Electrophilic reactivity in anti-Mills-Nixon systems

ELSEVIER

THEO CHEM

Journal of Molecular Structure (Theochem) 366 (1996) 173-183

Electrophilic reactivity in anti-Mills-Nixon

Mirjana Eckert-MaksiCaT*, Zoran Glasovaca, Zvonimir B. MaksiCbl”, Irena Zrinski”

systems

“Laboratory of Physical Organic Chemistry, Department of Chemistry, Ruder BoSkoviC Institute, P.O. Box 1016, 10001 Zagreb, Croatia

bQuantum Chemistry Group, Department of Chemistry, Ruder BoSkoviC Institute, P.O. Box 1016, 10001 Zagreb, Croatia

‘Faculty of Science and Mathematics, University of Zagreb, MaruliCev trg 19, 10000 Zagreb, Croatia

Received 20 December 1995; accepted 13 February 1996

Abstract

Structural properties and the electrophilic substitution susceptibility in some anti-Mills-Nixon (anti-MN) systems possessing fused small rings (3-5) are examined by employing HF/6-31G* and MP2(fc)/6-31G*//HF/6-3lG* theoretical models. The electrophilic substitution is simulated by protonation. It is shown that cr-Wheland intermediates are energetically more favourable than their /3-counterparts. This sort of behaviour is antipodal to the electrophilic reactivity exhibited by MN systems. The basic mechanism is, however, the same. It is related to the degree of matching of two distinct x-electron localization patterns. The first occurs in the ground state (GS). The second type of a-electron bond fixation is triggered by protonation. Compatibility of these two modes of bond localization in the transition structure (TS, the Wheland u-complex) determines the directional ability of the small annelated rings in the electrophilic substitution reactions. In anti-MN systems this synaction is greater for OL-

protonation. In addition, a-protonated forms 4a and 5a are energetically prefered because of the increased aromatic character of the fused small rings.

Keywords: Ab initio calculation; Anti-Mills-Nixon molecule; Electrophilic reactivity; Wheland complex

1. Introduction la and 10, respectively (Fig. 1). The former systems are termed Mills-Nixon (MN) compounds whereas

It was shown by Mills and Nixon as early as 1930 the latter are characteristic of anti-MN molecules. that a fused small ring exerted a decisive influence on The MN systems are the more thoroughly explored the directional selectivity of the benzene nucleus in so far. It has been established beyond reasonable

electrophilic substitution reactions [l]. More specifi- doubt that small annelated rings produce bond tally, the enhanced reactivity of the /3- relative to the alternation compatible with the dominant VB struc- cr-position in electrophilic substitution of P-hydroxy- ture LY as evidenced by a number of experimental indan in contrast to reversed reactivities in P-hydroxy- measurements [2-61 and theoretical studies [7-141.

tetralin was rationalized by partial x-bond fixation It was pointed out that C60 and other higher fullerenes within the aromatic benzene moiety as shown represent MN systems par excellence [15,16]. Addi-

schematically by the valence bond (VB) structures tional experimental evidence is provided by ESR measurements [17,18]. An interesting observation

* Corresponding author. was also made in studying tautomers of pyrazoles

0166-1280/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved

PII SO166-1280(96)04515-O

174 M. Eckert-MaksiL et al.Uournal of Molecular Structure (Theochem) 366 (1996) 173-183

\ CII / CD ‘I \ la 1P

Fig. 1. Schematic representation of the bond localization pattern in

(a) Mills-Nixon and (b) anti-Mills-Nixon systems.

fused to small rings. Martinez et al. [19] have shown

that tautomers possessing annelated bonds with

decreased double-bond character are favoured, which is in full accordance with the MN hypothesis.

Particularly strong support for the MN conjecture is offered by electrophilic reactivity investigations. It

was shown in a number of experimental studies that electrophilic substituents predominantly attack

the /3-position in MN systems la [20-241.’ A survey of the more recent experimental results is provided by

Taylor [25]. It is interesting to mention that the reactivity of buckminsterfullerene Cm can also be understood in terms of the MN effect [26]. The theoretical results are in full agreement with the experimental findings [27]. The electrophilic substituents were modelled by a proton and the corre-

sponding Wheland u-complexes [28] were examined, which in turn represented the relevant transition structures according to Hammond’s postu-

late [29]. It is shown that the difference in reactivity can be traced to the compatibility or incompatibility of two x-electron localization modes. The first is related to the ground state (GS) being induced by the angular strain of the small annelated carbocycle, thus reflecting a typical “memory” effect. It should be stressed in this connection that the rehybridization

effect at the carbon junction atoms could be reinforced either by the hyperconjugative interaction between the CH2 group of the small carbocycle and the aromatic sextet of the aromatic benzene moiety, or by the antiaromatic interaction induced by the

additional double bond/lone pair within the fused ring [10,30]. The second localization pattern is caused by protonation and concurrent formation of the sp3 centre within the aromatic fragment (Fig. 2) thus characterizing the transition structure (TS). The

1 *Geometric structures of molecular systems are denoted by Latin letters, whereas VB structures characterizing a particular coupling of the electronic spins are signified by Greek letters.

