Synthesis Of Novel Atropisomeric Bisphosphine Ligands And Their Applications In Asymmetric Catalysis

The design and development of effective chiral ligands have played a significant role in advancement of asymmetric catalysis, and have attracted a great deal of attention from both academia and industry. Axial chirality such as BINAP have been explored as effective templates for transition metal-catalyzed asymmetric reactions. Axial chirality results from restricted rotation around biaryl axis produced by ortho substituents. Generally, enantiomerically stable biaryls require at least three ortho-substituents to prevent the racemization. Therefore, for all of the traditional design of atropisomeric biphenyl ligands, there is at least one bulky group at 6- or 6’-position on biphenyls for sake of stable axial chirality. In order to develop more effective biphenyl ligands, a novel atropisomeric framework in which the biphenyls have only two coordinating groups next to the axis was reported. The axial chirality of biphenyls is expected to be retained by macro-ring strain produced by 5, 5’-linkage of biphenyls. Three different approaches have been developed.First, based on this design concept, a new family of atropisomeric ligands with a bridge across the 5, 5’-position of biphenyl have been developed. The absolute configuration of ligands was determined by CD spectrum. The axial chirality of this type of ligands can be retained by macro-ring strain produced by 5, 5’-linkage of biphenyls even without 6, 6’-substituents on biphenyls. These ligands showed good catalytic activities and enantioselectivities for Rh(I)-catalyzed asymmetric hydrogenation of (Z)-α-acetamido cinnamic acid (86% ee), which is much higher than that obtained with NAPHOS ligand (54% ee).Second, a new family of atropisomeric aminophosphine ligands with a bridge across the 5, 5’-position of biphenyl have been developed. The absolute configuration of ligands was determined by CD spectrum. The axial chirality of this type of ligands can be retained by macro-ring strain produced by 5, 5’-linkage of biphenyls even without 6, 6’-substituents on biphenyls. The ligand showed high catalytic activities and enantioselectivities (up to 95.3% ee and quantitative yields) for Rh(I)-catalyzed asymmetric hydrogenation of a variety of methyl (Z)-2-acetamido-3-arylacrylates, which is much higher than that of obtained with BDPAB (90% ee) and 41 (81% ee) as a ligand.Third, the new family of atropisomeric ligands have been used in Rh(I)-catalyzed asymmetric hydrogenation of enamides and Pd(II)-catalyzed asymmetric allylic alkylation, respectively. For the two types of the reactions, the ligands showed high catalytic activities and moderate enantioselectivities.