Radical Mechanism of IrIII/NiII-Metallaphotoredox-Catalyzed C(sp3)-H Functionalization Triggered by Proton-Coupled Electron Transfer: Theoretical Insight

Abstract

Photoredox catalysis can be induced to activate organic substrates or to modulate the oxidation state of transition-metal catalysts via unique single-electron transfer processes, so as to achieve challenging C(sp3)-H functionalization under mild conditions. However, the specific reaction mechanism and relevant electron transfer process still need to be clarified. Here, a highly regioselective IrIII/NiII-metallaphotoredox-catalyzed hydroalkylation of asymmetrical internal alkyne with an ether α-hetero C(sp3)-H bond has been investigated by density functional theory (DFT) calculations. A novel radical mechanism was predicted to merge oxidative quenching (IrIII-*IrIII-IrIV-IrIII) and nickel catalytic cycles (NiII-NiIII-NiI-NiIII-NiII) for this C(sp3)-H functionalization to construct C(sp3)-C(sp2) bonds. It consists of seven major steps: the single-electron transfer involved in the photoredox cycle for generating active Ni(I)-chloride complexes, proton-coupled electron transfer process to provide α-carbon-centered tetrahydrofuran (THF) radicals, radical capture by Ni(II), reductive elimination to obtain 2-chlorotetrahydro-furan, alkyne oxidative hydrometallation, inner-sphere electron transfer, and σ-bond metathesis to yield the desired alkyne hydroalkylation product. Importantly, both the thermodynamic performance for redox potentials and the kinetic exploration for energy barriers and electron-transfer rates have also been evaluated for the corresponding electron transfer processes. In addition, the steric effects play a major role in determining the regioselectivity of alkyne oxidative hydrometallation.