An enantiomerically pure tetracoordinate boron compound: Stereochemistry of substitution reactions at the chirogenic boron atom [22]

Full text of DOI:10.1021/ja0011764

J. Am. Chem. Soc. 2000, 122, 6329-6330
An Enantiomerically Pure Tetracoordinate Boron
6329
Compound: Stereochemistry of Substitution
Reactions at the Chirogenic Boron Atom
Tsuneo Imamoto* and Hideaki Morishita
Department of Chemistry, Faculty of Science
Chiba UniVersity, Yayoi-cho, Inage-ku
Chiba 263-8522, Japan
ReceiVed April 4, 2000
In contrast to extensive and exhaustive investigations on the
substitution reactions at a chirogenic tetracoordinate carbon atom,
less attention has been paid to the substitution reactions at an
isoelectronic chirogenic boron atom.1-4 Recently, the research
group of Mioskowski has synthesized diastereomerically pure
tetracoordinate boron compounds containing a chiral center at
the boron atom, and they provided direct evidence of an SN2 at
the boron atom.5 However, to our best knowledge, synthesis and
substitution reactions of enantiomerically pure tetracoordinate
boron compounds have not yet been reported. Here we report
the first synthesis of a such compound and the stereochemical
results of substitution reactions occurring at the boron atom.
As a model tetracoordinate boron compound bearing a leaving
group, we chose a phosphine-borane derivative, because com-
pounds of this kind are generally air- and moisture-stable and
can be easily handled without special caution.6 After various
synthetic trials, we succeeded in the synthesis of an enantiomeri-
cally pure B-chirogenic phosphine-borane bearing a bromine
atom at the boron atom, starting from tricyclohexylphosphine-
monoiodoborane (1). The synthetic route is shown in Scheme 1.
Compound 1 was reduced by 2 molar equiv of lithium 4,4′-
di-tert-butylbiphenylide (LDBB), followed by the reaction with
dimethyl carbonate to give methoxycarbonyl derivative 2 in good
yield. Subsequent reaction of 2 with bromine in methanol provided
compound 3. Hydrolysis of the methyl ester using aqueous HBr
afforded carboxylic derivative 4.7 This compound was mixed with
Figure 1. Molecular structure of (S)-3 showing only selected atoms.
Selected bond distances (Å) and angles (deg): B(1)-P(1) 1.957(7), B(1)-
C(1) 1.637(10), B(1)-Br(1) 1.990(8), C(1)-O(1) 1.188(9); P(1)-B(1)-
C(1) 112.7(4), P(1)-B(1)-Br(1) 110.3(4), Br(1)-B(1)-C(1) 108.4(4),
B(1)-C(1)-O(1) 130.1(4), B(1)-C(1)-O(2) 109.6(6), O(1)-C(1)-O(2)
120.2(7).
Scheme 1a
a Conditions: (a) (i) LDBB (2.5 equiv)-TMEDA, THF, -78 °C. (ii)
(MeO)2CO, 66%. (b) Br2, MeOH, 0 °C to room temperature, 88%. (c)
aq 48% HBr, THF, room temperature, 12 h, 57% after recrystallization
from AcOEt. (d) (S)-(-)-1-Phenylethanol, 120 °C, 10 min, fractional
recrystallization from hexane, (SB,S)-5, 24%; (RB,S)-5, 24%. (e) H2SO4
(cat.), MeOH-THF, room temperature, 4 h, 96-98%.
