Catalytically Relevant Organocopper(III) Complexes Formed through Aryl-Radical-Enabled Oxidative Addition

Stepwise oxidative addition of copper(I) complexes to form copper(III) species via single electron transfer (SET) events has been widely proposed in copper catalysis. However, direct observation and detailed investigation of these fundamental steps remain elusive owing largely to the typically slow oxidative addition rate of copper(I) complexes and the instability of the copper(III) species. We report herein a novel aryl-radical-enabled stepwise oxidative addition pathway that allows for the formation of well-defined alkyl-Cu^III species from Cu^I complexes. The process is enabled by the SET from a Cu^I species to an aryl diazonium salt to form a Cu^II species and an aryl radical. Subsequent iodine abstraction from an alkyl iodide by the aryl radical affords an alkyl radical, which then reacts with the Cu^II species to form the alkyl-Cu^III complex. The structure of resultant [(bpy)Cu^III(CF₃)₂(alkyl)] complexes has been characterized by NMR spectroscopy and X-ray crystallography. Competition experiments have revealed that the rate at which different alkyl iodides undergo oxidative addition is consistent with the rate of iodine abstraction by carbon-centered radicals. The Cu^II intermediate formed during the SET process has been identified as a four-coordinate complex, [Cu^II(CH₃CN)₂(CF₃)₂], through electronic paramagnetic resonance (EPR) studies. The catalytic relevance of the high-valent organo-Cu^III has been demonstrated by the C-C bond-forming reductive elimination reactivity. Finally, localized orbital bonding analysis of these formal Cu^III complexes indicates inverted ligand fields in σ(Cu-CH₂) bonds. These results demonstrate the stepwise oxidative addition in copper catalysis and provide a general strategy to investigate the elusive formal Cu^III complexes.