Organometallics 1995, 14, 1542-1544
1542
First X-ray Structure Analysis of cisltruns Paired Diastereoisomeric Chlorobismuthanes Bearing a Chiral Bismuth Center Induced by the (R)-b(Dimethy1amino)ethylGroup as a Chiral Source. Unexpected Axial Disposition of the Benzylic Methyl Group Causing the Equilibrium Isomer Ratio in Solution To Be Insensitive to Steric Congestion around the Bismuth Atom Toshihiro Murafuji" Department of Chemistry, Faculty of Science, Yamaguchi University, Yoshida, Yamaguchi 753, Japan
Nagao Azuma Department of Chemistry, Faculty of General Education, Ehime University, Bunkyo-cho, Matsuyama 790, Japan
Hitomi Suzuki" Department of Chemistry, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan Received December 1, 1994@ Summary: cis ltrans paired diastereoisomeric chlorop h e ny 1[2-[(R) - 1-(dimet hy 1a m i no)e t hy llp he ny l/bis muthanes, 2-[(R)-MeflCHMelC&&iPhCl (la),have been characterized by X-ray structure determination. The crystal structure involves the packing of two pairs of discrete mononuclear molecules in the unit cell, and the asymmetric unit contains a pair of diastereoisomeric molecules, the configurations of which are identical as to the chiral ligand but different as to the chirality of the bismuth atom. The benzylic methyl group of the chiral ligand prefers the w i a l disposition, which is in contrast to the related organotin bromide 2-(MeflCHMe)Ca4SnMePhBr (2), where the equatorial location is much preferred. Introduction Recently, we have reported the first synthesis of diastereoisomeric chlorobismuthane l a with a chiral bismuth center by introducing the 2-[(R)-l-(dimethy1amino)ethyllphenyl group as a chiral source onto the bismuth at0m.l The diastereoselection observed for
l a ; Ar = Ph l b ; Ar = +Naphthyl
2
3
compound la was only moderate (77:23). Expecting Abstract published in Advance ACS Abstracts, February 15,1995. (1)Suzuki, H.; Murafuji, T.; Matano, Y.; Azuma, N. J . Chem. SOC., Perkin Trans. 1 1993,2969. @
that an increase in steric crowdedness around the bismuth atom would enhance the degree of diastereoselection, we prepared compound lb similarly. Rather surprisingly, however, the diastereoisomeric ratio was found to be almost the same (78:22). This unexpected result may be accounted for by two possibilities, One is that the methyl group at the chiral benzylic position takes a favorable equatorial site in the chelate ring of compound 1, and its bulkiness is not enough to induce a high diastereoselection. The other is that the methyl group is forced to occupy an unfavorable axial position due to some steric reason, leading it to be a spectator substituent. The former is the case with the related chiral tin compound 2-(MezNCHMe)CsH4SnMePhBr (2).2,3 As an example of the latter case, we now report a full characterization of the cis I trans paired diastereoisomeric chlorobismuthanes l a by X-ray structure analysis. Comparison of the molecular structure of compound l a with that of compound 2 revealed that the disposition of the benzylic methyl group is closely related to the equatorial bond angles of the chiral metal center, i.e., axial for l a and equatorial for 2, the former disposition making the equilibrium ratio of the diastereoisomers in solution insensitive to the bulkiness of the aryl group at the equatorial site.
Results and Discussion According to our reported procedure,l the diastereoisomeric chlorobismuthane l a was synthesized from the corresponding lithiated dimethyl[(R)-l-phenylethyllamine and chlorodiphenylbismuthane. A single crystal of compound l a obtained by crystallization of the diastereoisomericmixture from methanol was examined (2) van Koten, G.; Jastrzebski, J . T. B. H.; Noltes, J. G.; Pontenagel, W. M. G. F.; Spek, A. L. J . Am. Chem. SOC.1978,100, 5021. (3) High diastereoselectivity ( > 98%) was realized by substituting the methyl group for the tert-butyl group. See: Jastrzebski, J . T. B. H.; Boersma, J.;van Koten, G. J . Organomet. Chem. 1991,413, 43.
