Page 437 - Advanced Organic Chemistry Part B - Reactions & Synthesis

410 Similarly, NaBH -CeCl reverses the stereochemistry relative to NaBH in the bicyclic
4
4
3
ketone 12. 122
CHAPTER 5
2
2
Reduction of O CH SO 2 Ph OH CH SO Ph
2
Carbon-Carbon Multiple
Bonds, Carbonyl
Groups, and Other
Functional Groups
H H
12
α:β
NaBH 4 20:80
NaBH , CeCl 3 95:5
4
Thus, NaBH -CeCl tends to give the more stable alcohol, but the origin of this
4 3
stereoselectivity does not seem to have been established. It is thought that these
reductions proceed through alkoxyborohydrides. 123 It is likely that equilibration occurs
by reversible hydride transfer.
5.3.2.2. Acyclic Ketones. The stereochemistry of the reduction of acyclic aldehydes
and ketones is a function of the substitution on the adjacent carbon atom and can be
63
predicted on the basis of the Felkin conformational model of the TS, which is based
on a combination of steric and stereoelectronic effects.
preferred direction
H – of approach H
M S M S
O R
HO R
L L
S, M, L = relative size of substituents

From a purely steric standpoint, minimal interaction with the groups L and M by
approaching from the direction of the smallest substituent is favorable. The stereo-
electronic effect involves the interaction between the approaching hydride ion and
the LUMO of the carbonyl group. This orbital, which accepts the electrons of the
incoming nucleophile, is stabilized when the group L is perpendicular to the plane of
the carbonyl group. 124 This conformation permits a favorable interaction between the
∗
LUMO and the antibonding orbital associated with the C−L bond.
H –
S C O
M
C
L

In the case of -substituted phenyl ketones, the order of stereoselectivity is C≡CH >
CH=CH > CH CH . 125 These results indicate a stereoelectronic as well as a steric
2 2 3
122
M. Leclaire and P. Jean, Bull. Soc. Chim. Fr., 133, 801 (1996).
123 A. C. Gemal and J.-L. Luche, J. Am. Chem. Soc., 103, 5454 (1981).
124 N. T. Ahn, Top. Current Chem., 88, 145 (1980).
125
M. Fujita, S. Akimoto, and K. Ogura, Tetrahedron Lett., 34, 5139 (1993).