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(ClO 4 ) 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 CH 2 ═CHX (X = CO 2 CH 3, CN, CONH 2, COCH 3, and SO 2 Ph) to afford polyfunctionalized products. Combined experimental and theoretical studies revealed that radicals couple directly with the C aryl atom of arylcopper(II) compounds to form C alkyl —C aryl 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.