Transfer Hydrogenation of Ketones, Nitriles, and Esters Catalyzed by a Half‐Sandwich Complex of Ruthenium

Half‐sandwich complexes [Cp(P i Pr 3 )Ru(CH 3 CN) 2 ]PF 6 ( 1 ; Cp=cyclopentadienyl) and [Cp*(phen)Ru(CH 3 CN)]PF 6 ( 2 ; Cp*=pentamethylcyclopentadienyl, phen=phenanthroline) catalyse the transfer hydrogenation of ketones to alcohols, aldimines to amines, and nitriles to imines under mild conditions. In the latter process, the imine products come from the coupling of the amines formed initially with acetone derived from the reducing solvent (isopropanol). Among functionally substituted nitriles, the aldo and keto groups are reduced concomitantly with the cyano group, whereas ester and amido groups are tolerated. Amides and alkyl esters are not reduced under these conditions even upon heating to 70 °C. However, phenylbenzoates and trifluoroacetates are reduced to alcohols. Kinetic studies on the reduction of acetophenone in isopropanol established that the reaction is first order in both the substrate and the alcohol. Stoichiometric mechanistic studies showed the formation of a hydride species. A hydride mechanism was proposed to account for these observations.