Radical Reactivity, Catalysis, and Reaction Mechanism of Arylcopper(II) Compounds: The Missing Link in Organocopper Chemistry

Organocopper­(I) compounds are recognized as carbon nucleophiles, while organocopper­(III) complexes are involved in copper catalysis as intermediates to undergo a cross-coupling reaction with various anionic nucleophiles. In contrast to the chemistry of organocopper­(I) and (III) compounds, organocopper­(II) chemistry is virtually a missing link in integral organocopper chemistry because structurally well-defined organocopper­(II) compounds have barely been isolated or studied. We report in this Article an investigation of the radical reactions of stable and structurally well-defined arylcopper­(II) compounds, obtained readily from the arene CH bond reaction of macrocyclic azacalix[1]­arene[3]­pyridines and Cu­(ClO4)2. We have found that arylcopper­(II) compounds acted as essentially radical species to undergo an efficient three-component reaction with radical initiators 2,2′-azobis­(isobutyronitrile) (AIBN) or 2,2′-azobis­(2,4-dimethylvaleronitrile) (ABVN) and α,β-unsaturated compounds CH2CHX (X = CO2CH3, CN, CONH2, COCH3, and SO2Ph) to afford polyfunctionalized products. Combined experimental and theoretical studies revealed that radicals couple directly with the Caryl atom of arylcopper­(II) compounds to form CalkylCaryl bonds through a Cu­(II)/Cu­(I) mechanism. Comprehension of the formation and radical reactivity of arylcopper­(II) compounds has allowed the development of a copper-catalyzed three-component radical reaction for arene CH bond functionalization.