Decoding the redox behaviour of copper in Ullmann-type coupling reactions

The copper-catalysed functionalization of aryl halides is one of the most preferred methods for forming carbon-carbon and carbon-heteroatom bonds¹. Yet the redox behaviour of the copper species in the catalytic cycle remains poorly understood and a subject of debate². We report experimental and theoretical mechanistic investigations into the reaction of a well-defined Cu(I) complex with an electron-poor aryl iodide, which leads to the formation of an isolable Cu(III)-aryl complex that subsequently reductively eliminates to form a C(sp²)-CF₃ bond. Our integrated experimental and theoretical findings indicate that the process proceeds through a Cu(I)/Cu(III)/Cu(II)/Cu(III)/Cu(I) redox sequence. By controlling the temperature, we managed to interrupt this sequence and capture the reactivity of the copper species through various spectroscopic methods, enabling in-depth mechanistic analysis. These findings shed light on the intricate behaviour of copper species and challenge the traditional mechanistic proposal for the reaction of Cu(I) with aryl iodide, thus providing fresh perspectives into the mechanistic aspect of the copper-catalysed coupling reactions.