Ruthenium-Catalysed Hydrogenation of Aromatic Ketones using Monodentate Phosphoramidite Ligands

A ruthenium pre-catalyst containing two equivalents of the bulky monodentate phosphoramidite 3,3’-dimethyl-PipPhos and one equivalent of a chiral diamine such as 1,2-diphenylethylenediamine or 1,2-diaminocyclohexane was used for the asymmetric hydrogenation of aromatic ketones. A range of substituted and unsubstituted aryl alkyl ketones was hydrogenated using only 0.1 mol% of this catalyst with full conversions and enantioselectivities up to 97%. The phosphoramidite and diamine ligands matched when both had the same configuration, i.e., S-phosphoramidite with S,S-diamine. In that case the product was obtained with high enantioselectivity and the R-configuration. The mismatched case produced the product in lower ee. The product configuration was determined by the configuration of the diamine ligand. A mechanistic proposal was made.