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2-Stannylpyridine-Borane Adducts - Multinuclear Magnetic Resonance Study and X-Ray Structure Determination of a l,4-Dihydro-4 a,l,4-azastannabora-naphthalene DerivativeBernd Wrackmeyer*, Heidi E. Maisel, Wolfgang Milius Laboratorium für Anorganische Chemie, Universität Bayreuth, D-95440 Bayreuth Z. Naturforsch. 50b, 809-815 (1995); received October 28, 1994 2-Trimethylstannyl-pyridine, Borane Adducts, Heterobicycle, X-Ray, NMR Spectra

The adduct formation between 2-trimethylstannyl-pyridine (1) and triethylborane, leading to 2a, and 9-borabicyclo[3.3.1]nonane, leading to 2b, was studied by ’H, “ B, 13C, 15N and 119Sn NMR in solution. Changes in the magnitude of the coupling constants 7(119Sn, 13C), with respect to the data for 1, were analysed. The absolute signs have been determined [all coupling constants " / (119Sn,13C) to methyl and pyridine carbon atoms in 1 to 3 possess a negative sign and the same is true for "y(119Sn,*H) of the pyridine hydrogen atoms] by various two-dimensional NMR experiments, and attributed to the influence of the lone pair of electrons at the nitrogen atom in 1. The NMR spectroscopic results for the 1,4-dihydro- 4a,l,4-azastannabora-naphthalene derivative 3, in which structural fragments are present analogous to those in the borane adducts 2, correspond to those for 1 and 2a,b. The molecu­lar structure of 3 has been determined by X-ray analysis [orthor’nombic; P 2 i2121; a = 713.9(2), b = 1566.0(2), c - 1578.4(2) pm]. Solid-state 13C and 119Sn CP/MAS NMR spectra prove that the molecular structures of 3 in the solid state and in solution are very similar.

Pyridine-type nitrogen atoms in azoles or azines are readily available for adduct formation with various boranes [1], Depending on steric require­ments, there can be fast exchange between the azole-(azine)-borane adduct and excess of either the azole (azine) or the borane. Recently, it was shown that this situation applies to azole-BEt3 ad­ducts if a Me3Sn group is present adjacent to the two-coordionated nitrogen atom [2], Such adducts are intermediates preceding the elimination of Me3SnEt and the formation of azaboles [2]. The NMR parameters of the intermediate azole-BEt3

adducts are of interest since the (313C data hardly respond to the adduct formation whereas the mag­nitude of the coupling constants l"7(1 1 9 Sn13C) I changes markedly, once E t3B has been added to the solution of the respective azole [2]. These changes can be attributed to the influence of the lone pair of electrons at the nitrogen atom [3]. In order to compare such changes with trends ob­served for coupling constants in pyridine [4] and in the pyridinium cation [5], we have now studied 2-trimethylstannylpyridine (1) and its E t3B-adduct

Reprint requests to Prof. Dr. B. Wrackmeyer.

(2 a) by multinuclear NMR spectroscopy ('H , n B, 1 3C, 1415N and 119Sn NMR) using two-dimensional (2 D) techniques for the determination of absolute signs of the coupling constants " /( '^ S n 'H ) and ”/ ( 1 1 9 Sn1 3C). A kinetically more stable N -B bond is encountered in 2b, the 9-borabicyclo[3.3.1]no- nane-adduct of 1, and in the fused heterobicycle 3. Compound 3 was also studied by 13C and 119Sn CP/MAS NMR in the solid state, and its molecular structure was determined by single crystal X-ray analysis.

n Me3Sn BEt3

■ ö . Ö 4 51 2 a

HN A fMe3Sn /B —

o

Ae Et

3

Results and Discussion

Synthesis

Reactions between 1 and E t3B or 9-borabicyclo-[3.3.3]nonane (9-BBN) were studied on a small scale in NMR tubes. 2-Trimethylstannyl-pyridine (1) reacts with E t3B to give an adduct which, at room temperature, readily exchanges with an ex­cess of 1 or E t3B. In contrast, 1 reacts very slowly

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810 B. Wrackmeyer et al. • 2-Stannvlpyridine-Borane Adducts

at room temperature with the dimeric 9-BBN. Fast and irreversible symmetrical cleavage of the BH2B

