Electroreduction of Carbonyl Compounds Catalyzed by a Manganese Complex

Chemical conversions are nowadays evaluated not only according to their functionality and originality but also regarding their atom and redox economy. Excess energy consumption, the origin of that energy, and waste production are becoming increasingly important factors in chemical synthesis development. This led to a re-emerging of electroorganic synthesis after decades of dormancy. Inspired by this, we combined organometallic catalysis and electrochemistry to develop a versatile and broadly applicable method for hydrogenation of ketones and aldehydes by electrons and trifluoroethanol as proton source under ambient conditions. Aromatic, aliphatic, and further functionalized ketones and aldehydes were converted to the corresponding alcohols in decent to excellent yields, with a catalyst loading of only 1%. The protocol is selective toward ketones and aldehydes over non-conjugated C═C bonds, esters, and carboxylic acids. As a base metal catalyst, we utilized a manganese complex with a proton relay, which was previously shown to electrohydrogenate CO2 via an Mn–H species. Mechanistic analysis showed that this hydride is also pivotal for the electrohydrogenation of C═O bonds in organic scaffolds and fosters ionic hydrogenations over radical-type PCET reactions, which leads to the observed selectivity toward polar substrates. Further mechanistic analysis shed light on the pKa of the metal hydride species, the kinetic rate of its formation, as well as the all-over catalysis. Chemical formation of the MnH species via H2 splitting under ambient conditions failed, which emphasizes that merging electrochemistry and organometallic catalysis can open different pathways for chemical catalysis under mild conditions.