Asymmetric Synthesis of Chiral Spiroketal Bisphosphine Ligands and Their Application in Enantioselective Olefin Hydrogenation

A series of chiral spiroketal bisphosphine ligands containing 1,1'-spirobi(3 H,3' H)isobenzofuran backbones was accessed through asymmetric synthesis and subsequently tested in enantioselective Rh-catalyzed hydrogenation of α-dehydroamino acid esters. The ligand providing the highest enantioselectivity (up to 99.5%) was obtained in seven steps in an overall 38% yield. The synthesis could be performed on a gram scale, and no kinetic resolution of enantiomers is required. Overall, the developed ligand provides an easily accessible alternative to SDP ligands as well as other chiral bisphosphine ligands.