Asymmetric 1,4-Conjugate Addition of Diarylphosphines to α,β,γ,δ-Unsaturated Ketones Catalyzed by Transition-Metal Pincer Complexes

An enantioselective asymmetric 1,4-addition of diarylphosphines to linear alpha,beta,gamma,delta-unsaturated dienones was developed. A series of chiral PCP- and PCN-transition-metal (Pd, Pt and Ni) pincers, themselves prepared catalytically via asymmetric hydrophosphination, were sequentially screened to reveal the roles of backbone architecture and metal ion in catalyst design. The selected ester-functionalized PCP-palladium pincer afforded the chiral 1,4-phosphine adducts in excellent yields with up to >99% ee. The same catalyst when utilized for the hydrophosphination of an alpha,beta,gamma,delta-unsaturated malonate ester also revealed the critical role played by the ester functionality on the ligand backbone in dictating the enantioselectivity of the 1,6-adduct.