Keyer, R. A.; Zhang, H.; Atwood, J. L. J. Chem. Soc., Chem. Commun. 1987, 837. (c) Rausch, M. D.; Moriarty, K. J. Organometallics 1986, 5, 1281. (d) {(Cp*2Sm)2[µ-η4-PhCH=C(O)-C(O)=CHPh]}: Evans, W. J.; Drummond, D. K. J. Am. Chem. Soc. 1988, 110, 2772.

108 (a) Gagné, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 275. (b) Giesbrecht, G. R.; Collis, G. E.; Gordon, J. C.; Clark, D. L.; Scott, B. L.; Hardman, N. J. J. Organomet. Chem. 2004, 689, 2177.

109 For examples, see: (a) La(NH-2,6-iPr2C6H3)3(C5H5N)2: Giesbrecht, G. R.; Gordon, J. C.; Clark, D. L.; Hay, P. J.; Scott, B. L.; Tait, C. D. J. Am. Chem. Soc. 2004, 126, 6387. (b) La(O-2,6-iPr2C6H3)3(NH3)4: Butcher, R. J.; Clark, D. L.; Grumbine, S. K.; Vincent-Hollis, R. L.; Scott, B. L.; Watkin, J. G. Inorg. Chem. 1995, 34, 5468. (c) [Cp3La(NCCH3)2]: Spirlet, M. R.; Rebizant, J.; Apostolidis, C.; Kanellakopulos, B. Acta Cryst. 1987, C43, 2322. (d) [Cp3La(NCCH2CH3)2]: Xing-Fu, L.; Eggers, S.; Kopf, J.; Jahn, W.; Fischer, R. D.; Apostolidis, C.; Kanellakopulos, B.; Benetollo, F.; Polo, A.; Bombieri, G. Inorg.Chim.Acta 1985, 100, 183.

110 Koch, W.; Hertwig, R. H. In The Encyclopedia of Computational Chemistry; Schleyer, P. v. R., Editor-in-Chief; Wiley: Chichester, 1998.

111 (a) Maron, L.; Eisenstein, O. J. Phys. Chem. A 2000, 104, 7140. (b) M. Dolg, Stoll, H. Handbook on the Physics and Chemistry of Rare Earths, Vol. 22; Elsevier, Amsterdam, 1996; Chapter 152. (c) M. Dolg, H. Stoll, Theor. Chim. Acta 1989, 75, 369. (d) Dolg, M.; Stoll, H; Preuß, H, J. Chem. Phys. 1989, 90, 1730. (e) Dolg, M.; Fulde, P.; Kijchle, W.; Neumann, C. S.; Stoll, H. J. Chem. Phys. 1991, 94, 3011. (f) Dolg, M.; Stoll, H.; Preuß, H. J. Mol. Struct. 1991, 235, 67. (g) Dolg, M.; Stoll, H.; Flad, H. T.; Preuß, H. J. Chem. Phys. 1992, 97, 1162. (h) Kaupp, M.; Schleyer, P. v. R.; Dolg, M.; Stoll, H. J. Am. Chem. Soc. 1992, 114, 8202. (h) Dolg, M.; Stoll, H.; Preuß, H., Theor. Chim. Acta 1993, 85, 441. (i) Brady, E. D.; Clark, D. L.; Gordon, J. C.; Hay, P. J.; Keogh, D. W.; Poli, R.; Scott, B. L.; Watkin, J. G. Inorg, Chem. 2003, 42, 6682.

112 (a) Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729. For examples of rare-earth metallocenes, see: (b) Steigerwald, M. L.; Goddard III, W. A.; J. Am. Chem. Soc. 1984, 106, 308. (c) Ortiz, J. V.; Hoffmann, R. Inorg. Chem. 1985, 24, 2095. (d) Rabaâ, H.; Saillard, J.-Y.; Hoffmann, R. J. Am. Chem. Soc. 1986, 108, 4327. (e) Rappé, A. K. J. Am. Chem. Soc. 1987, 109, 5605. (f) Ouddai, N.; Bencharif, M. J. Mol. Struct. (Theochem) 2004, 709, 109. (g) Ref. 114. (h) Ref. 115.

113 (a) Erker, G.; Rosenfeldt, F. Angew. Chem., Int. Ed. Engl. 1978, 17, 605. (b) Erker, G. Acc. Chem. Res. 1984, 17, 103 and references therein.

114 (a) Deelman, B.-J.; Teuben, J. H.; MacGregor, S. A.; Eisenstein, O. New J. Chem. 1995, 19, 691. (b) Sändig, N.; Koch, W. Organometallics 2002, 21, 1861.