15 pm above and the boron atom 18 pm below that plane. The atoms B, C(5), C(3), Sn form a plane within the experimental error. There are dis­torted tetrahedral surroundings for both the tin and the boron atom, with endocyclic bond angles being small [C(3)SnC(12) = 101.8(1)°] and fairly large [NBC(5) = 118.5(2)°]. The latter bond angle together with the rather long BN distance causes a wide bond angle BC(5)C(3) = 132.7(3)°. The BN bond [168.9(5) pm] is longer than all BC distances in 3, as in other pyridine-trialkylborane [10] or in trialkylamine-dialkylborane adducts [11]. The ar­rangement of the ethyl groups at the boron atom is noteworthy (see Fig. 1). The non-bonding dis­tances BC(9) and B C (ll) (266 pm) suggest that interactions between the hydrogen atoms at C(9) and C (ll) and the boron atom are conceivable which could account for the long BN distance and deviations from ideal bond angles.

Fig. 1. Molecular structure of the heterobicycle 3. Se­lected bond lengths [pm] and angles [°]: S n -C (l) 214.8(4), Sn-C (2) 214.5(4), Sn-C (3) 209.7(4), S n- C(12) 214.9(3), N -B 168.9(5), B -C (5) 163.0(5), B -C (8) 163.9(5), B -C (10) 164.7(5), C (3)-C (5) 134.6(5); C ( l) - Sn-C (2) 110.2(2), C (3)-S n-C (12) 101.8(1), S n -C (3 )- C(5) 118.7(3), S n -C (1 2 )-N 120.1(2), N -B -C (5 ) 118.5(2), B -N -C (1 2 ) 127.5(3), B -C (5 )-C (3 ) 132.7(3). B -C (8 )-C (9 ) 114.5(3), B -C (1 0 )-C (ll) 114.1(3).

NMR spectroscopic results

Relevant n B, 13C, 14/15N and 119Sn NMR data of the compounds 1 to 3 are given in Table II. The mutual assignment of 'H and l3C NMR signals is based on 2D 13C /'H heteronuclear shift correla-

moiety takes place at temperatures >50 °C and the adduct 3 is formed. Under these conditions, only a trace of Me3SnH can be detected ( ,19Sn NMR). Any exchange between 3 and an excess of either 1 or dimeric 9-BBN is slow on the NMR time scale.

The reaction of 2-pyridyllithium [6] with Me3SnCl affords 2-trimethylstannyl-pyridine (1), and with (£)-2-chloro(dimethyl)stannyl-3-di- ethylboryl-2-pentene [7] the heterobicycle 3 is ob­tained [eq. (1)] as a colourless, crystalline, mois- ture-sensitive solid. The reaction according to eq. (1) has been mentioned previously [8],

X Li. MeN X , / E tMe2Sn BEt2 ♦ \ = N ------ > JSn (1)

X CI O - LiCI M e V f O EtO

3

X-ray structure determination o f 3Data relevant to the X-ray analysis of 3 are

given in Table I [9]. The molecular structure of 3 is shown in Fig. 1 together with selected bond lengths and angles. The skeleton of the heterobi­cycle 3 is not exactly planar. Starting from the plane of the pyridine ring, the tin atom is located

Table I. Experimental data related to the single crystal X-ray analysis of 3.

Formula (molecular mass) C 1 6H 28B N S n (363.9)Crystal; Size colourless platelet;

0.24x0.24x0.14 mm3

Crystal system; space group; Z orthorhombic; P 2 12 121; 4Unit cell dimensions [pm] a = 713.9(2), 0 = 1566.0(2),

c = 1578.4(2)v [A3] 1764.6(6)Absorption coefficient [mm ’] 1.437Diffractometer Siemens P4Radiation [pm] MoK„, A = 71.073,

graphite-monochromatorTemperature [K] 1732 0 -range 4°< 26> <55°Scan type 2d - eScan range (co) 1.30° plus K„-separationMeasured reflections 3308Independent/observed reflections 3101 (F > 0.0cj(F), no

reflections omitted)Refined parameters 173Solution direct methods

(SHELXTL PLUS)Weighting scheme »v 1 = o 2 (F) + 0.0000F 2

/?-/w/?-value 0.029/0.024Max./min. residual electron 0 .4 9 /-0 .4 7

density [eA ~3]

B. Wrackmeyer et al. • 2-Stannylpyridine-Borane Adducts_________________________________________________811

Table II. n B, 13C, 14/15N and 119Sn NMR data3 the heterobicycle 3.

of 2-trimethylstannyl-pyridine (1) and the borane adducts 2 and of

CompoundNo.

