Selective Electrochemical Reductive Amination of Benzaldehyde Using a Silver Catalyst

The formation of carbon-nitrogen bonds is an essential step in the synthesis of important molecules ranging from small, specialty chemicals to large pharmaceuticals. One of the major tools used to form carbon-nitrogen bonds is reductive amination, where a carbonyl group is converted to an amine. In this work, we investigate an electrochemical reductive amination process to form carbon-nitrogen bonds. Electrochemical routes in industrial chemical synthesis are appealing due to increasingly available distributed sources of renewable electricity. We use benzaldehyde as a model carbonyl substrate to study electrochemical reductive amination; the industrial equivalent conversion of benzaldehyde to benzylamine proceeds at 100-150°C and 40-65 bar hydrogen gas. Previous research on electrochemical reductive amination focuses on outer sphere processes for proof-of-concept synthesis of amines and the production of secondary amines, whereas we investigate an inner-sphere route.

We demonstrate that ammonia and benzaldehyde react under a reductive potential to form benzylamine. This reaction proceeds through an inner-sphere mechanism that is catalyst dependent, in contrast to previous research on outer-sphere formation of amines. The inner-sphere pathway opens up a large phase space of catalysts to control reactivity and selectivity. We find that the partial currents across metals range from 0.5 mA/cm² to over 3 mA/cm² and Faradaic efficiencies from below 5% to above 75%. The reactivity as a function of catalyst is well-described by the charge density profile above the catalyst, a surface property analogous to an atomic radius and dependent only on the catalyst material. We also interrogated the mechanism for this reaction on a silver foil catalyst, one of the most active and selective catalysts, and find that the first electron transfer to benzylimine is the rate-determining step. Our work on this system opens the door to using electrochemical reductive amination as a tool for forming carbon-nitrogen bonds.