Alcohol-assisted base-free hydrogenation of acetophenone catalyzed by OsH(NHCMe₂CMe₂NH₂)(PPh₃)₂

Abstract

The hydrido–amido complex OsH(NHCMe2CMe2NH2)(PPh3)2 (1) catalyzes the base-free hydrogenation of ketones in benzene. Kinetic studies using acetophenone revealed that the system has an induction period, after which the rate of the reaction increases. A constant rate was observed when a critical amount of the product alcohol was added, indicating that the reaction is autocatalytic in 1-phenylethanol. Varying the initial conditions showed that the reaction rate is dependent on hydrogen and catalyst concentration and independent of ketone concentration. Above the critical concentrations of 1-phenylethanol, the reaction rate is independent of alcohol concentration. The rate law for pressures up to 5 atm was found to be rate = d[alcohol]/dt = −d[ketone]/dt = k[Os][H2], with k = 30 mol L−1 s−1 at 293 K and the temperature dependence provided energy of activation parameters. Therefore, the heterolytic splitting of dihydrogen is rate determining under these conditions. Only small kinetic isotope effects were measured in contrast with the analogous ruthenium system. Complex 1 reacts with the product alcohol 1-phenylethanol and is partially converted into the dihydride complex trans-OsH2(NH2CMe2CMe2NH2)(PPh3)2 and acetophenone; with excess alcohol, an osmium alkoxide is observed at low temperature. As expected from these results, 1 is a ketone transfer hydrogenation catalyst in isopropanol.

A theoretical study on the hydrogenation of CO2to methanol catalyzed by ruthenium pincer complexes

“…However, with the increase of concentration of intermediate HCOOH, the formate-assisted H 2 activation becomes a dominant dihydrogen activation pathway instead of the methanol-assisted one. This is consistent with experimental observation by Clapham et al 51 in 2014 that the reaction did not directly depend on the concentration of alcohol in the experiments. Therefore, the stability of complex D1 plays a decisive role in the dihydrogen activation.…”

Details of the Mechanism of the Asymmetric Transfer Hydrogenation of Acetophenone Using the Amine(imine)diphosphine Iron Precatalyst: The Base Effect and The Enantiodetermining Step

“…Base has been found to be an important component in the asymmetric pressure dihydrogen − and transfer hydrogenation ,− reactions catalyzed by metal complexes. There have been reports of the acceleration of the rate of transfer hydrogenation of ketones when excess base is added to ruthenium catalyst systems.…”

Iron Group Hydrides in Noyori Bifunctional Catalysis

“…The use of methyl groups on the diamine backbone prevents β‐hydride elimination and decomposition that is observed if the corresponding DACH amide complex is not kept under H 2 30. The isostructural osmium complex OsH(NHCMe 2 CMe 2 NH 2 )(PPh 3 ) 2 has also been prepared and found to be an active ketone hydrogenation catalyst 58,59. The rate law for the hydrogenation of acetophenone at 20°C, 1‐15 atm H 2 , [Ru] 10 −3 to 10 −4 M, [ketone] 0.02 M in benzene is shown in eq.…”