A DFT investigation of the mechanism of photoinduced radical borylation of alkyl bromides: elucidation of base-mediated photoinduced SET and SN2 processes

Abstract

The transition-metal-free borylation of alkyl halides has received widespread attention due to environmental and economic considerations. In particular, the visible-light-induced radical borylation of alkyl halides catalyzed by 4-phenylpyridine has been achieved, even for unactivated alkyl bromides. In these reactions, the visible-light-induced radical borylation of alkyl bromides has demonstrated the formation of nucleophilic products and proposed a mechanism where the activation of unreactive alkyl bromides occurs through an S_N2/photoinduced radical formation pathway. However, a systematic investigation of the mechanism of this type of reaction is lacking. Several crucial aspects necessitate further elucidation, including the formation of a super electron donor (SED), the competition between photoinduced single-electron transfer (SET) and S_N2 processes, and the precise nature of MeO⁻. In this study, we employed density functional theory (DFT) calculations to address these issues. The calculated results indicate that in the 4-PhPy/B₂cat₂/MeONa system, the generation of alkyl radicals mainly results from the two successive photoinduced SET processes alongside S_N2/photoinduced SET. Notably, in addition to the N-boryl pyridyl anion (INT-A) and the ate complex (INT-B), the resulting anionic radical (2C) and radical 1E as SEDs also play crucial roles in generating R˙ in the pathways subsequent to the SET of INT-A and INT-B, respectively. MeO⁻ plays a critical role in both the nucleophilic substitution and SET processes through the O^δ−⋯B^δ+ Lewis acid–base interaction, facilitated by decreased electron delocalization from sp² to sp³ of the B center, making it easier to donate electrons.