Kinetic studies of the asymmetric transfer hydrogenation of imines with HCOOH using Rh-chiral diamine catalysts reveal anomalous concentration dependences attributed to the participation of both reactant and product in the formation of formate salts. The resting state of the catalyst is suggested to be the Rh hydride species 5. The role of bases such as Et 3 N in metering HCOOH into the catalytic cycle is discussed. A practical reaction protocol for achieving high productivity based on slow addition of HCOOH is presented. These studies highlight the value of in situ reaction monitoring techniques in developing a detailed mechanistic picture of a complex reaction system.
IntroductionAsymmetric hydrogenation of CdO and CdN groups via transfer of hydrogen from an organic molecule has come under recent attention as an attractive alternative to reduction using gaseous hydrogen. Ru, Rh and Ir precursor complexes together with chiral diamine ligands have been shown to effect these reactions in high enantioselectivity and high efficiency. 1 Noyori and co-workers 2 have studied the mechanism of the reaction for ketones with both simple alcohols and formic acid as the source of hydrogen, and they were the first to propose that the reaction proceeds outside the coordination sphere of the metal. Such a concerted mechanism involving the transfer of both a metal hydride and a ligand proton has been supported by theoretical studies. 3 In addition, DFT (density functional theory) calculations 4 and deuterium-labeling studies 5 for ketone substrates suggested a similar mechanism for transfer hydrogenation and dehy-
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