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

Abstract

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.

Dual Nickel- and Photoredox-Catalyzed Asymmetric Reductive Cross-Couplings: Just a Change of the Reduction System?

“…It was proven that α-chloroboronates undergo oxidative addition with a monovalent nickel catalyst to generate alkyl radicals in-cage. This oxidative addition of Ni­(I) species to alkyl halides is not very common in Ni-catalyzed asymmetric RCCs, implying the crucial role of the dual catalysis system to accomplish the smooth transformation of this reaction. , As for alkyl iodides, they generate out-of-cage radical species, which could be catalyzed by both nickel catalysis and photocatalysis, as evidenced by the formation of homocoupling products with the single catalysis system, respectively. When utilizing α-bromo- or α-iodoboronates, no target product was formed.…”