<3n B <3I3CC(2) C(3) C(4) C(5) C(6) SnMe

<315N c)119Sn

l b - 173.4[-605.5]

131.6[-93.2]

133.4[-37.2]

122.4[-12.0]

150.6[-68.7]

-9 .3[-346.6]

-37 .6[+108.2]

-52 .9

2ac +5.0d 171.3[-500.0]

134.5[-66.4]

134.9[-35.0]

123.0[-9 .8]

148.6[-36.0]

-4 .1[-369.8]

-1 0 0 e -30 .4

2b f +3.6 173.7[-491.6]

133.9[-61.0]

135.7[-34.1]

123.5[-8 .1]

146.7[-26.0]

-5 .6[-381.5]

-109.8 -53 .5

+4.2 173.7[-354.0]

133.4[-39.5]

134.9[-22.5]

123.5[-6.3]

147.0[-21.5]

-8 .5[-325.6]

-109.3 -119.7

3h(solid state)

- 174.0 134.0 136.9 125.3 148.0 -8 .6[321.0]

— -122.3

a In C6D 6 (*» 10-20% , 25 ± 1 °C); coupling constants " /(119Sn,13C) (± 1 Hz) together with the sign (if experimentally determined) are given in square brackets; b See also Ref. [16] for d13C values and coupling constants / ( 119Sn,13C); the values 7 (119Sn,13C) given in [16] differ considerably from those reported here, probably because of a marked dependence on concentration; most of the data and signs of coupling constants given here stem from Ref. [15], however the data for C(3) and C(4) in Table I of [15] have to be exchanged. d'H [y(U9Sn,'H)] = 7.20 [-21 .4] (H 3);7.05 [-15.5] (H4); 6.30 [-10.6] (H 5); 8.80 [<4] (H6); 0.32 [+56.0] (SnMe3); c d'H [ / ( ^ S n .’H)] - 7.24 [-28.2] (H 3), 6.87 [-8 .4 ] (H4), 6.55 [-7 .4 ] (H 5), 8.45 [-6 .7 ] (H6), 0.29 [+56.5] (SnMe3); d Ratio of 1 and Et3B * l : l ; e 14N NMR in the presence of a five-fold excess of Et3B; ‘ Other d 13C data: 25.7 (broad, BCH), 37.3, 29.5 (CH2), 24.7, 23.6 ( -C H 2-) ; (3'H [ / ( 119Sn,‘H)] = 7.21 [-2 9 .0 ]'(H 3), 6.83 [-8 .5] (H4), 6.52 [-6 .7 ] (H5), 8.84 [-7 .4 ] (H 6), 2 .40-1 .90 m (CH2-C H 2-C H 2), 1.18 m (BCH), 0.37 [+56.3] (SnMe3); e Other <313C data: 128.1 [-624.0] (Sn-C =); 172.1 [broad] (B -C = ); 19.8 [broad], 10.6 (BEt); 26.3 [-96 .3], 14.8 [16.4] (Et); 21.2 [-99.0] (Me); d lH [ / (U9Sn13C)] = 6.90 [-19.5] (H 3), 6.69 [-5 .4] (H4), 6.40 [-4 .7 ] (H5), 8.60 [-9 .0] (H6), 2.62 [+3.0], 1.27 [2.7] (= C -E t), 2.15 [-67.1] (= C -M e), 1.18, 0.83, 0.80 (BEt-.), 0.25 [+53.7] (SnMe2); h Other d13C values from solid-state 13C CP/MAS measurements: 127.2 (Sn-C =); 173.0 (B -C = ); 22.0, 18.1, 10.6, 10.0 (BEt); 25.9, 15.9 (Et); 21.8 (Me).

tions (HETCOR), both for 1/ ( 13C 1H) and n7(13C1H) («>1).

