Mechanisms of Platinum Group Metal Catalysis Investigated by Experimental and Theoretical Methods.

摘要

The results of kinetic isotope determination and computational studies on Noyori-type catalytic systems for the hydrogenation of ketones are presented. The catalysts examined include RuH2(NHCMe2 CMe2NH2)(R-binap) and RuH(NHCMe 2CMe2NH2)(PPh3)2. These complexes are active catalysts for ketone hydrogenation in benzene without addition of an external base. The kinetic isotope effect (KIE) for catalysis by RuH2(NHCMe2CMe2NH2)( R-binap) was determined to be 2.0 +/- (0.1). The calculated KIE for the model system RuH(NHCH2CH2NH2)(PH 3)2 was 1.3, which is smaller than the experimentally observed value but does not include tunneling effects.;The tetradentate complexes trans-RuHCl[PPh2(ortho-C6 H4)CH2NHCH2)]2 and RuHCl[PPh 2(ortho-C6H4)CH2NHCMe2)]2 are known to be catalysts for the hydrogenation of acetophenone and benzonitrile in toluene when activated by KOtBu/KH. DFT studies were performed and a mechanism is proposed. The calculated rate limiting step for acetone hydrogenation was found to be heterolytic splitting of dihydrogen, which agrees well with experiment. The novel outer-sphere sequential hydrogenation of a C≡N triple bond and then a C=N double bond is proposed.;A mechanism is proposed, which is supported by DFT studies, to explain the selectivity observed in the nucleophilic attack of amines or aziridines on palladium pi-prenyl phosphines complexes. Calculations on based on a palladium complex with two phosphorus donor ligands indicated that the observed selectivity would not be produced. Using two new model intermediates with either THF or aziridine substituted for a phosphine ligand trans to the unhindered side of the prenyl ligand did predict the experimentally observed selectivity.;The complex OsH(NHCMe2CMe2NH2)(PPh 3)2 is known to display autocatalytic behaviour when it catalyzes the hydrogenation of acetophenone in benzene. Pseudo first-order reaction conditions are obtained via addition of the product alcohol at the beginning of each kinetic experiment. The KIE determined using various combinations of deuterium-labeled gas, alcohol and ketone was found to be 1.1 +/- (0.2). DFT calculations were used to explore the effect of the alcohol and the KIE. An induction period is observed at the start of the hydrogenation that is attributed to the formation of an alkoxide complex. A novel, diamine-orchestrated hydrogen-bonding network is proposed based on DFT calculations to explain how the alkoxide is converted back to the active catalyst.