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

Chiral Mn-1 and Mn-2 were prepared and evaluated as catalysts in asymmetric transfer hydrogenation (ATH) and asymmetric hydrogenation (AH) of ketones. Pincer complex Mn-1 proved the more effective and indeed it was able to hydrogenate a broad range of pro-chiral ketonic substrates (39 examples), to give their chiral alcohols in high yield and with excellent enantioselectivities (up to 99% ee). [Display omitted] • PNN - and NN -ligands incorporating chiral ferrocenyl units as chelating supports for manganese complexes. • PNN -pincer complex and its NN- bidentate analogue for the asymmetric hydrogenation (AH) and asymmetric transfer hydrogenation (ATH). • The manganese system bearing the tridentate PNN -ligand proving to be more effective than its bidentate counterpart. • Rich library of ketonic substrates differing in their steric and electronic profile. • The importance of π-π interactions and steric hindrance enhancing stereoselectivity. A new type of (R C , S P)-1-(2-diphenylphosphino)ferrocenylethylamine N -substituted with a (R C)-5,6,7,8-tetrahydroquinolinyl group (L PNN -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. [ABSTRACT FROM AUTHOR]