Page 445 - Advanced Organic Chemistry Part B - Reactions & Synthesis

418 C24
C23
CHAPTER 5
C25
Reduction of
Carbon-Carbon Multiple C22
Bonds, Carbonyl
Groups, and Other C3 C26
Functional Groups C2
C21
C4
C1
C5
H4
N1
01 C11
B2 C12
H1aa
C6 C16
C13
B1
H1bb H1cc C15
C14
Fig. 5.7. Crystal structure of borane complex of , -
diphenylprolinol oxazaborolidine catalysts. Reproduced from
Tetrahedron Lett., 33, 3429 (1992), by permission of Elsevier.

orientation of the ketone is dictated by the phenyl groups and the relatively rigid
geometry of the ring system. The enantioselectivity in these reductions is proposed
to arise from a chairlike TS in which the governing steric interaction is with the
alkyl substituent on boron. 153 154 There are experimental data indicating that the steric
demand of the boron substituent influences enantioselectivity. 154

Ph Ph
O H
R L O B
C H N +
R B
R S
H H

There have been ab initio studies of the transition structure using several model
catalysts and calculations at the HF/3-21G, HF/6-31G(d), and MP2/6-31G(d) levels. 155
The enantioselectivity is attributed to the preference for an exo rather than an endo
approach of the ketone, as shown in Figure 5.8.
According to B3LYP/6-31G computations of the intermediates and TSs, there are
∗
no large barriers to the reaction and it is strongly exothermic. 156 Measured E values
a
are around 10 kcal/mol. 157 The complexation of borane to the catalyst shifts electron
density from nitrogen to boron and enhances the nucleophilicity of the hydride. The
153
D. K. Jones, D. C. Liotta, I. Shikai, and D. J. Mathre, J. Org. Chem., 58, 799 (1993).
154
T. K. Jones, J. J. Mohan, L. C. Xavier, T. J. Blacklock, D. J. Mathre, P. Sohar, E. T. T. Jones, R. A.
Beaner, F. E. Roberts, and E. J. J. Grabowski, J. Org. Chem., 56, 763 (1991).
155 G. J. Quallich, J. F. Blake, and T. M. Woodall, J. Am. Chem. Soc., 116, 8516 (1994).
156 G. Alagona, C. Ghio, M. Persico, and S. Tomas, J. Am. Chem. Soc., 125, 10027 (2003).
157
H. Jockel, R. Schmidt, H. Jope, and H. G. Schmalz, J. Chem. Soc., Perkin Trans. 2, 69 (2000).