Q-Q-Q H H H H ‘H

20: - 2p 2y

Fig. 2. Partial x-electron localization in the protonated benzene as

given by the resonance structure.

competition between these two antagonistic n-electron localization modes is responsible for enhanced

reactivity of the P-sites over the cr-positions in the

MN molecules [27]. The anti-MN systems are less abundant. We found

that multiple fluorination in benzocyclopropenes [31]

and benzocyclobutenes [32] was capable of changing the dominant resonance structure la! into 10 thus

transforming the parent (hydrocarbon) MN molecular systems into anti-MN compounds (derivative). It has also been shown that anti-MN bond fixation took place in benzoborirene and the benzocyclopropenyl cation [33]. If this is the case, then these two molecules should exhibit a reversed pattern of electrophilic substitution reactivity, i.e., the o-position should be

more susceptible to electrophilic attack than the /3- position. This question is addressed in the present

paper.

2. Methodology

Standard ab initio molecular orbital calculations have been executed by using GAMESS [34] and ck4usmN 92 [35] computer programs. Optimization of the molecular structures has been achieved at the HF/6-31G* level of theory. Vibrational analysis was performed for each equilibrium geometry and the

zero-point energies (ZPEs) were obtained by multi- plying the HF/6-31G* values by a common scaling factor of 0.89 [36]. Since we are interested in energetic properties the electron correlation should be explicitly taken into account. This was attained by Moller-Plesset perturbation theory of the second order by carrying out single-point MP2(fc)/6-31G*// HF/6-31G* calculations, where (fc) stands for the frozen inner-core electrons. Details can be found in a book by Hehre et al. [37]. Results are discussed in terms of conventional a-bond orders and local

M. Eckert-MaksiC et al./Journal of Molecular Structure (Theochem) 366 (1996) 173-183 175

hybridization indices estimated by natural bond orbi-

tal (NBO) analysis [38]. However, it was of some

interest to examine a potential use of the semi-

empirical AM1 wavefunctions for interpretive

purposes. Hence, the single-point AMl//HF/6-31G* calculations have been executed and the resulting first-order density matrix was used to extract the

hybridization parameters following the method of Trindle and Sinanoglu [39]. Finally, the gross electron

charges are given as described by tiwdin [40]

and NBO [38] procedures. Although atomic charge cannot be defined in a unique way [41], it provides

a useful qualitative index of the chemical bonding,

being one of the most important descriptors of the

modified atoms embedded in the molecular environ- ment [42].

3. Results and discussion

3.1. Structural properties

We commence discussion with the modelled anti-MN system par excellence: edge-protonated 1,2-benzyne (Fig. 3, Table 1). The additional proton

is bound to a formal triple bond in the plane of the molecule, thus mimicking the annelated three-membered ring. This represents a clear-cut case since the

u- and T-electrons are separated and perturbation takes place in the molecular plane. Hence, changes in the r-electron distribution are caused solely by changes in the u-skeleton, which in turn are triggered by protonation. Let us denote the CC bond distances

as ipso, ortho, meta and para starting with the “fused” C(1) = C(2) bond. An important diagnostic tool in these planar systems is provided by the r-bond

orders. They alternate in a typical reversed-MN way being higher along the ipso and meta CC bonds and

lower in the remaining ortho and para bonds. Surpris- ingly, alternation of the CC bond distances is not so regular since ortho and meta bond lengths are practically equal. Another point of some interest is the similarity of charges of C(3) and C(4) atoms suggesting in a simple naive approach their equal susceptibility to electrophilic attack. We shall see later that this is not

the case. The angular deformation of the benzene moiety is considerable; the central C(2)-C(3)-C(4) angle is very sharp (104’7, whereas both peripheral angles

J-

L -

3b

Lb

-5 5a 5b

Fig. 3. Molecular structures of the systems studied.

are widened to 125 and 130”. This finding is remark- able; both bond lengths and bond angles of the benzene skeleton exhibit substantial variation and yet the

r-bond orders indicate that the aromatic character of the ring is preserved to a large extent. Thus, a-bond orders differ relatively little from the free benzene value of 0.667, which is determined by the symmetry. This is in accordance with a general belief that the

aromatic stabilization is robust and persistent. As a final comment, it should be mentioned that there is a substantial shift of the s-character into the ortho bonds

as evidenced by the sp ’ hybridization ratio (5 1.1%) of the hybrid A0 placed at the C(2) atom and directed towards the C(3) carbon. It should be noted that the average s-character of this bond is the highest and yet the corresponding r-bond order is the lowest. Appar- ently, u- and 7r-electrons act here in a non-concerted way. The high average s-character in the ortho CC

bond is responsible for a distance which is slightly shorter than the meta CC bond in spite of the lower r-bond order.

