Understanding Formation and Roles of Ni <sup>II</sup> Aryl Amido and Ni <sup>III</sup> Aryl Amido Intermediates in Ni-Catalyzed Electrochemical Aryl Amination Reactions
Jian Luo, Michael T. Davenport, Chad Callister, Shelley D. Minteer, Daniel H. Ess, Tianbiao Liu
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 Ni II -amine dibromide and Ni II 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 Ni II catalyst before the cathodic reduction and oxidative addition steps, (2) a stable Ni II 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 Ni I -based pathway to a Ni 0 -based pathway, and (4) redox-active bromide in the supporting electrolyte functions as a redox mediator to promote the oxidation of the stable Ni II aryl amido intermediate to a Ni III aryl amido intermediate. Subsequently, the Ni III 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.