The HETCOR experiments also serve for the comparison of signs of the coupling constants [12,13] 7(119Sn13C) and y(119Sn'H). A t least one so-called “key coupling constant” must be avail­able in order to obtain the absolute signs. In the case of methyltin compounds, such a “key coup­ling constant” is 2/ ( 119Sn1H Me) (>0 [14]). The strategy for obtaining the absolute signs is shown in Scheme 1 for compound 3 by the experiments A -G for some relevant isotopomers. The HETCOR experiments based on long-range cou­pling constants 7(13C,JH) are particularly useful, as shown in Fig. 2 for experiment A, in Fig. 3 for experiment B and in Fig. 4 for experiment G. There are always two “active” spins, e.g. 13C and 'H in the 13C /1H HETCOR experiments, and one “passive” spin, 119Sn. This allows to compare the signs of the coupling constants 7(119Sn,13C) and 7(119Sn,'H ), and the relative signs, alike or oppo­

site, are revealed by the tilt of the cross peaks in the 2D contour plots (see Fig. 2 -4 ). For the pulse sequences employed, a positive tilt indicates alike signs [Fig. 3 (experiment G in Scheme 1) and Fig. 4], whereas a negative tilt indicates opposite signs [Fig. 2 (experiment A in Scheme 1)]. Since y119Sn<0, the comparison of signs refers to the reduced coupling constants K [K(A,B) = 4;r2-y(A ,B )-(yA -yB -h)-1] rather than to J.

15N NMR spectra of 2 b and 3 were best ob­tained using the basic INEPT sequence [17] with­out ‘H-decoupling. This method was more effi­cient than the refocused version of INEPT with 'H-decoupling [18]. Owing to scalar relaxation of the second kind (partially relaxed scalar 15N - n B coupling [19]) T2(15N) becomes fairly short and a great deal of 15N-magnetization is lost during the additional delays in the refocused INEPT se­quence with 'H-decoupling. This method also ena­bled us to determine the coupling constant 27(15N,’H6) ~ 7.5 ± 2 Hz in 2b and 3. Because of

812 B. Wrackmeyer et al. • 2-Stannylpyridine-Borane Adducts

Isotopomer Experiment Result Conclusion

A 13c / 1h hetcor 2K(119Sn,1H) 0*>oc4 5

(3J(13C *,1H) 1K(119Sn,13C*)K( Sn, C ) > 0

'> c B 13c / 1h hetcor

[3J(13C *,1H)]4K(119Sn.1H 4) . 0 1K(119Sn,13C*)

4K(119Sn,1H 4) > 0

c 13c / 1h hetcor

[1J(13C *,1H)]4K(119Sn,1H) ; 0

3K(119Sn,13C *)3K(119Sn,13C*) > 0

’>Oc,H

D 1H /1H COSY-45 [3J(1H,1H)]

3K(119Sn ,1H3) 4K(119Sn,1H4) '

3K(119Sn,1H3) > 0

E 13c / 1h hetcor

[1J(13C *,1H)]2K(119Sn,13C*)3 K(119Sn,1H) '

2K(119Sn,13C*) > 0

">0c■p*F 13c / 1h hetcor

[3J(13C *,1H)]

3K(119Sn,13C*)4K(119Sn,1H 4)

3K(119Sn,13C*) > 0

">QC0-’HG 13c / 1h hetcor

[3J(13C *,1H)]

1K(119Sn,13C*) 5 0

4K(119Sn,1H 6)4K(119Sn,1H6) > 0

Scheme 1. A non-exhaustive list of isotopomers (the asterisk marks a 13C nucleus) relevant for the determination of absolute signs of coupling constants for the pyridine ring in 3. It starts with experiment A (see Fig. 2) where the absolute sign of 2K (119Sn,1H Me) is known to be negative. The same strategy ap­plies to 1 [10] and 2, and in 3 it is also applicable to the -(M e)C = C (E t)- moiety (for results see Table II).

the broadened 15N NMR signals observed for 2 b and 3 it was not possible to determine the coupling constant 2/ ( 119Sn,15N) from the 15N NMR spectra.

