Kinetic Hydrogen/Deuterium Effects in the Direct Hydrogenation of Ketones Catalyzed by a Well-Defined Ruthenium Diphosphine Diamine Complex
2009-08-12T00:00:00Z (GMT) by
The trans-dihydride complex trans-RuH2(NH2CMe2CMe2NH2)((R)-binap) (1) is an active catalyst for the homogeneous hydrogenation of ketones in benzene under pressure of H2 gas. The mechanism of the catalysis is proposed to occur through a rapid transfer of a hydride from the ruthenium and a proton from the amine on 1 to the carbonyl of the ketone to give the product alcohol and a hydrido-amido intermediate RuH(NHCMe2CMe2NH2)((R)-binap) (2). The dihydride is then regenerated by the turnover-limiting heterolytic splitting of dihydrogen by complex 2. In this work the kinetic isotope effect (KIE) is measured to be 2.0 (±0.1) for the reduction of acetophenone to 1-phenylethanol catalyzed by 1 using 8.0 atm of H2 versus D2 gas. DFT calculations predict a KIE of 2.1 for the described mechanism using the simplified catalyst RuH(NHCH2CH2NH2)(PH3)2 with H2 or the catalyst RuD(NDCH2CH2ND2)(PH3)2 with D2. This supports the observation that deuterium scrambles into the catalyst when a pressure of D2 is used. We discuss the significance of these results relative to the KIE of 2 that was reported by Sandoval et al. (J. Am. Chem. Soc. 2003 125, 13490) for the hydrogenation/deuteriation of acetophenone catalyzed by trans-RuH(η1-BH4)((S)-tolbinap)((S,S)-dpen) in basic isopropanol/isopropanol-d8.