(1) Synthesis and substitution reactions of (()-amine-borane complexes
bearing a chirogenic boron atom have been reported. (a) Mills, W. J.; Todd,
L. J.; Huffmann, J. C. J. Chem. Soc., Chem. Commun. 1989, 900-902. (b)
Mills, W. J.; Sutton, C. H.; Libby, E.; Todd, L. J. Inorg. Chem. 1990, 29,
302-308. (c) Miller, N. E. Inorg. Chem. 1991, 30, 2228-2231. (d) Gyo¨ri,
B.; Kova´cs, Z.; Emri, J.; Berente, Z. J. Organomet. Chem. 1994, 484, 225-
231. (e) Gyo¨ri, B.; Kova´cs, Z.; Emri, J.; La´za´r, I. Inorg. Chim. Acta 1994,
218, 21-26. (f) Sutton, C. H.; Baize, M. W.; Todd, L. J. Inorg. Chem. 1994,
33, 4221-4225.
(S)-1-phenylethanol (97% ee, 2.5 molar equiv) and was heated
at 120 °C for 10 min to give a mixture of diastereomers,8 which
were separated by fractional recrystallization to diastereomerically
pure (SB,S)-5 (mp 135-136 °C dec, [R]D -29.2° (c 0.50, THF))
and (RB,S)-5 (mp 133-134 °C dec, [R]D -39.3° (c 0.50, THF)).
The absolute configuration of the boron atom of (SB,S)-5 was
determined to be S by single-crystal X-ray analysis. Therefore,
another diastereomer (RB,S)-5 should possess R configuration of
the boron atom. Compound (SB,S)-5 was converted to enantio-
merically pure, tetracoordinate boron compound (S)-3 in almost
quantitative yield by dissolving it in MeOH-THF containing a
catalytic amount of H2SO4. In a similar manner, another enanti-
omer (R)-3 was obtained in enantiomerically pure form.
The structure of (S)-3 was confirmed by X-ray crystal-
lography. The ORTEP drawing shown in Figure 1 apparently
indicates a tetrahedral, chirogenic boranate structure.9 The absolute
configuration of the boron atom determined by the Flack
parameter method is S, the same as the configuration of compound
(SB,S)-5.
(2) Stereochemical studies on B-chirogenic, tetracoordinate boron com-
pounds: (a) Vedejs, E.; Fields, S. C.; Schrimpf, M. R. J. Am. Chem. Soc.
1993, 115, 11612-11613. (b) Vedejs, E.; Fields, S. C.; Lin, S.; Schrimpf, M.
R. J. Org. Chem. 1995, 60, 3028-3034. (c) Vedejs, E.; Fields, S. C.; Hayashi,
R.; Hitschcock, S. R.; Powell, D. R.; Schrimpf, M. R. J. Am. Chem. Soc.
1999, 121, 2460-2470. (d) Brown, H. C.; Ramachandran, P. V. J. Org. Chem.
1989, 54, 4504-4511.
(3) Studies on the SN2 reaction occurring at tetracoordinate boron: (a)
Toyota, S.; Futawaka, T.; Ikeda, H.; Ohki, M. J. Chem. Soc., Chem. Commun.
1995, 2499. (b) Toyota, S.; Futawaka, T.; Asakura, M.; Ikeda, H.; Ohki, M.
Organometallics 1998, 17, 4155-4163.
(4) Hawthorne et al. studied the substitution reactions at the tetracoordinate
boron atom using achiral amine-borane complexes and presented good kinetic
evidence for the SN2 and SN1 nucleophilic displacement mechanism at
tetrahedral boron. (a) Hawthorne, M. F.; Budde, W. L. J. Am. Chem. Soc.
1964, 86, 5337. (b) Hawthorne, M. F.; Budde, W. L.; Walmsley, D. J. Am.
Chem. Soc. 1964, 86, 5337-5338.
(5) Vedrenne, P.; Le Guen, V.; Toupet, L.; Le Gall, T.; Mioskowski, C. J.
Am. Chem. Soc. 1999, 121, 1090-1091.
(6) Representative reviews dealing with phosphine-boranes: (a) Carboni,
B.; Monnier, L. Tetrahedron 1999, 55, 1197-1248. (b) Ohff, M.; Holz, J.;
Quirmbach, M.; Bo¨rner, A. Synthesis 1998, 1391-1415. (c) Imamoto, T. Pure
Appl. Chem. 1993, 65, 655-660. (d) Imamoto, T. J. Synth. Org. Chem. Jpn.
