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Direct synthesis of high-valent aryl-Cu(II) and aryl-Cu(III) compounds: mechanistic insight into arene C-H bond metalation.

Copper and its salts are abundant, inexpensive, and eco-friendly and have been used as the surrogates of noble metals to effect arene C-H bond activation and transformations. Despite of the recent significant progress of the study, syntheses of high-valent arylcopper(II-III) compounds are still very rare and mechanisms of copper(II)-catalyzed reactions remain elusive. With the use of azacalix[1]arene[3]pyridines as a platform, a number of arylcopper(II) compounds were synthesized efficiently from the reaction of Cu(ClO4)2 under ambient conditions. The resulting aryl-Cu(II) compounds, which contain an unprecedented (substituted) phenyl-Cu(II) σ-bond, were stable under atmospheric conditions and can undergo facile oxidation reaction by free copper(II) ions or oxone to afford arylcopper(III) compounds in good yields. Both arylcopper(II) and arylcopper(III) compounds were characterized unambiguously by means of XRD, XPS, and NMR methods. Experimental evidence including reaction kinetics, LFER and KIE, and theoretical calculations indicated that the Cu(ClO4)2-mediated arene C-H bond activation proceeds plausibly through an electrophilic aromatic metalation pathway. The synthesis of high-valent arylcopper compounds and the reaction mechanism reported here highlight the diversity and richness of organocopper chemistry.