115 Perrin, L.; Maron, L.; Eisenstein, O. New. J. Chem. 2004, 28, 1255 and references therein. 116 (a) Ahlrichs, R.; Bär, M.; Baron, H.-P.; Bauernschmitt, R.; Böcker, S.; Ehrig, M.; Eichkorn, K.;

Elliott, S.; Furche, F.; Haase, F.; Häser, M.; Hättig, C.; Horn, H.; Huber, C.; Huniar, U.; Kattannek, M.; Köhn, A.; Kölmel, C.; Kollwitz, M.; May, K.; Ochsenfeld, C.; Öhm, H.; Schäfer, A.; Schneider, U.; Treutler, O.; Tsereteli, K.; Unterreiner, B.; Von Arnim, M.; Weigend, F.; Weis, P.; Weiss H. Turbomole Version 5, january 2002. Theoretical Chemistry Group, University of Karlsruhe. (b) Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346. (c) Turbomole basisset library, Turbomole Version 5, see 1a. (d) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571. (e) Andrae,


69

D.; Häussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (f) Schäfer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.

117 Although the fluorine atom is larger than the hydrogen atom, both the pentafluorophenyl group and the phenyl group are planar structures.

118 It can be anticipated that the decrease of electron density at the triple bond will weaken the interaction with the electrophilic lanthanide metal. On the other hand, polarization of the triple bond, as to induce a partial positive charge at the β-position and a partial negative charge at the α-position of the four-centered insertion transition state is well-known to facilitate insertion into electrophilic metal-carbon bonds. For examples, see: (a) Burger, B. J.; Santarsiero, B. D.; Trimmer, M. S.; Bercaw, J. E. J. Am. Chem. Soc. 1988, 110, 3134. (b) Doherty, N.; Bercaw, J. E. J. Am. Chem. Soc. 1985, 107, 2670. (c) Halpern, J.; Okamoto, T.; Zakhariev, A. J. Mol. Catal. 1976, 2, 65-68. (d). Lin, Z.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 5515. (e) Ref. 102a.

119 For a review of insertion reactions in organo rare-earth metal chemistry, see: (a) Zhou, X.; Zhu, M. J. Organomet. Chem. 2002, 647, 28. For examples, see: (b) Evans, W. J.; Wayda, A. L.; Hunter, W. E.; Atwood, J. L. J. Chem. Soc., Chem. Commun. 1981, 706. (c) Ref. 122.

120 Deelman, B.-J,; Stevels, W. M.; Teuben, J. H.; Lakin, M. T.; Spek, A. L. Organometallics 1994, 13, 3881.

121 To the best of our knowledge, only one example of CO insertion into a Ln-C(alkenyl) bond has been reported in literature, see: Evans, W. J.; Hughes, L. A.; Drummond, D. K.; Zhang, H.; Atwood, J. L. J. Am. Chem. Soc. 1986, 108, 1722.

122 Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091.

123 den Haan, K. H.; de Boer, J. L.; Teuben. J. H.; Spek, A. L.; Kojiĉ-Prodiĉ, B.; Hays, G. R.; Huis, R. Organometallics 1986, 5, 1726.

124 Vogel, A. I.; Rogers, V.; Hannaford, A. J.; Furniss, B. S. Vogel's Textbook of Practical Organic Chemistry, 4th ed.; Longman: London, 1978; p. 409.

125 Vogel, A. I.; Rogers, V.; Hannaford, A. J.; Furniss, B. S. Vogel's Textbook of Practical Organic Chemistry, 4th ed.; Longman: London, 1981; p. 285.

126 (a) Kaufman, G. B.; Pinnell, R. P. Inorg. Synth. 1960, 6, 3. (b) Hiller, W.; Zybill, C. E. In Synthetic Methods of Organometallic and Inorganic Chemistry, Breitinger, D. K.; Herrmann, W. A. (Eds.); Georg Thieme: Stuttgart, 1999; Vol. 5., Chapter 1, p. 6.

127 For propynyllithium, see: (a) Midland, M. J. Org. Chem. 1975, 40, 2250. (b) Taschner, M. J.; Rosen, T.; Heathcock, C. H. Org. Synth. Coll. Vol. 7, 226. (c) Stang, P. J.; Boeshar, M.; Wingert, H.; Kitamura, T. J. Am .Chem. Soc. 1988, 110, 3272. (d) Marshall, J. A.; Wang, X. J. Org. Chem. 1991, 56, 960.

128 Brandsma, L.; Vasilevsky, S.F.; Verkruijsse, H. D. Application of Transition Metal Catalysts in Organic Synthesis, Springer: Berlin, 1998; Chapter 10, p. 4.

129 Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals; 2nd ed.; Pergamon Press: Oxford, 1980.

130 Maercker, A.; Fischenich, J. Tetrahedron 1995, 51, 10209. 131 Chen, T. R.; Anderson, M. R.; Grossmann, S.; Peters, D. G. J. Org. Chem. 1987, 52, 1231. 132 A. Koch Magn. Reson. Chem. 2000, 38, 216. 133 SMART, SAINT, SADABS, XPREP and SHELXK/NT. Smart Apex Software Reference Manuals 2000,

Bruker AXS Inc., Madison, Wisconsin, USA. 134 Beurskens, P. T.; Beurskens, G.; De Gelder, R.; García-Granda, S.; Gould, R. O.; Israël, R.; Smits, J.

M. M. The DIRDIF-99 Program System 1999, University of Nijmegen, The Netherlands.

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