Asymmetric transfer hydrogenation of acetophenone using bis-imine rhodium complexes: Kinetic study and modeling

Abstract

Asymmetric transfer hydrogenation of acetophenone (APh) to (R)-1-phenylethanol (PhE) was studied in 2-propanol solution in the presence of Rh(1,5-COD)(μ-Cl)]2+L, where COD = cyclooctadiene and L is a bis-aldimine ligand based on (R,R)-1,2-cyclohexanediamine and pyridinecarboxaldehyde. This rhodium (I) complex bearing optically active ligands achieved a maximum turnover frequency (TOF) of 268 h−1 at 78 °C and a moderate maximum enantioselectivity (ee) of 55 % for (R)-1-PhE. A formal kinetic study of the process was carried out using the Rh(1,5-COD)(N,N'-(R,R)-1,2-cyclohexane-1,2-diyl-bis-(1-(pyridine-2-yl)-methanimine)-Cl complex. The induction period for catalyst activation is proposed to involve the rapid reaction of the pre-catalyst with excess iso-propoxide to give the catalytically active Rh(1,5-COD)(N,N'-(R,R)-1,2-cyclohexane-1,2-diyl-bis-(1-(pyridine-2-yl)-methanimine)-OiPr, which leads to the transfer hydrogenation of 1,5-COD. Reaction profiles were obtained by varying the initial concentrations of APh, pre-catalyst, base, acetone, and the temperature. These data were fitted to a kinetic model for the proposed mechanism, and numerical simulation led to a set of rate constants and thermodynamic parameters.