Catalytic Asymmetric Hydride Transfer Reduction of Ketones with Rhodium and Chiral Diamine Ligands: Approach of the Active Species Structure by DFT Calculations

The structure of RhH(diamine)(C2H4)2 complexes involved in the title reaction was studied by DFT calculations. The structures of all the possible diastereoisomers of the Rh complexes were optimized with either the (R,R)-N,N‘-dimethyl-1,2-dimethylethylenediamine or the (R,R)-N,N‘-dimethyl-1,2-diphenylethylenediamine (in both cases with the threo diamine). The diastereoisomer with all the substituents of the nitrogen atoms arranged trans to each other relative to the mean plane was found to be the most stable. Therefore, the active species involved in the title reaction has unambiguously a structure presenting a C2-symmetry axis with stereogenic nitrogen atoms. This can be the key to enantioselectivity.