Mechanism of Asymmetric Hydrogenation of Ketones

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Chiral diphosphine/1,2-diamine Ru(II) complexes catalyze the rapid, productive, and enantioselective hydrogenation of simple ketones. The reaction proceeds smoothly even at an atmospheric pressure of H_2 at room temperature and is further accelerated by addition of an alkaline base or a strong organic base. Most importantly, the hydrogenation rate is initially increased to a great extent with an increase in base molarity, but subsequently decreases. Without a base, the rate is independent on H_2 pressure in the range of 1-16 atm, while in the presence of a base, the reaction is accelerated with increasing H_2 pressure. The extent of enantioselection is unaffected by hydrogen pressure, the presence or absence of base, the kind of base and coexisting metallic or organic cations, the nature of the solvent, or the substrate concentrations. The reaction with H_2/(CH_3)_2CHOH proceeds 50-times faster than that with D_2/(CD_3)_2CDOD in the absence of base, but the rate differs only by a facto … More r of 2 in the presence of KO-t-C_4H_9. These findings indicate that dual mechanisms are in operation, both of which are dependent on reaction conditions and involve heterolytic cleavage of H_2 to form a common reactive intermediate. The key [RuH(diphosphine)(diamine)]^+ and its solvate complex have been detected by ESI-TOFMS and NMR spectroscopy. The hydrogenation of ketones is proposed to occur via a nonclassical metal-ligand bifunctional mechanism involving a chiral RuH_2(diphosphine)(diamine), where a hydride on Ru and a proton of the NH_2 ligand are simultaneously transferred to the C=O function via a six-membered pericyclic transition state. The NH_2 unit in the diamine ligand plays a pivotal role in the catalysis. The reaction occurs in the outer coordination sphere of the 18e RuH_2 complex without C=O/metal interaction. The enantiofaces of prochiral aromatic ketones are kinetically differentiated on the molecular surface of the coordinatively saturated chiral RuH_2 intermediate rather than in a coordinatively unsaturated Ru template. Less