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 Ni^III and C(sp²)-C(sp³) reductive elimination from bipyridine Ni^II 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 Ni^II dihalide intermediates, radical capture by a Ni^II(aryl)(halide) resting state, and key C-C bond formation from Ni^III. Oxidative addition to Ni^I, as opposed to Ni⁰, and rapid Ni^III/Ni^I 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.