Radical/Ylide-Tunable Pyridinium Salt Enables Photo/Thermocatalytic Divergent [3 + 2] Cycloadditions

Pyridinium salts are valuable building blocks in synthetic design, yet most derivatives display only one reactive mode. We report an α-carboxyalkylpyridinium salt that functions as a dual mode reagent, acting either as a photoactivated radical precursor or as a thermally generated pyridinium ylide. The salt is readily prepared from amino acid and permits divergent [3 + 2] cycloadditions by simply changing the energy input (visible light versus heat). Two photoinduced electron transfer pathwaysintermolecular (with the photocatalyst) and intramolecular (within the pyridinium salt)generate different radical intermediates, which subsequently undergo distinct reaction sequences, affording the same γ-butyrolactone. Thermal activation induces decarboxylation to pyridinium ylides, allowing access to tetrahydroindolizines via 1,3-dipolar cycloaddition, which subsequently undergo an unprecedented photocatalytic 1,2-phenyl migration to yield fluorescent indolizines.