Page 216 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

196

CHAPTER 2
Stereochemistry,
Conformation,
and Stereoselectivity

Fig. 2.20. Optimized (HF/3-21G) structures of the exo and endo transition states for reduction
of t-butyl methyl ketone by a model catalyst. The exo structure is favored by 2.1 kcal, in accord
with an experimental e.e of 88%. Reproduced from J. Am. Chem. Soc., 116, 8516 (1994), by
permission of the American Chemical Society.

been suggested that the enantioselectivity in these reductions arises from a chair-like
TS in which the governing steric interaction is the one with the alkyl substituent on
boron. 154

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

There also 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 2.20.

CH 3 Ph Ph Ph

HN O HN O
B B
CH 3 CH 3

2.5.3. Enantioselective Epoxidation of Allylic Alcohols

Certain transition metal complexes catalyze oxidation of allylic alcohols to the
corresponding epoxides. The most useful procedures involve t-butyl hydroperoxide as

154 D. K. Jones, D. C. Liotta, I. Shinkai, and D. J. Mathre, J. Org. Chem., 58, 799 (1993); L. P. Linney,
C. R. Self, and T. H. Williams, J. Chem. Soc., Chem. Commun. 1651 (1994).
155
G. J. Quallich, J. F. Blake, and T. M. Woodall, J. Am. Chem. Soc., 116, 8516 (1994).