Remarkable co-catalyst effects on the enantioselective hydrogenation of unfunctionalised enamines: both enantiomers of product from the same enantiomer of catalyst

During studies on the enantioselective hydrogenation of unfunctionalised enamines, a very surprising switch in enantiopreference was observed; [((R,R)-Et-DUPHOS)-Rh(COD)]BF4 hydrogenates an enamine to give (R)-amine with up to 73% ee, but when iodine is added as a co-catalyst, the (S)-amine is formed with up to 61% ee. Mechanistic studies implicate a protonationiminium ion reduction pathway.