Synthesis of diastereomeric 1,4-diphosphine ligands bearing imidazolidin-2-one backbone and their application in Rh(I)-catalyzed asymmetric hydrogenation of functionalized olefins

The diastereomeric 1,4-diphosphine ligands, (S,S,S,S)-1a, (R,S,S,R)-1b and (R,S,S,S)-1c, with the imidazolidin-2-one backbone were synthesized, and utilized for an investigation of the effects of backbone chirality on the enantioselectivity in the Rh(I)-catalyzed hydrogenation of various functionalized olefinic substrates. It was found that the catalytic efficiencies are largely dependent on the configurations of the α-carbons to phosphine. Thus, the Rh complex of the pseudo-C₂-symmetrical diphosphine, (R,S,S,S)-1c, showed excellent enantioselectivities (93.0-98.6% ees) in the hydrogenations of a broad spectrum of substrates, and especially in the hydrogenations of methyl α-(N-acetyamino)-β-arylacrylates (95.3-97.0% ees). However, the enantioselectivities obtained with the C ₂-symmetrical (R,S,S,R)-1b were largely dependent on the substrate (19.8-97.3% ees). The Rh complex of ligand 1a having the (S,S,S,S)-configuration showed the lowest catalytic efficiency for all of the substrates examined (0-84.8% ees).