Divalent Organocopper Complexes: Masked Radicals for Effective Electrochemically Driven Atom Transfer Radical Addition

Copper-catalyzed electrochemical atom transfer radical addition (eATRA) is an emerging synthetic approach to C–C bond formation. This method hinges on an electrochemically generated organocopper(II) complex [CuIILR]+ (L is a tetradentate N-donor ligand) a rare and highly reactive intermediate, which acts as a controlled-release source of organic radicals (R·) through reversible CuII–C bond homolysis. A variety of organic halides (RX) have been added across the C═C double bonds of aromatic alkenes. X-ray crystallography, cyclic voltammetry, and UV/vis spectroelectrochemical analysis provided deep understanding and rationale for which functional groups best promote formation of the active organocopper(II) intermediate, which in turn mediate effective C–C bond formation. Nitrile, keto, and ester functional groups have been explored and, when complexed as their carbanions, act as masked radicals, suppressing undesirable radical homocoupling reactions that typically plague this chemistry. Polyhalogenated compounds are shown to accelerate eATRA and greatly increase product yields. Mild optimized reaction conditions are reported, expanding the utility of this synthetic approach beyond conventional copper-catalyzed C–C bond formation methodology.