and DIFLUORPHOS ® vs BINAPs in Rh(I)-Catalyzed Asymmetric

Abstract: The electronic and steric effects of chiral biaryl diphosphine ligands on the Rh(I) catalyzed asymmetric Pauson-Khand type reaction were examined. We demonstrated that enantioselectivity and reaction yield were influenced by the electronic density on phosphorus, the dihedral angle of ligands and the electronic density of the alkyne substrate. Ligands bearing a narrower dihedral angle than Binap, such as Synphos (L4) and Difluorphos (L5), were found to increase substantially the enantioselectivity of the reaction, compared to Binap-type ligands. Ligands having deshielded phosphine, such as p-CF3-Binap (L3) and Difluorphos (L5) provided better enantioselectivity than Binap, with reduced formation of side products, especially with electron-poor alkyne substrates. 2 Key Words; Pauson-Khand reaction, enantioselective, atropisomeric ligands, electronic effects