0276-733319512314-1542$09.00/0 0 1995 American Chemical Society
Notes
Organometallics,Vol. 14,No. 3, 1995 1543 Table 1. Selected Bond Lengths
(A)and Angles (deg) for
&/trans Paired Diastereoisomeric Chlorobismuthanes laa
cis isomer
trans isomer
Bi( 1)-N( 1) Bi( 1)-C( 11) Bi( 1)-C( 1) Bi( 1)-C1( 1) N( 1)-Bi( 1)-Cl( 1) N( 1)-Bi( 1)-C( 1) N( 1)-Bi( 1)-C( 11) C( 1)-Bi( 1)-C( 11) C(7)-N(l)-Bi( 1) N( l)-C(7)-C(2)
2.55(2) 2.25(2) 2.30i2j 2.646(6) 162.7(4) 72.7(7) 88.9(6) 95.6(7) 106(1) 105(2)
2.57(2) 2.26(2) 2.16(2) 2.634(6) 161.7(4) 72.0(6) 86.6(6) 98.0(8) 102(1) 107(2)
Numerals in parentheses are estimated standard deviations.
Figure 1. Crystal structure of the trans isomer of compound la.For the definition of cis and trans isomers, see text.
Figure 2. Crystal structure of the cis isomer of compound la. For the definition of cis and trans isomers, see text. by X-ray analysis, which revealed that the asymmetric unit contains a pair of independent molecules with a configuration different at the chiral bismuth center, where the benzylic methyl and equatorial phenyl groups are disposed in a cis or trans fashion as shown in Figures 1 and 2, respectively. The bismuth center of both isomers was shown t o have a distorted-pseudotrigonal-bipyramidal configuration with two aryl groups in an equatorial plane. The lone pair of electrons is considered t o occupy the remaining equatorial position. The apical positions are occupied by the nitrogen and chlorine atoms through the formation of a hypervalent three-center-four-electron bond.4 The intramolecular Bi-N distance of the trans isomer (2.55(2) A) is a bit shorter than that of the cis isomer (2.57(2)A), suggesting the operation of a stronger Bi-N interaction in the (4) Suzuki, H.; Murafuji, T.; Azuma, N. J. Chem. SOC,Perkin Trans.
I 1993,1169.
former compound (Table l h 5 In accordance with this, the Bi-C1 bond length of the trans isomer (2.646(6)A) is longer than that of the cis isomer (2.634(6)A). The C(Ar)-Bi-C(Ph) angle of the cis isomer (98.0(8)") is larger than those of the trans isomer (95.6(7)")and of the chlorobismuthane 3 (93.7(2)").l All these observations support the view that the cis isomer is forced into a more strained geometry than the trans isomer by the presence of the methyl group at the benzylic position. The short intermolecular bismuth-chlorine bond distances of 3.717(7) A for Bi(l)-C1(2) and 3.696(8) A for Bi(2)-C1(1) indicate the operation of strong interaction between the pairs of cisltrans isomers, which are linked together by the sharing of chlorine atoms into the infinite chains in the crystallographic a-axis direction. The most unusual structural feature of compound la concerns the methyl group at the benzylic position. Despite the steric compression by the neighboring dimethylamino group and nearby ortho hydrogen atom of the phenylene ring, this methyl group has a tendency to adopt an apparently unfavorable axial position in the five-membered chelate ring. This is in contrast to a similar situation observed for compound 2, where the benzylic methyl group preferentially occupies the equatorial position. This discrepancy in the preferred configurations may well be attributed to the difference of the equatorial bond angles of the respective chiral metal centers. The large C(Ar)-Sn-C(Ph) angle of 116.1(3)' in compound 2 would lessen the steric repulsion between the methyl and phenyl groups, making the equatorial location of the methyl group more favorable in the organotin compound 2. In contrast, the smaller C(Ar)-Bi-C(Ph) angle of 98.0(8)" in compound la would give rise to high steric congestion between these two groups, thus enforcing the benzylic methyl group to occupy the axial position in both diastereomers. The reason the methyl group does not prefer the equatorial position in the trans isomer may be attributed to the stereoelectronicrepulsion between the methyl group and the lone pair of electrons, which should arise when such a position is adopted. lH NMR inspection of the diastereoisomeric mixture of la revealed that the structural feature in the solid state is also reflected in solution. When dissolved in CDCls, compound la forms an equilibrium mixture of two diastereoisomers (77:23) through the Bi-N dissociation-association process. The methine proton signal of the ethylidene group of the major isomer was observed at appreciably high field (6 3.57) relative t o that of the minor isomer (6 3.891, showing the former methine proton to be subject t o the shielding effect of (5) The corresponding Bi-N distance and Bi-C1 bond length in chlorobismuthane 3 are 2.525(6) and 2.700(2) A, respectively.
