Photoinduced C-Br bond cleavage enables difluoroketene-mediated ground-state [2 + 2] cycloaddition: a TD-DFT study

This work presents a TD-DFT computational study of the visible-light-driven [2 + 2] cycloaddition between alkynes and bromodifluoroacetylsilanes, leading to gem-difluorocyclobutenones. Visible-light irradiation of bromodifluoroacetylsilanes was recently shown by Zhou et al. to generate difluoroketene in situ for [2 + 2] cycloaddition with alkynes; the electronic origin of this photoactivation, however, had not been established. The results show that irradiation at 450 nm is vertically resonant only with the T₁ triplet state of the brominated precursor, consistent with heavy-atom-enhanced spin-orbit coupling, although the specific population mechanism remains unresolved. On the triplet surface, barrierless C-Br homolysis generates an acyl radical and Br˙. We propose a S_H2-type radical fragmentation mechanism in which the bromine radical attacks silicon, releasing difluoroketene through a low-barrier (ΔG^‡ = 5.7 kcal mol^-1) and highly exergonic process (ΔG = -25.0 kcal mol^-1). The resulting difluoroketene subsequently undergoes a concerted, asynchronous, and barrier-moderate [π2_s + π2_s] cycloaddition with several alkynes on the ground-state surface (ΔG^‡ ≈ 15.3-16.5 kcal mol^-1), producing highly stable cyclobutenone products (ΔG ranging from -57.0 to -59.0 kcal mol^-1). ELF analysis indicates a sequential bond-formation process initiated by nucleophilic attack at the electrophilic ketene carbonyl carbon. Overall, this study provides an electronic-structure and mechanistic analysis of photoexcitation, C-Br bond cleavage, silicon-mediated radical fragmentation, and ground-state ketene cycloaddition for this transformation, offering a computational basis to support the design of visible-light-driven fluorinated [2 + 2] cycloadditions.