Chiral induction effects in ruthenium(II) amino alcohol catalysed asymmetric transfer hydrogenation of ketones: An experimental and theoretical approach

The enantioselective outcome of transfer hydrogenation reactions that are c atalysed by ruthenium(Ii) amino alcohol complexes was studied by means of a systematically varied series of ligands. It was found that both the substi tuent at the 1-position in the 2-amino-1-alcohol ligand and the substituent at the amine functionality influence the enantioselectivity of the reactio n to a large extent: enantioselectivities (ee values) of up to 95% were obt ained for the reduction of acetophenone. The catalytic cycle of ruthenium(I I) amino alcohol catalysed transfer hydrogenation was examined at the densi ty functional theory level. The formation of a hydrogen bond between the ca rbonyl functionality of the substrate and the amine proton of the ligand, a s well as the formation of an intramolecular H ... H bond and a planar H-Ru -N-H moiety are crucially important for the reaction mechanism. The enantio selective outcome of the reaction can be illustrated with the aid of molecu lar modelling by the visualisation of the steric interactions between the k etone and the ligand backbone in the ruthenium(II) catalysts.