Notes
1544 Organometallics, Vol. 14, No. 3, 1995
Table 2. Chemical Shifts (6) of the 1-@imethylamino)ethyl Group" NMe2
isomer
la
lb
a
Av,Hz
a-Me
a-H
major (trans) minor (cis)
2.36 2.03
2.39 2.70
6 134
1.31 1.09
3.57 3.89
major (trans) minor (cis)
2.12 1.84
2.37 2.67
50 166
1.26 1.01
3.62 3.84
Determined in CDC13 at room temperature.
Table 3. Positional Parameters and Isotropic Thermal Parameters (AZ) for la" atom
Y
Z
0 -0.23315(5) -0.0371(5) -0.1909(5) 0.079( 1) -0.3 18(1) 0.140(1) 0.188( 1) 0.276(2) 0.304(2) 0.250(2) 0.167(2) 0.160(1) 0.225(2) 0.090(2) 0.033(2) -0.017(1) -0.073( 1) -0.085(2) -0.038(2) 0.013(2) 0.036(2) -0.364( 1) -0.423( 1) -0.508( 1) -0.540(2) -0.487(2) -0.402( 1) -0.387( 1) -0.448(2) -0.349( 1) -0.263(2) -0.214(2) -0.258( 1) -0.247(2) -0.177(2) -0.129(2) -0.150(2)
0.83 191(6) 0.81021(6) 0.8853(6) 0.8796(6) 0.769(1) 0.727( 1) 0.838(2) 0.803(2) 0.821(2) 0.876(2) 0.920(2) 0.899(2) 0.722(2) 0.696(3) 0.874(2) 0.686(2) 0.648(1) 0.581(2) 0.472(2) 0.427(2) 0.485(2) 0.599(2) 0.824(1) 0.823(1) 0.843(2) 0.864(2) 0.863(2) 0.844(2) 0.809(2) 0.791(2) 0.61 l(2) 0.737(2) 0.629( 1) 0.576(2) 0.46l(2) 0.404(2) 0.460(2) 0.569(2)
X
0.5955(1) 0.10 10(1) 0.3234(8) -0.1560(8) 0.818(2) 0.301(2) 0.541(3) 0.639(3) 0.635(4) 0.509(3) 0.419(3) 0.438(3) 0.748(2) 0.871(4) 0.932(3) 0.884(3) 0.476(2) 0.529(2) 0.458(3) 0.336(3) 0.281(2) 0.355(3) 0.049(2) 0.177(2) 0.152(3) 0.015(4) -0.109(3) -0.094(2) 0.337(2) 0.450(3) 0.246(3) 0.446(3) -0.032(2) -0.164(3) -0.249(3) -0.183(3) -0.056(3) 0.022(3)
+
+
+
+
Experimental Section Chlorophenyl[2-[(R)-l-~dimethylamino~ethyllphenyllbismuthane (la) was obtained in a crystalline form of a 1:l diastereoisomeric mixture according to our reported procedure.l 'H NMR spectra were recorded in CDC13 on a Varian Gemini-200 (200 MHz) spectrometer with tetramethylsilane as an internal standard. X-ray Crystallography of Compound la. A crystal of dimensions 0.230 x 0.380 x 0.350 mm grown from methanol at ambient temperature was used for X-ray crystallography. Crystal Data. Cl&gNBiCl, Mr = 469.77, monoclinic, space group P21, a = 8.672(5) A, b 16.041(7)A, c = 12.058(4)A, p = 102.63(3)", V = 1637(1) A3, 2 = 4, D, = 1.906 g ~ m - ~ , colorless prisms, p(Mo Ka, A = 0.710 69 A) = 108.93 cm-'. Intensity data were collected on a Rigaku AFC5R diffractometer with graphite-monochromated Mo Ka radiation and a 12 kW rotating-anode generator using the w-28 scan technique to a maximum 28 value of 55.0". Scans of (1.47 0.30 tan 0)"were made at a speed of 16.0" min-l (in 0). Of the 4142 reflections which were collected, 3898 were unique (Ri,t = 0.093). Data were corrected for Lorentz, polarization, and absorption effects. Empirical correction for the absorption was made on the basis of azimuthal or q scans6 (transmission factors 0.57-1.00). The structure was solved by the Patterson m e t h ~ d .The ~ non-hydrogen atoms were refined anisotropically. The final cycle of full-matrix least-squares refinement was based on 2174 observed reflections (Z > 3.00dZ)), and 343 variable parameters converged with unweighted and weighted agreement factors of R = 0.041 and R, = 0.036. The residual electron densities in the final difference Fourier map ranged from -1.20 to 1.10 e/A3. The weighting