Manganese(I)-catalyzed asymmetric (transfer) hydrogenation of ketones: An insight into the effect of chiral PNN and NN ligands

A new type of (RC,SP)-1-(2-diphenylphosphino)ferrocenylethylamine N-substituted with a (RC)-5,6,7,8-tetrahydroquinolinyl group (LPNN-1) was successfully employed as a chiral chelating ligand in both Mn-catalyzed asymmetric transfer hydrogenation (ATH) and asymmetric hydrogenation (AH) of a broad range of ketonic substrates (39 examples), leading to high conversions and excellent enantioselectivities for their product alcohols. In particular, PNN-pincer complex Mn-1 and its NN-bidentate analogue Mn-2 have been isolated and their comparative performance as catalysts studied with Mn-1 proving more effective in both ATH and AH. Moreover, Mn-1 generally imparted higher degrees of enantiomeric excess (ee) in both hydrogenation processes which can reach up to 99% in ATH and 93% in AH for propiophenone-type substrates. DFT calculations highlight the importance of π-π interactions and steric hindrance between catalyst and substrate which manifests itself in enhancements in ee for propiophenone over acetophenone substrates. Furthermore, a possible mechanism for the Mn-catalyzed ATH has been proposed on the basis of a joint DFT and experimental investigation.