Electroreduction of Activated Esters Catalyzed by a Manganese Complex.

Electrochemical hydrogenation (electrohydrogenation) using electrons and protons as H2 surrogates offers a sustainable alternative to conventional reduction methods. Herein, we report the manganese-catalyzed electroreduction of activated esters employing a derivative of Mn-(bpy)-(CO)3 (bpy = 2,2'-bipyridine) featuring a proton relay in the ligand. Using low catalyst loadings (1 mol%), a range of fluorinated esters were reduced to the corresponding alcohols under mild electrochemical conditions. Experimental and quantum-chemical mechanistic studies provide an understanding of the reaction pathway. The key catalytic intermediate, a manganese hydride species, is generated by electrochemical reduction of the precatalyst followed by protonation. Computational analysis indicates that hydrogenation proceeds via an outer-sphere H-/H+ transfer pathway. These findings expand the scope of base-metal electrocatalytic hydrogenation to esters and provide insight into manganese-hydride-mediated electroreduction mechanisms.