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 the formation of carbon-carbon and carbon-heteroatom bonds.1 Yet, the redox behaviour of the copper species in the catalytic cycle remains elusive and a subject of considerable debate.2 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 forge a C(sp2)-CF3 bond. Our integrated experimental and theoretical findings indicate that the process proceeds through a redox sequence of Cu(I)/Cu(III)/Cu(II)/Cu(III)/Cu(I). Additionally, we managed to interrupt this sequence by temperature control and captured the reactivity of the copper species through various spectroscopic methods, facilitating 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.