The 119Sn NMR spectrum of 1 shows a single broad resonance, and it was possible to measure 2/ ( 119Sn,15N) = +108.2 Hz from the 15N satellites in the ,19Sn NMR spectra by using Hahn-echo ex­tended (HEED) INEPT pulse sequences [15,20]. In contrast, the 119Sn NMR signal of 2 b is rather sharp (h1/2<10 Hz) which means that the magni­tude of l2/ ( 119Sn,14N) I and therefore also that of l2/ ( 119Sn,15N) I must be small. The 1,9Sn NMR sig­nal of 3 is somewhat broader than for 2 b. How­ever this broadening is due to non-resolved scalar 119S n - n B coupling across three bonds rather than to 2/ ( 119Sn,14N).

The presence or absence of the lone pair of electrons at nitrogen affects various coupling con­stants in pyridine, pyridine derivatives and in pyri- dinium cations [3-5]. The trends of / ( ^ S n / H ) or of / ( 119Sn,13C) in 1 -3 should correspond to those found for 'H - 13C or 1H - 1H couplings in pyridine and in the pyridinium cation. Since all signs of the relevant coupling constants in 1 -3 have been de­termined, this comparison is now feasible. Re­moval of a “cis lone pair” normally causes a decrease in !K [3]. This is not found in pyridine (1/ ( 13C2,1H) = +177.4 Hz) and in the pyridinium cation ('./(^C V H ) = +190.7 Hz) because of com­pensation of increased a-inductive effects in the cation. However, in the borane adducts 2 a, 2 b and 3 this expectation is fulfilled since the

B. Wrackmeyer et al. • 2-Stannylpyridine-Borane Adducts 813

135.2—i 135.0

I-134.8 S 13C

Fig. 4. Contour plot of the 2 D 13C4/*H HETCOR spectrum based on the long range coupling constant 3/ ( 13C V H 6) ( ~ 6 Hz). The positive tilt of the satellite cross peaks indicates that the signs of 4K (119Sn,'H 6) (>0) and 3K (119Sn,13C4) (>0) are alike.

Fig. 2. Contour plot of the 2D 13C2/ ‘H HETCOR spectrum of 3 based on the long range coupling constant 3y (13C2,'H SnMe) (« 1 .5 Hz; see experiment A in Scheme 1). The negative tilt of the satellite cross peaks indicates that the signs of 2K (U9Sn,1HSnMe) (known to b e< 0) and ‘K (U9Sn,13C2) (>0) are opposite.

174.0 173.5 173.0 S 13C

175.0 ' 173.0 ' s«C

Fig. 3. Contour plot of the 2D 13C2/'H HETCOR spectrum of 3 based on the long range coupling con­stants 3/ ( 13C2,xH6) (= 6 .0 Hz; see experiment G in Scheme 1). The positive tilt of the satellite cross peaks indicates that the signs of 'K (119Sn,13C2) (>0) and 4K (119Sn,'H 6) (>0) are alike.

'K (119Sn,13C2) values are all smaller than in 1. The 2K (119Sn,15N) value in 1 is large and positive, whereas it is small (sign and magnitude not deter­mined) in 2 b and 3. This trend agrees with that for 2K(15N,1H 2,6) in pyridine and in the pyridinium cation. There is a markedly positive contribution of the lone pair of electrons at the nitrogen atom to 2K (119Sn,13C3) in 1 as compared with 2 and 3, and this is again true for 2K (13C3,1H2) in pyridine and in the pyridinium cation [3]. There is only a small difference in the 3K (119Sn,13C4) values be­tween 1 and 2, similar to the situation for 3K (13C4,1H2) in pyridine and [C5H 5NH]+. The vici­nal 119S n -C 4 and 119S n -H 3 couplings in 3 differ significantly from those in 2 a and 2 b. This points towards the influence of the deviation from the ideal planar bicyclic structure (vide supra). If the lone pair of electrons is attached to the interven­ing nitrogen atom (1), there is a strong positive contribution to 3K (119Sn,13C6), in accord with the findings for 3K (13C6,1H2) in the parent compounds (pyridine: V p C V H 2) = 8.5 Hz; [C5H 5NH1+: 3/ ( 13C V H 2) = 5.10 Hz).

