Transfer Hydrogenation of Carbonyl Compounds Catalyzed by a Ruthenium-Acetamido Complex: Evidence for a Stepwise Hydrogen Transfer Mechanism Chae S. Yi* and Zhengjie He Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881 Ilia A. Guzei Department of Chemistry, University of WisconsinsMadison, Madison, Wisconsin 53706-1396 Received June 25, 2001 Summary: The ruthenium-acetamido complex, pre- pared from the reaction of [(PCy 3 ) 2 (CO)(CH 3 CN) 2 RuH] + - BF 4 - with KOH in 2-propanol, was found to be an effective catalyst for the transfer hydrogenation of car- bonyl compounds and imines. Observation of both the inverse deuterium isotope effect and the competitive inhibition by added phosphine provided strong evidence for a stepwise mechanism of proton and hydride transfer via a coordinatively unsaturated ruthenium-amido species. The transition-metal-catalyzed transfer hydrogena- tion of ketones and imines has been shown to be an ef- fective method for forming chiral alcohols and amines. 1-3 For example, Noyori achieved highly enantioselective transfer hydrogenation reactions of ketones and imines by using chiral Ru II -TsDPEN catalysts, and recently proposed a concerted mechanism of hydrogen transfer involving metal-to-ligand “bifunctional” hydrogen acti- vation on the basis of both experimental and computa- tional results. 2 Several other Ru and Rh catalysts with chiral nitrogen ligands have also been shown to give high enantioselectivity toward the hydrogenation of ketones and imines. 3 The “N-H effect” of the amide ligand has been suggested as an important factor for promoting the catalyst activity for these systems. 4 On the basis of a detailed kinetic and mechanistic study of benzaldehyde reduction mediated by Shvo’s bimetallic HOC 5 Ph 4 -Ru II -hydride complex, Casey recently pro- posed a concerted transfer mechanism of proton and hydride via a monomeric ruthenium-hydride complex. 5 As part of ongoing efforts to study ruthenium-catalyzed hydrogenation reactions, 6 we have begun to explore the catalytic activity of coordinatively unsaturated ruthe- nium-amido complexes. Here we report the synthesis of the ruthenium-acetamido complex [(PCy 3 ) 2 (CO)(CH 3 - CONH)(i-PrOH)RuH] (1) and its catalytic activity to- ward the transfer hydrogenation of carbonyl compounds and imines. The acetamido complex 1 was prepared from the nucleophilic addition of OH - to the cationic complex [(PCy 3 ) 2 (CO)(CH 3 CN) 2 RuH] + BF 4 - (2) (eq 1). 7 Thus, the treatment of 2 (200 mg, 0.23 mmol) with KOH (26 mg, 2.0 equiv) in 2-propanol at room temperature cleanly formed the ruthenium-acetamido complex 1, which was isolated in 75% yield as a colorless crystalline solid after recrystallization in 2-propanol at 0 °C. The initially colorless crystals of 1 slowly turned to a pale yellow powder, apparently due to desolvation at room temper- ature, but the desolvated complex 1 was found to exhibit the same spectral features as the solvent-coordinated one. 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