Marked deuterium isotope effects on the enantioselectivity in rhodium-catalyzed asymmetric hydrogenation of enamides

Deuterium isotope effects on the enantioselectivity in the rhodium-catalyzed asymmetric hydrogenation of enamides and related substrates have been studied. Distinct deuterium isotope effects were observed in the hydrogenation of aryl-substituted enamides having an ortho substituent that is capable of forming a hydrogen bond. The observed isotope effects are interpreted in terms of the competitive reactions of two dihydride intermediates and dideuteride intermediates that exist in equilibrium in the catalytic cycle.