The changes in the <513C(pyridine) values in 2a, 2 b and 3 with respect to 1 are small and non-char-

814 B. Wrackmeyer et al. ■ 2-StannyIpyridine-Borane Adducts

acteristic. The solid-state c313C values of 3 are very similar to the data in solution. Nitrogen nuclear magnetic shielding increases significantly if the lone pair of electrons becomes engaged in the co- ordinative N -B bond. This is expected, con­sidering the deshielding influence of B„-induced circulation of charge connected with n^-jr* transi­tions on nitrogen nuclear magnetic shielding [1 c,21]. The (5119Sn value of 3 (d -119.7) is close to the range known for l-stanna-2,5-cyclohexa- dienes [22] and must be regarded as typical of that particular structural unit. The (3119Sn value for a solid sample of 3 (<5 -122.3) differs little from the solution-state value, in agreement with the solu­tion- and solid-state (313C values for 3. Considering the great sensitivity of 119Sn nuclear shielding to small structural changes [22], this strongly suggests that there can be only minor differences between the surrounding of the tin atom in liquid and solid state. The similarity of the <3119Sn values of 1 (d -52.9) and 2b (d -53.9) cannot be explained as yet and the same is true for the U9Sn deshield­ing in the case of 2a ((3 -30.4). The values I 19Sn,13CMe) I in the liquid (325.6 Hz) and in the solid state (321 Hz) are similar, as expected from the chemical shift data. The proposed rela­tionship between 11/ ( 119Sn,13CMe)l and the bond angle CMeSnCMe [23] does not include compound 3. We have already observed several failures of this relationship [24],

ExperimentalAll preparative work and handling of samples

was carried out in an inert atmosphere (Ar or N2). Compound 1 [16], E t3B [25], dimeric 9-borabi- cyclo[3.3.1]nonane [26] and (£)-2-chlorodimethyl- stannyl-3-diethylboryl-2-pentene [7] were pre­pared according to literature procedures. NMR measurements in solution were performed using Bruker ARX 250, AC 300 and AM 500 instru­ments, all equipped with multinuclear units: *H/

13C NMR [<3 values relative to Me4Si: dlH(C6D5H) = 7.15; (313C(C6D 6) = 128.0]; n B NMR [external standard: BF3-O E t2, E ^ 'B ) = 32.083971 MHz]; 14N, 15N NMR [external stan­dard: neat M eN 02, £ (14N) = 7.226455 MHz. E (15N) = 10.136767 MHz]; 119Sn NMR [external standard: Me4Sn; E (119Sn) = 37.290665 MHz).

Solid-state 13C and 119Sn CP/MAS NMR spectra of compound 3 were measured using a Bruker MSL 300 spectrometer, and the compound was packed into an airtight insert [27] fitting exactly into the commercial Z r0 2-7 mm-rotor for the double bearing probehead.

The 9-borabicyclo[3.3.1]nonane adduct 2b was isolated as a colourless powder (m.p. 88 °C) after recrystallization from pentane. - El-MS (70 eV): m/e (%) = 365 M+ (18), 228 C7H 10SnN (100). - 500 MHz lH NMR: see footnotes Table II.

3,4,4-Triethyl-l ,4-dihydro-l ,1,2-trimethyl-4 a,l ,4- azastannaborci-naphthalene (3)

After cooling a solution of 0.79 g (5 mmol) of 2-bromopyridine in 30 ml of ether to -7 8 °C, 3.12 ml of a solution of /7-BuLi in hexane (5 mmol) was added, and the reaction mixture was stored at -7 8 °C for 12 h. Then a solution of 1.6 g (5 mmol) of (E)-2-chloro(dimethyl)stannyl-3-diethylboryl-2- pentene [7] in 10 ml of THF was added at -7 8 °C. The reaction mixture was allowed to reach room temperature. After filtration and removal of all volatile material a yellowish solid was left. This was treated with hexane, and from the hexane-so- lution 1.65 g (91%) of compound 3 was obtained as a colourless, crystalline material (m.p. 67 °C). - 500 MHz !H NMR, see footnotes Table II.

A cknowledgem en tsSupport of this work by the Deutsche

Forschungsgemeinschaft and the Fonds der Che­mischen Industrie is gratefully acknowledged. We thank Prof. Dr. R. Köster (Mülheim a. d. Ruhr) for a generous gift of triethylborane and Dr. A. Sebald (Bayerisches Geoinstitut, Bayreuth) for re­cording the 13C and ll9Sn CP/MAS NMR spectra.

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B. W rackmeyer et al. • 2-Stannylpyridine-Borane Adducts 815

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