Chiral Metal Template Induced Asymmetric Synthesis of a Mixed Phosphine−Phosphine Oxide Ligand

Abstract

The asymmetric hydrophosphination reaction of 1,1-bis(diphenylphosphino)ethene and diphenylphosphine promoted by a chiral organopalladium(II) complex derived from ( S )- N, N -dimethyl-1-(1-naphthyl)ethylamine proceeded stereoselectively to generate an equilibrium mixture of four diastereomeric triphosphine palladium(II) template products in a ratio of 17:5:3:2. Alternatively, the direct coordination of 1,1,2-tris(diphenylphosphino)ethane to the chiral organopalladium template generated the same equilibrium mixture of diastereomeric products with the same stereoselectivity. Subsequent asymmetric oxidation of the diastereomeric template products with hydrogen peroxide proceeded stereoselectively to generate four diastereomeric monooxidation products in the ratio of 14:3:3:1. The effects of solvent, temperature, and alternative oxidizing agents were also studied. The naphthylamine auxiliary was removed chemoselectively from the template products by treatment with concentrated hydrochloric acid to form an enantiomerically enriched mixture of the corresponding dichloro complexes, which upon subsequent repeated recrystallization gave the predominant enantiomeric complex in its optically pure form. Further ligand displacement of the enantiomerically pure dichloro complex with aqueous cyanide liberated the free mixed phosphine−phosphine oxide ligand in quantitative yield.