176 M. Eckert-MaksiC et al.lJournal of Molecular Structure (Theochem) 346 (19%) 173-183

Before proceeding further it is important to recall C(4)-C(5) bond is also increased. On the other that protonation of benzene introduces localization of hand, it becomes significantly lower in the ortho the CC bonds vicinal to the site of proton attack [27]. C(2)-C(3) bond assuming a value close to that Concomitantly, their distance is lowered relative to characterizing hyperconjugation (namely the C(l)- the benzene value. It is, therefore, not unexpected C(6) and C(5)-C(6) bonds). Relatively close bond that the C(l)-C(2) bond is additionally shortened in distances of the metu (C(3)-C(4)) and puru C(4)- the protonated form 3a. The same holds for the second C(5) bonds is easily understood in terms of the similar vicinal C(4)-C(5) bond. The hybridization para- hybridization parameters and r-bond orders. It is meters are hardly changed, rehybridization occurring noteworthy that protonation induces additional at the C(6) carbon atom (sp2 - sp3) attacked by a sharpening of the central C(l)-C(6)-C(5) angle proton being a notable exception. It follows that the (100’) which is significantly lower than the tetra- structural changes should be ascribed to the redistri- hedral value. Obviously, some bent bonding occurs bution of the r-bond orders. This is indeed the case here. It is interesting to mention that the protonated since the x-bond orders exhibit a pronounced varia- carbon atom has the highest electron density, which in tion For example, the r-bond order of the C(l)-C(2) turn is increased by 0.35 tel. This finding could be annelated bond approaches complete localization interpreted by charge reorganization; the electron (7rb0(l,2) = 0.91). The n-bond order of the para density of the remaining carbon atoms drifts towards

Table 1

Relevant structural parameters (distances in Angstroms, angles in degrees) of 3 and its protonated species as calculated by the HF/6-31G*

model. Hybridization s-characters are estimated by the HF/6-31G* (NBO) and AMl/iHF/6-31G* procedures (AM1 results are in parentheses).

Atomic charges extracted from HF/6-31G* wavefunctions by using Liiwdin and NBO approaches are compared with the AM1 results

Molecule Bond/Angle HF/6-31G* s-character r-b.o. Atom Atomic charge

Liiwdin NBO AMI

3

3a

G(ltc(2) 1.231

G(2tc(3) 1.387

G(3tc(4) 1.392

G(4PW 1.425

W)_H, G(2)--H 1.358

G(3)-H 1.069

G(4tH 1.075

G(ltc(2)_H 63.1

G(ltc(2kc(3) 130.4

c(2W3kC(4) 104.4

c(3kG(4W(5) 125.2

W-G(2) 1.206

c(2kc(3) 1.440

G(3W4) 1.392

G(4W5) 1.408

W-G(6) 1.522

c(6Wl) 1.486

W-H 1.358

c(2)_H 1.358

G(3)--H 1.077

C(4tH 1.077

(X5)-H 1.077

C(6tH 1.081

W-G(2)_H 63.6

C(2)++H 63.6

G(ltc(2tc(3) 130.8

G(2tc(3W4) 107.3

37.2-37.2 (38.7-38.7)

51.1-28.0 (48.7-25.9)

35.8-34.8 (33.9-32.2)

35.0-35.0 (31.6-31.6)

- (18.3)

36.0 (39.0)

30.1 (34.3)

36.8-38.3 (38.3-39.4)

48.0-27.0 (46.9-24.1)

36.7-33.5 (34.2-31.2)

33.7-36.7 (30.7-32.3)

32.3-25.4 (28.6-23.6)

21.7-51.3 (19.1-47.5)

- (20.2)

- (19.8)

36.2 (39.4) 32.7 (36.4)

30.8 (34.8)

26.5 (30.9)

0.74 (0.74)

0.55 (0.58)

0.73 (0.73)

0.58 (0.59)

0.89 (0.91)

0.34 (0.37)

0.71 (0.71)

0.63 (0.66)

0.29 (0.32)

0.23 (0.25)

C(l) 0.06 0.09 0.00

C(6) -0.07 -0.20 0.04

G(5) -0.10 -0.17 -0.09

HW) 0.31 0.36 0.29

H(G3) 0.23 0.30 0.22

H(G4) 0.24 0.29 0.22

G(1) 0.15 0.30 0.12

G(2) 0.27 0.04 0.04

c(3) 0.00 0.16 0.30

G(4) 0.07 -0.36 -0.20

c(5) 0.18 0.27 0.17

c(6) -0.16 -0.55 -0.08

HW) 0.18 0.44 0.37

HG(3) -0.21 0.39 0.27

HG(4) 0.38 0.35 0.27

HG(5) 0.29 0.35 0.27

HC(6) 0.29 0.32 0.27

M. Eckert-MaksiC et aLlJournal of Molecular Structure (Theochem) 366 (19%) 173-183 177

the site of attack to shield the positive charge of the incoming proton. This is a general feature which was found in all other protonated species.

