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

Abstract

The work by MacMillan et al. (Angew. Chem., Int. Ed.2018571254312548) developed an IrIII/NiII-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 (IrIII/*IrIII/IrII/IrIII) and a Ni0-initiated catalytic cycle (Ni0/NiI/NiIII/NiI/Ni0 or Ni0/NiII/NiIII/NiI/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 NiII precatalyst. Moreover, the calculations also rationalize the observed stoichiometric effect of CHF2Br in the reactions of aryl bromides with different substituted groups.

State of knowledge in photoredox-catalysed direct difluoromethylation

“…The reductive quenching cycle (Ir III /*Ir III /Ir II / Ir III ) and the Ni 0 -initiated catalytic cycle were confirmed. 53 Electron-deficient heterocyclic N-oxides as redox auxiliaries could activate carboxylic acid derivatives such as anhydrides or acyl chlorides by inducing reductive decarboxylation and radical formation. This approach was applied in 2018 by Stephenson and co-workers with commercially available difluoroacetic anhydride for the visible-light-promoted intramolecular fragment-coupling decarboxylative difluoromethyl-Scheme 37 Visible-light-mediated difluoromethylation of alkynoates.…”

“…The reductive quenching cycle (Ir III /*Ir III /Ir II /Ir III ) and the Ni 0 -initiated catalytic cycle were confirmed. 53…”

Mechanistic views and computational studies on transition-metal-catalyzed reductive coupling reactions

“…11). 129 The photoexcited species *Ir III is not only an oxidant but also a reductant. [130][131][132] The photoredox catalytic cycle proceeds via a reductive quenching mechanism, which involves an Ir III /*Ir III /Ir II /Ir III cycle.…”

Understanding Ir(III) Photocatalyst Structure–Activity Relationships: A Highly Parallelized Study of Light-Driven Metal Reduction Processes

“…Light absorption rapidly generates exceptionally reactive excited state species capable of converting photons into chemical energy for solar energy conversion, organic transformations, micropollutant degradation, CO 2 reduction, and photodynamic therapy applications while circumventing the need for harsh reaction conditions or reagents. − Following the example of nature, photochemical reactions can be initiated through absorption of visible light by a transition metal complex acting as a photocatalyst (PC). After Franck–Condon absorption upon illumination, internal conversion (IC) and, in some cases, intersystem crossing (ISC) results in the lowest energy excited state configuration (PC*).…”