Searching for Practically Useful P-Chirogenic Phosphine Ligands

In this account, the design, synthesis, and application of P-chirogenic phosphine ligands that have been mainly carried out in our laboratory over the last three decades are described. Various enantiopure P-chirogenic phosphine ligands have been efficiently prepared by using phosphine boranes as intermediates. Conformationally rigid and electron-rich P-chirogenic phosphine ligands possessing C₂ symmetry as well as a bulky alkyl group and a small group at the phosphorus atoms exhibit excellent enantioselectivities and catalytic efficiency in a variety of transition-metal-catalyzed asymmetric reactions. Enantiopure 2,3-bis(tert-butylmethylphosphino)quinoxaline (QuinoxP*) is an air-stable crystalline solid that shows superior enantioinduction ability in catalytic asymmetric syntheses. Mechanistic studies of Rh-catalyzed asymmetric hydrogenation using structurally simple P-chirogenic phosphine ligands, such as tBu-BisP*, are briefly described.