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

Abstract

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 SH2-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 [π2s + π2s] 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.