Catalytic asymmetric aldol reactions. Use of a chiral (acyloxy)borane complex as a versatile Lewis-acid catalyst

Abstract

In the presence of 20 mol% of a chiral (acyloxy)borane (CAB) complex prepared from (2R,3R)-2-O(2,6-diisopropoxybenzoyl)tartaric acid and borane-tetrahydrofuran, various allyltrimethylsilanes react with achiral aldehydes to afford the corresponding homoallylic alcohols in good yields with high diastereo- and enantioselectivities. Furthermore, the reactivity of allylation can be improved without reducing the enantioselectivity by using 10-20 mol% of the CAB complex prepared from 3,5-bis(trifluoromethyl)phenylboronic acid and chiral tartaric acid derivative. The observed selectivities and re-face attack of nucleophiles on the carbonyl carbons of aldehydes imply that the extended transition-state model is applicable