Kinetic Studies on the Asymmetric Transfer Hydrogenation of Acetophenone Using a Homogeneous Ruthenium Catalyst with a Chiral Amino-Alcohol Ligand

Abstract

The overall kinetics of the asymmetric transfer hydrogenation of acetophenone to 1-phenyl-ethanol using a Noyori-type homogeneous Ru-catalyst with a chiral amino-alcohol ligand ((1R,2S)-(+)-cis-1-amino-2-indanol) were determined in a batch reactor with on-line FT-IR spectroscopy. Data analysis showed that the transfer hydrogenation is an equilibrium reaction with additional inhibition terms of both reactants and products. The rate equation is best expressed as: RA = −(k1CACB − k-1CCCD)/(k1CA + k2CB + k-1CC + k-2CD) where CA, CB, CC, and CD are the concentration levels of acetophenone (A), 2-propanol (B), phenyl ethanol (C), and aceton (D), repectively. The overall kinetics are in agreement with the proposed mechanism of transfer hydrogenation of ketones by Noyori. The equilibrium constant for the transfer hydrogenation was about 0.19 at T = 33 °C. The enantiomeric excess of the asymmetric conversion was high (ee = 0.92) and almost no reduction was observed in the course of the reaction. The kinetic data have been applied to optimise the production of enantiomerically pure 1-phenyl-ethanol in a batch reactor setup.