Asymmetric hydrogenation of ketones with planar chiral manganese(i) complexes

Abstract

Tridentate P,N,N ferrocenyl ligands 4a-b with planar chirality only have been prepared and evaluated in the manganese-catalysed hydrogenation of aromatic ketones. The Mn^I complexes 5a-b, prepared in situ from ligands 4a-b and [MnBr(CO)₅], showed a complex mixture of stereoisomers and cationic tricarbonyl/neutral dicarbonyl species. The catalytic activity of both complexes is excellent, with a total conversion of acetophenone in 4 h in the presence of 0.1 mol% catalyst and the mild K₂CO₃ base, under 30 bar of H₂ at 50 °C. Ees of up to 69% were obtained with bulky pivalophenone in the presence of 1 mol% catalyst 5b. The results of a computational study, carried out on the Mn-catalysed hydrogenation mechanism, agree with the greater activity and lower enantioselectivity of 4a with respect to previously reported P,N,N ferrocenyl ligands bearing both planar and central chirality, highlighting the useful role of each chiral source.