The effects of ligand variation on enantioselective hydrogenation catalysed by RuH2(diphosphine)(diamine) complexes

Abstract

Density functional theory calculations have been used to investigate the hydrogenation of acetophenone (ACP) catalysed by the RuH₂(diphosphine)(diamine) complexes with emphasis on the effect of the structure of the diphosphine and diamine ligands on the enantioselectivity. The computed reaction coordinate diagrams of RuH₂(diphosphine)[(S,S)-DPEN] catalysed reactions with different (S)-diphosphine ligands (XylBINAP, TolBINAP, and BINAP) show that the presence of two methyl groups in the meta position is critical to obtaining a high difference in activation energy for the reaction pathways associated with the (R)- and (S)- alcohols, and consequently high enantioselectivity. The effect of the diamine structure while keeping the TolBINAP and XylBINAP fixed has also been analysed. To enhance the enantioselectivity of the TolBINAP system, the addition of two methyl groups and the removal of a phenyl group of the diamine (DMAPEN) offer the necessary steric interactions. We conclude by reporting a correlation between the enantiomeric excess and the difference in the computed activation energies of the two most favourable (S) and (R) reaction pathways, which shows that the computational procedure adopted could be used to predict the enantiomeric excess of ketone hydrogenation reactions catalysed by the Noyori-type catalysts, and assist in the choice of ligand when optimising the enantiomeric excess.