Asymmetric aldol reaction of 2-cyanopropionates catalyzed by a trans-chelating chiral diphosphine-rhodium(I) complex: Highly enantioselective construction of quaternary chiral carbon centers at α-positions of nitriles

The aldol reaction of 2-cyanopropionates with aldehydes proceeded under neutral conditions in the presence of a catalytic amount of the rhodium complex generated in situ from Rh(acac)(CO)₂ and triphenylphosphine, to give the corresponding β-hydroxy-α-cyanocarboxylates bearing a quaternary chiral carbon center at the α-position of the cyano group. A high degree of asymmetric induction for the aldol reaction was achieved by use of trans-chelating chiral diphosphine ligands, (R,R)-2,2″-bis[(S)-1-(diarylphosphino)ethyl]-1,1″-biferrocenes (TRAPs). The asymmetric aldol reactions gave optically active β-hydroxy-α-cyanocarboxylates with up to 94% ee.