Copper-Carbon Homolysis Competes with Reductive Elimination in Well-Defined Copper(III) Complexes.

Despite the recent advancements of Cu catalysis for the cross-coupling of alkyl electrophiles and the frequently proposed involvement of alkyl-Cu(III) complexes in such reactions, little is known about the reactivity of these high-valent complexes. Specifically, although the reversible interconversion between an alkyl-Cu III complex and an alkyl radical/Cu II pair has been frequently proposed in Cu catalysis, direct observation of such steps in well-defined Cu III complexes remains elusive. In this study, we report the synthesis and investigation of alkyl-Cu III complexes, which exclusively undergo a Cu-C homolysis pathway to generate alkyl radicals and Cu II species. Kinetic studies suggest a bond dissociation energy of 28.6 kcal/mol for the Cu III -C bonds. Moreover, these four-coordinate complexes could be converted to a solvated alkyl-Cu III -(CF 3 ) 2 , which undergoes highly efficient C-CF 3 bond-forming reductive elimination even at low temperatures (-4 °C). These results provide strong support for the reversible recombination of alkyl radicals with Cu II to form alkyl-Cu III species, an elusive step that has been proposed in Cu-catalyzed mechanisms. Furthermore, our work has demonstrated that the reactivity of Cu III complexes could be significantly influenced by subtle changes in the coordination environment. Lastly, the observation of the highly reactive neutral alkyl-Cu III -(CF 3 ) 2 species (or with weakly bound solvent molecules) suggests they might be the true intermediates in many Cu-catalyzed trifluoromethylation reactions.