Distinctly different changes are found in the /3- protonated form 3b. Whereas cr-protonation has led to a shoaening of the already short annelated bond by 0.02 A, /3-protonation elongates this bond by the same amount. One expects on intuitive grounds that &protonation is energetically less favourable. Once again the hybridization s-characters are, mutatis mutandis, transferable to a large extent and the main changes are due to x-electron density variation. For instance, the u-bond order of the “fused” C(l)-C(2) bond drops from the initial value of 0.89 in 3 to 0.63 in 3b thus leading to C(l)-C(2) bond stretching.

Examination of the structural data and u-bond orders of the planar benzoborirene 4 (Table 2) shows that it is a typical anti-MN system. Its diferen- tia specifica compared to the model compound 3 is in

Table 1 Continued

the first place the extended x-system, which now encompasses the boron atom. The corresponding a- bond orders C(l)-B are substantial, being practically equal to that found in thepara bond. Since the L.iiwdin x-populations of C(1) and B atoms are 0.836 and 0.361, respectively, one is tempted to conclude that a local three-membered “aromatic” ring is formed; the total x-electron population of the carbon junction atoms and boron is 2.03 lel, which is delocalized over the small fused ring. Of course, the term aromatic is used here conditionally since neither bond distances nor a-bond orders are identical. Nevertheless, we find it justified since 2x-electrons are distributed over three centres.

The geometric and bonding features follow a similar pattern as in the model system 3. Some of them are less pronounced, however, as for example the angular deformations of the benzene fragment. A notable difference is hybridization of the annelated bond which

Molecule Bond/Angle HF/6-31G* s-character *-b.o. Atom Atomic charge

tiwdin NBO AM1

3b

C(3W4N5) 123.4

C(4N5N6) 127.9

C(5tc(6>c(l) 100.2

C(6)c(ltc(2) 130.4 H-C(6)-H 107.8

c(1kw) 1.249

C(2)-~(3) 1.370

C(3N4) 1.400

C(4N5) 1.504

C(5N6) 1.519

C(6kW) 1.343

C(l)+ 1.389

C(2)-H 1.389

C(3)_H 1.073

C(4)_H 1.081

C(5tH 1.101

C(6)_H 1.078

C(l)-C(2)-H 63.3

c(2)-W)-H 63.3

c(lk~(2>c(3) 135.7

c(2)c(3)-c(4) 101.8

C(3)c(4Fx5) 127.1

C(4)+5>c(6) 120.1

C(5>c(6Nl) 104.1

C(6kWW2) 131.3

H-C(S)-H 102.5

37.0-37.2 (40.3-39.1) 52.0-28.1 (49.1-25.7) 33.2-35.6 (32.1-32.2) 33.5-27.9 (29.2-24.4) 25.9-31.9 (23.4-29.6) 31.W9.5 (27.2-48.4)

38.7 (41.1) 30.7 (35.2) 23.1 (28.7) 36.9 (40.0)

0.6 3 (0.64) C(1) 0.04 -0.03 -0.08

0.6 1 (0.63) C(2) 0.24 0.30 0.20

0.6 9 (0.70) C(3) -0.08 -0.28 0.01

0.3 1 (0.34) C(4) 0.11 0.17 0.09

0.3 1 (0.34) C(5) -0.27 -0.60 -0.24 0.69(0.70) C(6) 0.18 0.19 0.28

H(C1) 0.36 0.43 0.32

H(C3) 0.28 0.36 0.27

H(C 4) 0.27 0.33 0.27

H(C 5) 0.29 0.39 0.28

H(C 6) 0.29 0.34 0.27

178 M. Eckert-MaksiC et al.iJournal of Molecular Structure (Theochem) 366 (1996) 173-183

assumes an s-character lower than in the canonical sp3 state. More specifically, the s-content of 23.7-23.7% compares with the value 25.7-25.7% found in benzo- cyclopropene [43]. A slight decrease in the average s- character and a significant increase in the s-content of the hybrid directed towards the boron atom is consis- tent with the electropositive character of boron. According to the Walsh-Bent rule [44], electropositive atoms prefer vis-li-vis hybrid AOs possessing low p-character. The hybridization in the B-H bond is shifted to the sp’ state, which is compatible with bent bonding occurring in the small ring and requiring higher p-character.

