Divalent Organocopper\nComplexes: Masked Radicals for\nEffective Electrochemically Driven Atom Transfer Radical Addition

Copper-catalyzed electrochemical atom transfer radical\naddition\n(<i>e</i>ATRA) is an emerging synthetic approach to C–C\nbond formation. This method hinges on an electrochemically generated\norganocopper(II) complex [Cu<sup>II</sup>LR]<sup>+</sup> (L is a tetradentate\nN-donor ligand) a rare and highly reactive intermediate, which acts\nas a controlled-release source of organic radicals (R<sup>·</sup>) through reversible Cu<sup>II</sup>–C bond homolysis. A variety\nof organic halides (RX) have been added across the CC double\nbonds of aromatic alkenes. X-ray crystallography, cyclic voltammetry,\nand UV/vis spectroelectrochemical analysis provided deep understanding\nand rationale for which functional groups best promote formation of\nthe active organocopper(II) intermediate, which in turn mediate effective\nC–C bond formation. Nitrile, keto, and ester functional groups\nhave been explored and, when complexed as their carbanions, act as\nmasked radicals, suppressing undesirable radical homocoupling reactions\nthat typically plague this chemistry. Polyhalogenated compounds are\nshown to accelerate <i>e</i>ATRA and greatly increase product\nyields. Mild optimized reaction conditions are reported, expanding\nthe utility of this synthetic approach beyond conventional copper-catalyzed\nC–C bond formation methodology.