Ligand Electronic Effects in Asymmetric Catalysis: Enhanced Enantioselectivity in the Asymmetric Hydrocyanation of Vinylarenes

Abstract

The enantioselectivity of the nickel-catalyzed, asymmetric hydrocyanation of vinylarenes using glucose- derived, chiral phosphinite ligands, L, increases dramatically when the ligands contain electron-withdrawing P-aryl substituents. The substrate and solvent also strongly influence the enantioselectivity, with the highest ee's (85-91% for 6-methoxy-2-vinylnaphthalene (MVN)) obtained for the hydrocyanation of electron-rich vinylarenes in a nonpolar solvent such as hexane. Mechanistic studies suggest the catalytic cycle consists of an initial HCN oxidative addition or vinylarene coordination to NiL, followed by insertion to form an (q3-benzy1)nickel cyanide complex, and irreversible reductive elimination of the nitrile. A kinetic analysis of the NiL.(COD) (L., P-aryl = 3,5-(CF3)2C6H3) catalyzed hydrocyanation of MVN indicates that as the HCN concentration is increased the catalyst resting state shifts from NiL,(COD) to a complex containing both MVN and HCN, presumably the (q3-benzy1)nickel cyanide intermediate NiL,(q3-CH3CHCloH60CH3)CN. A 31P NMR analysis of the intermediate NiL(MVN) shows little ground state differentiation of the MVN enantiofaces and suggests that the enantioselectivity is determined later in the mechanism. Deuterium labeling studies suggest that electron-withdrawing P-aryl substituents increase the rate of reductive elimination of the product nitrile from the (q3-benzy1)nickel cyanide intermediate and, on this basis, a rationale for the ligand electronic effect is proposed.