Ipso and para bonds in the o-protonated benzo- borirefe 4a are considerably shrunk by 0.03 and 0.04 A, respectively, which is in accordance with the concomitant increase in the n-bond orders. It is

important to note that the aromatic character is appar- ently diminished in the annelated ring. A qualitative insight into the relative aromatic stabilization of the fused rings is offered by the bond strength criterion defined by the sum of the corresponding r-bond orders:

where the summation is extended over the Al3 bonds of the small ring. The BS, values are 1.58, 1.63 and 1.31 for systems 4,4a and 4b, respectively. This finding provides a hint that the /3-protonated form might be less stable than its a-protonated competitor. Varia- tion of the corresponding hybridization parameters along the series 4-4b is rather small in accordance with the general picture.

Distribution of CC bond distances and x-bond

Table 2

Relevant structural parameters (distances in Angstroms, angles in degrees) of 4 and its protonated species as calculated by the HF/6-31G*

model. Hybridization s-characters are estimated by the HF/6-31G* (NBO) and AMl//HF/6-31G* procedures (AM1 results are in parentheses).



Liiwdin NBO AM1

4 C(lW2) 1.374 C(2)c(3) 1.413

C(3W(4) 1.360

C(4kW) 1.434

C(l)_B 1.472 C(3)_H 1.075

C(4)-H 1.076

B-H 1.179 C(l)C(2tB 59.9

C(l)-C(2)-C(3) 122.2 C(2)-C(3)-C(4) 115.6

C(3)-C(4)-C(5) 122.2 4a WI-P) 1.345

C(2)-c(3) 1.434

C(3W4) 1.392

C(4)+5) 1.397

C(5)c(6) 1.488 C(lkC(6) 1.472

C(ltB 1.501 C(2)-B 1.472 BH 1.169 C(3kH 1.076 C(4tH 1.073 C(5tH 1.076 C(6tH 1.092

C(ltc(2)_B 64.2

C(2kWkB 62.0

23.7-23.7 (22.6-22.6)

38.7-32.8 (35.7-29.1)

36.2-36.9 (32.8-34.2)

33.7-33.7 (30.3-30.3)

37.7-28.4 (40.1-28.6)

30.9 (35.7)

29.3 (32.3)

43.1 (48.3)

0.62 (0.67) C(1) -0.02 -0.30 -0.21

0.48 (0.47) C(3) -0.14 -0.21 -0.07

0.79 (0.82) C(4) -0.14 -0.20 -0.12

0.52 (0.48) B -0.14 0.54 0.18

0.48 (0.49) HB 0.08 -0.06 0.05

HC(3) 0.17 0.23 0.13

HC(4) 0.17 0.23 0.13

25.3-25.7 (25.7-25.7)

35.4-33.0 (33.6-29.9)

35.6-33.6 (32.3-31.0)

34.2-36.5 (31.3-32.6)

33.5-26.2 (29.823.6)

37.4-27.5 (33.1-24.1)

37.1-25.6 (38.0-24.4)

38.8-27.8 (38.4-26.9) 46.4 (56.0)

31.0 (34.1)

32.0 (34.8)

29.9 (33.1)

23.2 (27.6)

0.72 (0.77) C(1) 0.07 -0.11 -0.15

0.38 (0.45) C(2) -0.06 -0.43 -0.34

0.65 (0.64) C(3) 0.14 0.20 0.24

0.70 (0.74) C(4) -0.21 -0.39 -0.24

0.29 (0.31) C(5) 0.09 0.19 0.10

0.21 (0.26) C(6) -0.27 -0.57 -0.16

0.42 B -0.03 0.66 0.42

0.49 HB 0.13 -0.03 0.15

HC(3) 0.24 0.27 0.19

HC(4) 0.22 0.29 0.21

HC(5) 0.22 0.27 0.21

HC(6) 0.22 0.33 0.21

M. Eckert44akd et aLlJournal of Molecular Structure (Theochem) 366 (1996) 173-183 179

orders in the benzene ring in the benzocyclopropenyl

cation 5 (Table 3) reveals a typical anti-MN bond

fixation. Its protonated (Y- and /3- forms follow the same pattern as in the benzoborirene protonated

species 4a and 4b, respectively. It is of some impor- tance to note a substantial aromatic character of the

fused small ring in 5. The corresponding bond strength value BS, is 1.72 which should be compared with BS, values of 1.85 and 1.64 in 5a and 5b, respec-

tively. It appears that a-protonation increases the

aromatic character of the annelated ring whereas the opposite is the case in the fl-Wheland u-complex. An

interesting distinction of the 5-5b series relative to

the protonated benzoborirenes is Coulomb repulsion between the atoms forming the three-membered ring, which should lead to lower proton affinities (PAS).

