Understanding Formation and Roles of NiII Aryl Amido and NiIII Aryl Amido Intermediates in Ni-Catalyzed Electrochemical Aryl Amination Reactions.

Abstract

Ni-catalyzed electrochemical aryl amination (e-amination) is an attractive, emerging approach to building C-N bonds. Here, we report in-depth experimental and computational studies that examined the mechanism of Ni-catalyzed e-amination reactions. Key NiII-amine dibromide and NiII aryl amido intermediates were chemically synthesized and characterized. The combination of experiments and DFT calculations suggest (1) there is coordination of an amine to the NiII catalyst before the cathodic reduction and oxidative addition steps, (2) a stable NiII aryl amido intermediate is produced from the cathodic half-reaction, a critical step in controlling the selectivity between cross-coupling and undesired homo-coupling reaction pathways, (3) the diazabicycloundecene additive shifts the aryl halide oxidative addition mechanism from a NiI-based pathway to a Ni0-based pathway, and (4) redox-active bromide in the supporting electrolyte functions as a redox mediator to promote the oxidation of the stable NiII aryl amido intermediate to a NiIII aryl amido intermediate. Subsequently, the NiIII aryl amido intermediate undergoes facile reductive elimination to provide a C-N cross-coupling product at room temperature. Overall, our results provide new fundamental understandings about this e-amination reaction and guidance for further development of other Ni-catalyzed electrosynthetic reactions such as C-C and C-O cross-couplings.