Triplet Energy Transfer (EnT)-Promoted 1,3-Dipolar Cycloaddition Reactions of <i>N</i>-(Trimethylsilyl)methylphthalimide with Electron-Deficient Alkynes and Alkenes

Triplet energy transfer (EnT)-promoted photochemical pathways, arisen by visible light and its photosensitizers, have gained significant attention as a complementary strategy for initiating organic transformations in photochemical reactions that are unlikely to occur through a single electron transfer (SET) process. In the present study, we investigated the triplet EnT-promoted 1,3-dipolar cycloaddition reactions of <i>N</i>-(trimethylsilyl)methylphthalimide with electron-deficient alkynyl and alkenyl dipolarophiles. The triplet excited state of <i>N</i>-(trimethylsilyl)methylphthalimide, promoted by the triplet EnT from thioxanthone (TXA) photosensitizer, underwent sequential intramolecular SET and carbon-to-oxygen migration of the silyl group to form azomethine ylide. This generated ylide cycloadded to alkynes or alkenes to regioselectively and stereospecifically produce a nitrogen-containing benzopyrrolizidine scaffold with multiple stereogenic centers. Crucially, the stereoselectivity of these cycloaddition reactions (i.e., endo versus exo addition) was influenced by the nature of the dipolarophiles.