Enhanced Enantioselectivity Achieved at Low Hydrogen Pressure for the Asymmetric Hydrogenation of Methyl Acetoacetate over a Tartaric Acid NaBr-Modified Raney Nickel Catalyst: A Kinetic Study

Abstract

To ensure high enantiopurity of the product, enantio-differentiating hydrogenation of methyl acetoacetate over a (R,R)-tartaric acid-modified Raney nickel catalyst is normally performed under elevated H2-pressure (∼10 MPa). In this study, higher enantioselectivity than previously reported for methyl acetoacetate was achieved (92% ee) under low H2-pressure of 0.42 MPa. Effects of reaction conditions on the enantioselectivity and hydrogenation rate were investigated using a low-pressure reaction system (<0.5 MPa of H2). It was found that impurities in the solvent greatly reduce the enantioselectivity of MAA. The low-pressure reaction system enabled a satisfactory kinetic approach. The reaction rate was well described by Langmuir-Hinshelwood formalism, verifying the previous assumption that the addition of adsorbed hydrogen to the substrate interacting with surface tartrate is a rate-determining step.