DFT Mechanistic Study of Ir^III/Ni^II-Metallaphotoredox-Catalyzed Difluoromethylation of Aryl Bromides

The work by MacMillan et al. (Angew. Chem., Int. Ed. 2018, 57, 12543−12548) developed an Ir^III/Ni^II-metallaphotoredox-catalyzed difluoromethylation strategy of aryl bromides using CHF₂Br as the CHF₂ reagent in the presence of tris­(trimethylsilyl)­silane. Here, we present a density functional theory (DFT)-based computational study to understand special dual catalysis promoting the C­(sp²)–C­(sp³) coupling. The calculated results show that the energetically more favorable pathway involves the reductive quenching of a photocatalyst (Ir^III/*Ir^III/Ir^II/Ir^III) and a Ni⁰-initiated catalytic cycle (Ni⁰/Ni^I/Ni^III/Ni^I/Ni⁰ or Ni⁰/Ni^II/Ni^III/Ni^I/Ni⁰). The calculations reveal not only the mechanistic details delivering the difluoromethylarene product but also the molecular-level picture of the generation of Ni⁰ species from the Ni^II precatalyst. Moreover, the calculations also rationalize the observed stoichiometric effect of CHF₂Br in the reactions of aryl bromides with different substituted groups.