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Lewis bases as catalysts in the reduction of imines and ketones with silanes (n → σ*)
chapterposted on 20.09.2016 by Pavel Kocovsky, Andrei Malkov
Division of a book, which in a scholarly context usually treats a part of a larger subject in a stand-alone manner.
Silanes are widely recognized as efficient reagent for reduction of the carbonyl and imine functionality. In the case of alkyl- and aryl-silanes, the reaction requires catalysis by transition metal complexes or Lewis acids, which activate the electrophilic substrate. However, the enantioselectivity dropped dramatically in the case of the pyridyl derivative, presumably owing to the coordination of the pyridine nitrogen of the achiral substrate to Cl3SH, which thus competes with the chiral catalyst. Significantly, 105 proved to perform equally well in the reduction of both aromatic ketimines 1 and ketones 2, which place this catalyst among the rare systems that are applicable to both imines and ketones. The ready formation of esters of p-amino acids by reduction of the corresponding imines/enam-ines, which in turn can be prepared from the readily available p-keto esters, allowed an expedient synthesis of SCH48461, a potent, orally active inhibitor of cholesterol absorption.