A brief general comment on the bonding

Table 2 Continued

descriptors estimated at the semiempirical AM1

level is of some interest. The local hybridization para-

meters obtained by the HF/6-31G* and AM1 wave-

functions are in qualitative accordance despite the fact that they are estimated by widely different NBO and

Trindle-Sinanoglu procedures, respectively. They define different scales but within each scale the same general pattern emerges. This indicates that

the hybridization reflects something genuine in the

chemical bonding process although it does not have an absolute meaning. It should be pointed out that the

NBO method fails to provide the hybridization para-

meters for the long C(l)-H and C(2)-H bonds in 3a and 3b. The Trindle-Sinanoglu procedure is more useful in this respect. Another point of interest is the

x-bond orders which constitute an important mode of description of bonding in planar molecular systems.


Lawdin NBO AM1

C(l)-C(2)-C(3) 122.3 C(2)-C(3)-C(4) 118.3 C(3)-C(4)-C(5) 120.1 C(4)-C(S)-C(6) 124.7 C(S)-C(6)-C(l) 111.1 C(t+C(l)-C(2) 123.5 H-C(6)-H 103.9

4b Wkw) 1.383 C(2)-~(3) 1.418 C(3)c(4) 1.345 C(4>c(5) 1.496 C(5W6) 1.476 C(6VJl) 1.376 C(l)-B 1.483 C(2)-B 1.541

B-H 1.168 CO)_H 1.072 C(4)_H 1.075 C(5)-H 1.096 C(6)-H 1.077 C(lEC(2kB 60.7 C(2tC(ltB 64.9

C(lw(2)-C(3) 126.0 C(2)-C(3)-C(4) 114.5 C(3)-C(4)-C(5) 123.7 C(4)-C(5w(6) 117.6

C(5)-C(6)-C(l) 116.8 C(6)-C(l)-C(2) 121.4 H-C(5)-H 102.5

24.0-25.0 (24.1-24.4) 40.5-31.2 (35.8-28.9) 35.8-37.8 (32.8-34.3) 31.6-28.6 (28.3-24.8) 27LL33.0 (24.1-29.6) 35.3-37.6 (31.435.4) 38.3-28.6 (38.2-27.4) 34.3-24.7 (36.1-23.4) 46.7 (56.3) 33.0 (35.9) 30.5 (33.7) 22.3 (26.8) 31.6 (34.8)

0.58 (0.60) 0.47 (0.50) 0.82 (0.83) 0.26 (0.26) 0.32 (0.33) 0.61 (0.67) 0.44 (0.36) 0.29 (0.25)

C(l) -0.11 -0.53 0.39

C(2) 0.12 -0.04 -0.08

C(3) -0.16 -0.30 -0.11

C(4) -0.02 0.00 0.00

C(5) -0.29 -0.60 -0.23

C(6) 0.16 0.27 0.20 B 0.04 0.74 0.47 HB 0.14 -0.030 0.15

HC(3) 0.22 0.28 0.20

HC(4) 0.21 0.27 0.20

HC(5) 0.24 0.27 0.21

HC(6) 0.22 0.34 0.20

180 M. Eckert-MaksiC et al.IJournal of Molecular Structure (Theochem) 366 (1996) 173-183

Their values are very similar implying that u-bond order is not very sensitive to the quality of the wave-

function. This is a desirable feature of the qualitative bond index. Finally, atomic charges exhibit the stron-

gest dependence on the wavefunctions and the method employed for their extraction. L(jwdin HF/6-31G*

and AM1 charges reflect a broad mutual similarity,

whereas NBO (HF/6-31G*) atomic charges are at var-

iance in a number of cases with these two sets of numbers. More specifically, NBO charges suggest

highly pronounced intramolecular transfer which is probably not realistic.

3.2. Energetic properties

Total molecular energies (E,s), zero-point energies (ZPEs) and proton affinities (PAS) are listed in Table 4.

PAS are calculated by Eq. (2):

PA = E,(n) + ZPE(n) - [E,, (n), + ZPE+ (n),]

where n assumes values 3, 4 and 5 and x stands for

positions a! or /3. It appears that PAS estimated by the HF/6-31G* model are higher than those predicted by

the more involved MP2(fc)/6-31G*//HF/6-3lG*

model. The same holds for differences P(n),&‘(n),

which are 1.8 (6.2), 8.2 (10.6) and 21.2 (25.9) for n = 3, 4 and 5, respectively, where HF/6-31G* values

are given in parentheses. The cY-Wheland intermediate is predicted to be more stable in all the systems and by both models employed. Hence, the anti-MN systems indeed exhibit a reversed behaviour compared to the

much discussed MN compounds, as far as electrophilic substitutions are concerned. This theoretical prediction awaits experimental confirmation, but we

(2)

Table 3

Relevant structural parameters (distances in kgstr(ims, angles in degrees) of 5 and its protonated species as calculated by the HF/6-31G*

model. Hybridization s-characters are estimated by the HF/6-31G’ (NBO) and AMl//HF/6-31G* procedures (AM1 results are in parentheses).


