Robust and efficient transfer hydrogenation of carbonyl compounds catalyzed by NN -Mn(I) complexes.

A series of manganese(I) carbonyl complexes bearing structurally related NN - and NNN -chelating ligands have been synthesized and assessed as catalysts for transfer hydrogenation (TH). Notably, the NN -systems based on N -R functionalized 5,6,7,8-tetrahydroquinoline-8-amines, proved the most effective in the manganese-promoted conversion of acetophenone to 1-phenylethanol. In particular, the N -isopropyl derivative, Mn1, when conducted in combination with t -BuONa, was the standout performer mediating not only the reduction of acetophenone but also a range of carbonyl substrates including (hetero)aromatic-, aliphatic- and cycloalkyl-containing ketones and aldehydes with especially high values of TON (up to 17 200; TOF of 3550 h -1 ). These findings, obtained through a systematic variation of the N -R group of the NN ligand, are consistent with an outer-sphere mechanism for the hydrogen transfer. As a more general point, this Mn-based catalytic TH protocol offers an attractive and sustainable alternative for producing alcoholic products from carbonyl substrates.