Exploring the Alcohol Stability of Bis(phosphine) Cobalt Dialkyl Precatalysts in Asymmetric Alkene Hydrogenation

Cobalt complexes bearing enantiopure, bidentate bis(phosphine) ligands exhibit extraordinary activity and stereoselectivity for the hydrogenation of enamides. Optimal performance was observed in polar protic solvents such as methanol, an industrially preferred green solvent but a medium that is often a poison for reduced Earth abundant metals. The interaction of the low-spin cobalt(II) dialkyl complex, (R,R)-(Pr-i-DuPhos)Co(CH2SiMe3)(2), with alcohols including 4-methoxyphenol, pinacol, and CH3OH was studied. With the alcohols lacking beta-hydrogens, cobalt bis(alkoxide) complexes were isolated and structurally characterized. With methanol, protonolysis of the alkyl ligands was again observed followed by dehydrogenation of the alcohol and [(R,R)-(Pr-i-DuPhos)Co](2)(mu-CO)(2) was isolated. Both solid-state and solution EXAFS studies were conducted to establish the spectroscopic signatures of bis(phosphine) cobalt(II) and cobalt(0) complexes relevant to catalytic hydrogenation and also to probe the role of phosphine dissociation in methanol.