Molecule Bond/Angle HF/6-31G’ s-character r-b.o. Atom Atomic charge

Liiwdin NBO AM1

5

5a

c(lK(2) 1.373

c(2~(3) 1.415

C(3W(4) 1.375

c(4W(5) 1.465

W-c(7) 1.351

C(7)_H 1.072

c(3)_H 1.071

c(4)-H 1.075

c(l~(2~(7) 59.5

c(l)-c(2~(3) 124.8

c(2K(3>c(4) 111.1

C(3)-c(4)-W) 124.2

c(l)c(2) 1.341

c(2W3) 1.463

c(3WX4) 1.365

c(4kW 1.429

W-C(6) 1.495

c(6W(l) 1.477

W-c(7) 1.360

c(2W(7) 1.352

c(7)--H 1.077

C(3)-H 1.076

C(4)_H 1.075

C(5)-H 1.080

c(6)-H 1.095

c(lK(2H7) 60.7

c(2~(l)c(7) 60.1

20.4-20.4 (21.421.4)

44.3-29.5 (41.3-27.0)

36.3-36.9 (33.8-33.3)

32.7-32.7 (29.7-29.7)

35.1-29.6 (34.7-26.6)

40.5 (43.6)

34.1 (37.0)

30.3 (33.7)

21.9-22.2 (22.9-16.2)

41.5-28.5 (39.0-24.5)

36.9-35.2 (34.0-32.2)

31.8-35.5 (29.3-31.3)

33.625.7 (29.0-22.6)

24.8-43.1 (20.5-39.2)

34.8-28.2 (34.6-25.2)

35.3-28.7 (34.7-25.8)

42.8 (45.7)

34.5 (37.7)

32.9 (37.6)

30.8 (35.8)

24.8 (34.3)

0.48 (0.46)

0.45 (0.49)

0.81 (0.80)

0.41 (0.45)

0.62 (0.64)

0.62 (0.65)

0.27 (0.30)

0.77 (0.77)

0.55 (0.58)

0.28 (0.33)

0.22 (0.25)

0.61 (0.60)

0.62 (0.64)

C(1) 0.10 0.10 0.04

c(6) -0.13 -0.10 -0.04

c(5) -0.06 -0.25 -0.04

C(7) 0.07 0.07 0.19

HC(3) 0.22 0.29 0.21

HC(4) 0.21 0.28 0.21

HC(7) 0.25 0.30 0.26

C(1) 0.16 0.21 0.04

C(2) 0.11 0.07 -0.03

C(3) 0.10 0.09 0.47

C(4) -0.14 -0.34 -0.18

C(5) 0.22 0.35 0.23

C(6) -0.26 -0.49 -0.09

C(7) 0.17 0.17 0.01

HC(3) 0.27 0.33 0.25

HC(4) 0.26 0.33 0.25

HC(5) 0.27 0.30 0.24

HC(6) 0.28 0.32 0.26

HC(7) 0.30 0.34 0.32

M. Eckert-MaksiC et aLlJournal of Molecular Structure (Theochem) 366 (1996) 173-183 181

feel confident that our conclusion is correct. Rationalization of the energetic preponderance of the cY-Wheland intermediates over the /I-counterparts follows the same lines as in the MN systems [27]. There are two antagonistic cz-localization patterns which interfere in Wheland u-complexes. One represents the “molecular memory effect” being a characteristic of the GS. The second is imposed by protonation. It is obvious that both modes of bond fixation are more orchestrated for o-attack. A question arises as to what is the origin of the pronounced variation of the PAS within a set of the studied molecules 3-5. The answer at the qualitative level should be sought in the interactions involving the “hetero- atoms” X = H’ , BH and C’ H and their nearest neighbours. We discussed earlier the considerable aromatic character of the small fused rings for

Table 3 Continued

X = BH and C’ H which was missing in the model system 3. This feature should increase the PAS. In particular, a substantial increase in the a-electron bond strength BS, was observed in 5a. There is, however, an additional mode of interaction. This is Cou- lomb interaction between the members of the fused rings, which is attractive for the subset of molecules 4-4b and repulsive for the rest of the considered systems. Examination of the Coulomb interactions in the point-charge approximation indicates that they increase PAS in benzoborirene 4 and decrease PAs in 3 and 5.

