Aminoquinoline-based Ruthenium catalyst for reduction of ketones by hydrogen transfer

Asymmetric reduction of C=O and C=N bonds to give chiral alcohols and amines is one of the more relevant applications of transition metal catalysis1. A striking improvement in hydrogenation or reduction by hydrogen transfer is due to Noyori and more recently to Rigo and co-workers with the development of Ru(II) catalysts bearing chiral diamine ligands2. Here we present our results in hydrogen transfer reduction of aryl ketones with (S)- or (R)-8-amino-5,6,7,8-tetrahydroquinoline (CAMPY). We demonstrated that the enantiomers of CAMPYs were themselves possible alternatives to other diamines as ancillary ligands for AHT even with triphenylphosphine alone. CAMPYs, whose more relevant features are a flat scaffold and a rigid alkyl fused ring, gave highly efficient catalysts when combined with an achiral chelating diphosphine. We were able also to describe the three-dimensional structure of trans and cis-[RuCl2(CAMPY)PP] by 2D-NMR experiments and to define the chirality of the metal centre of the unique diastereoisomer coming from transcis isomerisation either when the phosphorous ligand was triphenylphosphine or chelating DPPB and DPPP. We found a relationship between the ring size of the chelating diphosphines and the enantioselctivity of the catalysts, assigning to a six-member ring the best performances in term of activity and enantioselectivity