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.