From Photoexcitation to Cycloaddition: Computational Mechanism of gem-Difluorocyclobutenone Formation

This work presents a comprehensive TD-DFT computational investigation of the visible-light- driven [2+2] cycloaddition between alkynes and bromodifluoroacetylsilanes leading to gem- difluorocyclobutenones. The results show that irradiation at 450 nm selectively populates the T1 triplet state of the brominated precursor, a process strongly facilitated by the bromine heavy-atom effect. On the triplet surface, barrierless C–Br homolysis generates an acyl radical and Br• . We propose a SN 2-type radical fragmentation mechanism in which the bromine radical attacks silicon, releasing difluoroketene through a low-barrier (∆G‡ = 5.7 kcal mol−1 ) and highly exergonic process (∆G = −25.0 kcal mol−1 ). The resulting difluoroketene subsequently undergoes a concerted, asyn- chronous, and almost barrierless [π2s + π2s ] cycloaddition with several alkynes on the ground-state surface (∆G‡ ≈ 15.3–16.5 kcal mol−1 ), producing highly stable cyclobutenone products (∆G ranging from −57.0 to −59.0 kcal mol−1 ). ELF analysis indicates a sequential bond-formation process initi- ated by nucleophilic attack at the electrophilic ketene carbonyl carbon. Overall, this study provides a comprehensive mechanistic framework connecting photoexcitation, C–Br bond cleavage, silicon- mediated radical fragmentation, and ground-state ketene cycloaddition, offering a computational basis to support the design ofvisible-light-driven fluorinated [2+2] cycloadditions.