Chemoselective Hydrogenation of Aldehydes under Mild, Base-Free Conditions: Manganese Outperforms Rhenium

Several hydride Mn­(I) and Re­(I) PNP pincer complexes were applied as catalysts for the homogeneous chemoselective hydrogenation of aldehydes. Among these, [Mn­(PNP-iPr)­(CO)2(H)] was found to be one of the most efficient base metal catalysts for this process and represents a rare example which permits the selective hydrogenation of aldehydes in the presence of ketones and other reducible functionalities, such as CC double bonds, esters, or nitriles. The reaction proceeds at room temperature under base-free conditions with catalyst loadings between 0.1 and 0.05 mol% and a hydrogen pressure of 50 bar (reaching TONs of up to 2000). A mechanism which involves an outer-sphere hydride transfer and reversible PNP ligand deprotonation/protonation is proposed. Analogous isoelectronic and isostructural Re­(I) complexes were only poorly active.