Enantioselective Hydrogenation of N-H Imines

Significance

Chiral amines are common structural motifs in a large number of commercially important compounds. Catalytic hydrogenation of imines represents an important synthetic tool for introduction of the amine moiety. Despite a number of successful examples of asymmetric hydrogenation, many systems leave quite a bit of room for improvement of enantiocontrol. The necessity of using protective groups in many cases decreases the synthetic value of the corresponding methods. This paper provides a fundamentally important synthetic approach to chiral amines via hydrogenation of unprotected ketoimines, which can be easily prepared on multigram scale in the form of hydrochlorides. Surprisingly, these compounds received no attention so far as substrates for asymmetric hydrogenation. High yields and good to excellent enantioselectivities were obtained for a number of variously substituted ketimines (R^¹ = Ar, R^² = Me, Et, n-Bu, t-Bu, Ph).