4. Conclusion

We have shown that systems 3-5, which exhibit an


5b

C(l>c(2~(3) 125.8

CWC(3N4) 113.3

C(3N4N5) 122.2

c(4)-W-C(6) 126.6

C(5N6Nl) 107.0

C(6PW-C(2) 125.2 H-C(6)-H 104.5

c(ww 1.357

C(2vx3) 1.411

C(3W4) 1.358

c(4kw) 1.512

C(VC(6) 1.458

C(6Wl) 1.438

c(l)-c(7) 1.344

C(2w7) 1.402

C(7)-H 1.079

C(3)-H 1.073

c(4tH 1.077

c(5)-H 1.107

C(6)-H 1.083

C(lw(2~7) 58.3

C(2)-c(lwx7) 62.5

C(ltc(2vx3) 128.8

C(2W3N4) 110.4

c(3W4N5) 125.6

C(4N5N6) 120.2

C(stc(6Nl) 111.3

C(6PW)-c(2) 123.7 H-C(S)-H 98.4

23.1-22.7 (23.0-23.0) 45.9-28.8 (42.1-26.5) 35.4-37.5 (33.0-34.0) 31.0-29.5 (27.5-25.7) 26.5-34.9 (24.1-30.8) 30.4-40.0 (22.1-32.0) 36.7-29.7 (36.0-26.3) 31.2-27.1 (31.9-24.1) 43.0 (45.8) 35.7 (38.6) 31.3 (34.8) 22.1 (28.4) 34.6 (38.0)

0.54 (0.52) 0.49 (0.55) 0.80 (0.77) 0.26 (0.29) 0.43 (0.47) 0.36 (0.43) 0.66 (0.67) 0.44 (0.46)

c(1) 0.02 -0.12 -0.20

c(2) 0.18 0.21 0.07

C(3) -0.13 -0.30 -0.04

C(4) 0.03 0.06 0.06

C(5) -0.30 -0.67 -0.27

C(6) 0.29 0.43 0.34

C(7) 0.22 0.27 0.37

HC(3) 0.25 0.33 0.27

HC(4) 0.25 0.31 0.28

HC(5) 0.30 0.41 0.30

HC(6) 0.28 0.32 0.27

HC(7) 0.30 0.34 0.31

L&din NBO AM1

182 M. Eckert-MabiC et al.lJournal of Molecular Structure (Theochem) 366 (1996) 173-183

Table 4

Total molecular energies, zero point energies and proton affinities in the anti-MN systems 3-5 as estimated by the HF/6-31G* and MP2(fc)/6-

3lG*//~/6-3~G* models””

Molecule Total energy ZPE(HF/&31G*) PAb

3 -229.65498 -230.40563 54.0

3a -229.75041 -230.47615 60.5 37.8 (53.4)

3b -229.74049 -230.47337 60.5 36.0 (47.2)

4 -254.74420 -255.56633 61.0

4a -255.04903 -255.84540 67.6 168.5 (184.7)

4b -255.03123 -255.83133 67.0 160.3 (174.1)

5 -267.65088 -268.49410 63.3

5a -267.75841 -268.57873 69.4 47.0 (61.4)

5b -267.71486 -268.54253 67.9 25.8 (35.5)

HF/6-31G*

a Total molecular energy and proton affinity (PA) in a.u. and kcal mol-‘, respectively

b Proton affinities are calculated at the MP2(fc)/6-31G*//HF/6-3lG* and HF/6-31G* (in parentheses) levels by employing eqn (2). ZPEs used

in eqn (2) are those of the HF/6-31G’ procedure multipied by 0.89

anti-MN pattern of x-electron bond fixation, prefer electrophilic attack at the cu-position. Hence, their behaviour is diametrically opposite to that established in the MN systems. Interpretation of the electrophilic reactivity in both families of compounds is based on the same general mechanism. It originates in greater or lesser matching of two a-electron localization modes. The first is present already in the GS, whilst the second is induced by protonation. Interference of these two antagonistic n-electron distribution patterns in the TS modelled by Wheland’s intermediates determines the electrophilic reactivity. It appears that this competition is more favourable for a-protonation, which is preferred over P-attack in the anti-MN systems. It is worth noting that a-protonation leads to an increase in the aromatic character of the fused small rings in compounds 4 and 5. Actual values of the PAS are affected by Coulomb interactions within the fused small rings. The present and earlier results [7-19,27,30-331 conclusively show that statements about the chemical irrelevance [45,46] of the MN effect should be considered as premature.

Acknowledgements

This work was supported in part by the Ministry of Science and Technology of the Republic of Croatia. We thank the University of Zagreb Computation

Centre and the IBM project “Academic Initiative for Croatia” for computation time and excellent service.

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Documents

Electrophilic reactivity in anti-Mills-Nixon systems