Amino Alcohol Coordination in Ruthenium(II)-Catalysed Asymmetric Transfer Hydrogenation of Ketones

Abstract

The nature of ruthenium-amino alcohol precursors in the catalytic cycle of asymmetric hydrogen transfer reactions was studied using two C2-symmetrical tetradentate ligands (1 and 2) that were synthesised from (nor)ephedrine. The structure of the catalyst precursor was examined through catalysis and NMR experiments. It was shown that the active catalyst contains one ligand per metal, which coordinates in a didentate N,O fashion (9). In addition, a RuIICl2 complex, in which N,N′-bis(2-hydroxy-1-methyl-2-phenylethyl)-1,2-diaminoethane (1) coordinates through two nitrogen atoms, was structurally characterised by X-ray diffraction (8). – Based on the results of this study a series of new amino alcohol ligands was synthesised from easily available starting materials. Optimisation of the amino alcohol ligand structure resulted in the most effective chiral amino alcohol ligand (5) so far that is capable of reducing acetophenone at 0 °C with up to 97% ee in the RuII-catalysed transfer hydrogenation.