Rhodium-catalyzed asymmetric hydrogenation of unprotected NH imines assisted by a thiourea

Abstract

Asymmetric hydrogenation of unprotected NH imines catalyzed by rhodium/bis(phosphine)-thiourea provided chiral amines with up to 97% yield and 95% ee. (1)H NMR studies, coupled with control experiments, implied that catalytic chloride-bound intermediates were involved in the mechanism through a dual hydrogen-bonding interaction. Deuteration experiments proved that the hydrogenation proceeded through a pathway consistent with an imine.

Keywords: asymmetric catalysis; hydrogenation; ion pairs; reaction mechanisms; rhodium.