1987, 45, 592-602. (e) Power, P. P. Angew. Chem., Int. Ed. Engl. 1990, 29,
449-460. (f) Arbusov, B.; Nikonov, G. N. ReV. Heteroatom. Chem. 1990, 3,
1-23. (g) Schmidbaur, H. J. Organomet. Chem. 1980, 200, 287-306.
(7) Formation of tricyclohexylphosphine-chloro(carboxyl)borane occurred
when concentrated HCl was used instead of HBr.
(8) It is noted that this esterification occurred smoothly even in the absence
of catalysts such as sulfuric acid or condensation reagents.
10.1021/ja0011764 CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/17/2000

6330 J. Am. Chem. Soc., Vol. 122, No. 26, 2000
Communications to the Editor
To reveal the stereochemical aspect of the nucleophilic
substitution reactions at the chirogenic boron atom, compound
(S)-3 was allowed to react with lithium cyanide or lithium
phenylthiolate. The reaction with LiCN proceeded smoothly in
DMF-THF (2:3) at 50 °C for 4 h to give a substitution product
6 in 87% yield (eq 1). The enantiomeric excess of the product
was determined to be 96% ee by HPLC analysis using a chiral
column. The absolute configuration of this compound was
determined to be R by the Flack parameter method. These results
clearly demonstrate that the nucleophilic substitution reaction took
place with inversion of configuration at the boron atom in 96%
stereospecificity. The reaction of (S)-3 with lithium phenylthiolate
also afforded in 83% yield the corresponding substitution product
7, whose ee was determined to be 99% ee (eq 2). Unfortunately,
the product was unstable and did not give rise to good crystals
for X-ray crystallography. Although the absolute configuration
was not determined, we envisage that this substitution reaction
also takes place via an SN2 manner with inversion of configuration
in exceedingly high stereospecificity.
(eq 3). HPLC analyses of these products using chiral columns
demonstrated that they all were racemates.
These reactions are considered to involve a boranyl radical and
a boron dianion species which is in isoelectronic relationship with
tricoordinate carbanions. The formation of completely racemized
products is due to the rapid racemization of the intermediate
boranyl radical or the boron dianionic species.
We considered that compound (S)-3 would be a most suitable
substrate for the stereochemical study on electrophilic substitution
reactions at a boron atom,10 and it was reacted with LDBB,
followed by reaction with iodomethane, 1-bromo-2-methylpro-
pane, or diphenyl disulfide at -78 °C through to room temper-
ature. From the reaction mixtures, B-substitution products, 8a,
8b, and 8c were isolated in 62, 55, and 54% yields, respectively
Acknowledgment. This work was supported by the Research for the
Future program, the Japan Society for the Promotion of Science. We
thank Drs. M. Nishiura and H. Tsuruta for X-ray analyses.
(9) Crystal data for (S)-3: C20H37BBrO2P; space group P21 (No. 4); a )
7.666(2) Å, b ) 17.200(2) Å, c ) 8.955(2) Å, â ) 109.84(2)°, V ) 1110.8-
(4) Å, Z ) 2, Dcalcd ) 1.289 g/cm3, µ(Cu KR) ) 32.71 cm-1, λ (Cu KR) )
1.54178 Å; temperature of data collection 296 K.; 4517 reflections measured,
4217 observed (I > 1.50σ(I)); 227 variables; R ) 0.096, Rw ) 0.129, GOF )
1.87, flack parameter ) 0.01(4).
Supporting Information Available: Experimental details and full
characterization data for all new compounds; X-ray structural information
on compounds (SB,S)-5, (S)-3, and 6 (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
(10) Imamoto, T.; Hikosaka, T. J. Org. Chem. 1994, 59, 6753-6759.
JA0011764