Readily available chiral phosphine–aminophosphine ligands derived from 1-phenylethylamine for Rh-catalyzed enantioselective hydrogenations

A series of new chiral phosphine–aminophosphine ligands have been prepared via a two- or three-step transformation from commercially available and inexpensive (S)-1-phenylethylamine, and successfully used in the rhodium-catalyzed asymmetric hydrogenation of various enamides, β-dehydroamino acid esters, and dimethyl itaconate. The results show that the ligand structure plays an important influence on both the reactivity and enantioselectivity. Ligand 2d bearing a N–H proton and two F-atoms on the 3,5-positions of the phenyl ring of the aminophosphino moiety was most effective for the hydrogenation of enamides and (Z)-β-aryl-β-(acylamino)acrylates, whereas ligand 1b showed the highest enantioselectivities in the hydrogenation of (Z)-β-alkyl-β-(acylamino)acrylates and dimethyl itaconate.