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Effect of the Secondary Reduction on the Enantioselectivity and Function of Additives in the Chiral Oxazaborolidine-Catalyzed Asymmetric Borane Reduction of Ketones

✍ Scribed by Han Liu; Da-Ming Du; Jiaxi Xu

Publisher
John Wiley and Sons
Year
2006
Tongue
German
Weight
79 KB
Volume
89
Category
Article
ISSN
0018-019X

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✦ Synopsis

Abstract

The secondary reduction in the direct and oxazaborolidine‐catalyzed asymmetric borane reduction of ketones was investigated by the use of GC/MS tracing titration and control experiments. The results indicate that the secondary reduction affects the enantioselectivity only in noncoordinated solvents at low temperature and not under the usual catalytic reduction conditions because the intermediate alkoxyborane is unstable and quickly converts to borane and dialkoxyborane. The function of an alcohol additive in the asymmetric borane reduction of ketones is to consume excess borane in the reduction system thus inhibiting noncatalytic reduction, which leads to increased enantioselectivity in the catalytic reduction.

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