scheme w = l/uz(Fo) was employed. Neutral atom scattering factors were taken from Cromer and Waber.8 Anomalous dispersion effects were included in Fc;9 the values for Af' and Af" were those of Cromer.'O All calculations were performed on a VAXstation 3200 computer using the TEXSAN" crystallographic software package from Molecular Structure Corp. The PLUTO program12 was used t o obtain the drawings in Figures 1 and 2. Selected bond lengths and bond angles are given in Table 1, and atomic coordinates are given in Table 3.
+
Numerals in parentheses are estimated standard deviations. B(eq) = 1.33 [a2Bll b2B22 ~*B33 ab(cos ?)Biz UC(COS P ) B I ~ bc(cos a
both diastereoisomers. Thus, we have finally reached a unique answer to the puzzling phenomenon why the diastereoisomeric ratios of compounds la and lb are so insensitive to the steric bulkiness of another aryl group attached to the chiral bismuth center.
+
a)Bd
the aryl group at the equatorial position (Table 2). This indicates that the major and minor isomers possess the trans and cis geometry, respectively, for the benzylic methyl and equatorial phenyl groups in the fivemembered chelate ring. A high-field shift of the benzylic methyl absorption of the cis isomer may be accounted for by flipping of the puckered chelate ring. The N-methyl groups are also subject to the anisotropic effect of the aryl ring, but the degree is considerably dependent on the geometry of the respective isomers. As has been shown in Figure 2, one of the N-methyl groups in the cis isomer is located above the aromatic ring, which is well reflected by a marked difference between the Av values for the diastereotopic N-methyl protons of both diastereoisomers (Table 2). Such a remarkable anisotropic effect was not observed for the trans isomer. These observations confirm that the preferred conformation of compound la in solution is similar t o that in the solid state, the benzylic methyl group being preferentially fixed at the axial position in
Acknowledgment. The present work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education and Culture of Japan (No. 05236101 and 06740550). T.M. thanks the Japan Society for the Promotion of Science for the Fellowship (No. 1339). Supplementary Material Available: Full details of crystal data, fractional atomic coordinates, bond lengths, bond angles, hydrogen coordinates, and thermal parameters for l a and unit cell and ORTEP diagrams (29 pages). Ordering information is given on any current masthead page. OM940921G (6)North, A. C.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr., Sect. A 1968,24, 351. (7) Structure solution method: Calabrese, J. C. PHASE: Patterson Heavy Atom Solution Extractor. Ph.D. Thesis, University of Wisconsin-Madison, 1972. (8) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV, Table 2. 2 A. (9) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781. (10) Cromer, D. T. International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV,Table 2.3.1. (11)TEXSAN-TEXRAY Structure Analysis Package, - Molecular Structure Corp., The Woodlands, TX, 1985.(12) Motherwell, S.; Clegg, W. PLUTO Program for Plotting Molecular and Crystal Structures; University of Cambridge, Cambridge, England, 1978.
