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 () and the ate complex (), the resulting anionic radical () and radical as SEDs also play crucial roles in generating in the pathways subsequent to the SET of and , 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.