Recent Advances in Catalytic Asymmetric Addition to Imines and Related C=N Systems

Abstract

Addition reactions to imines are relatively less developed compared to those of carbonyl compounds due to low electrophilicity and ease of α-deprotonation. With the use of an appropriate activator, which can coordinate to the imine nitrogen atom, the electrophilicity of the imine can be enhanced and a range of nucleophilic addition reactions becomes possible. When the activator is chiral, it will also create chiral environment and will direct the approach of the nucleophile to one face of the imine over the other resulting in enantioselectivity. The potential of catalytic asymmetric addition to imines in organic synthesis is enormous, but is relatively underused. With the development of many highly effective chiral catalysts for imine additions, the situation will soon be changed. The present review will cover selected literature on catalytic asymmetric additions to aldimines, ketimines and related compounds including hydrazones, oximes and nitrones from 1999 to 2004. The reactions of interest include hydrogenation, alkylation, Mannich and related reactions such as aza-Baylis-Hillman and aza-Henry reaction, Strecker reaction, hydrophosphonylation, and 3-6 membered ring forming reactions from imines. Some applications of these methodologies in synthesis of nitrogen-containing biologically active compounds will also be presented. Keywords: nucleophilic addition, chiral auxiliary, tautomerization, hydrogenation, hydride transfer reactions, hydrosilylation, alkyllithium reagents, alkylzinc reagents