Electrochemical Dehydrogenation Pathways of Amines to Nitriles on NiOOH

Nitriles are highly important synthetic intermediates with applications in a wide variety of organic reactions including production of pharmaceuticals, fine chemicals, and agricultural chemicals. Thus, developing effective green routes to oxidize amines to nitriles is of great interest. One promising method to achieve the oxidation of primary amines to nitriles is through electrochemical oxidation on NiOOH electrodes. This reaction has long been thought to occur through an indirect mechanism consisting of a series of potential independent hydrogen atom transfer steps to catalytic Ni³⁺ sites in NiOOH, which reduces NiOOH to Ni(OH)₂. The role of the applied potential in this mechanism is simply to regenerate NiOOH by oxidizing Ni(OH)₂. In this work, we demonstrate that a second, potential-dependent pathway recently found to apply to alcohol and aldehyde oxidation on NiOOH and consisting of potential-dependent hydride transfer to Ni⁴⁺ sites is the dominant pathway for the oxidation of amines using propylamine and benzylamine as model systems. After qualitatively and quantitatively examining the contributions of indirect and potential-dependent oxidation pathways to amine oxidation on NiOOH, we also examine the effect the amine concentration, solution pH, applied bias, and deuterium substitution have on the two pathways, further clarifying their mechanisms and exploring what factors control their rate. This work provides a comprehensive understanding of the mechanism of primary amine oxidation on NiOOH.