Page 215 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Table 2.6. Enantioselective Reduction of Ketones 195
Reagent Ketone % e.e. Configuration SECTION 2.5
Alpine-Borane ©a 3-Methyl-2-butanone 62 S Enantioselective
NB-Enantride ©b 2-Octanone 79 S Reactions
Eapine-Hydride c 2-Octanone 78 S
Ipc 2 BCl d 2-Acetylnaphthalene 94 S
Ipc t-Bu BCl e Acetophenone 96 R
Ipc 2 BCl f 2,2-Dimethylcyclohexanone 91 S
Eap 2 BCl g 3-Methyl-2-butanone 95 R
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derived from the amino acid proline. 150 The enantiomer is also available. An adduct
of borane and D is the active reductant. This adduct can be prepared, stored, and used
as a stoichiometric reagent, if so desired. 151

Ph
Ph
+ Ph
N Ph + BH 3 H B – N O
3
B O B
H C D H 3 C
3

A catalytic amount (5–20 mol %) of this reagent, along with additional BH as the
3
reductant, can reduce ketones such as acetophenone and pinacolone in > 95% e.e.
There are experimental data indicating that the steric demand of the boron substituent
influences enantioselectivity. 152 The enantioselectivity and reactivity of these catalysts
can be modified by changes in substituent groups to optimize selectivity toward a
particular ketone. 153
Computational studies have explored the mechanism and origin of the enantiose-
lectivity of these reactions. Based on semiempirical MO calculations (MNDO), it has

150
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153
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