Mechanism of Ni-Catalyzed Photochemical Halogen Atom-Mediated C(sp3)–H Arylation

Within the context of Ni photoredox catalysis, halogen atom photoelimination from Ni has emerged as a fruitful strategy for enabling hydrogen atom transfer (HAT)-mediated C­(sp3)–H functionalization. Despite the numerous synthetic transformations invoking this paradigm, a unified mechanistic hypothesis that is consistent with experimental findings on the catalytic systems and accounts for halogen radical formation and facile C­(sp2)–C­(sp3) bond formation remains elusive. We employ kinetic analysis, organometallic synthesis, and computational investigations to decipher the mechanism of a prototypical Ni-catalyzed photochemical C­(sp3)–H arylation reaction. Our findings revise the previous mechanistic proposals, first by examining the relevance of SET and EnT processes from Ni intermediates relevant to the HAT-based arylation reaction. Our investigation highlights the ability for blue light to promote efficient Ni–C­(sp2) bond homolysis from cationic NiIII and C­(sp2)–C­(sp3) reductive elimination from bipyridine NiII complexes. However interesting, the rates and selectivities of these processes do not account for the productive catalytic pathway. Instead, our studies support a mechanism that involves halogen atom evolution from in situ generated NiII dihalide intermediates, radical capture by a NiII(aryl)­(halide) resting state, and key C–C bond formation from NiIII. Oxidative addition to NiI, as opposed to Ni0, and rapid NiIII/NiI comproportionation play key roles in this process. The findings presented herein offer fundamental insight into the reactivity of Ni in the broader context of catalysis.