Annulation of α-Diazo Sulfonium Salts with α-Vinylanilines via a Dicationic Intermediate toward Quinolines

α-Diazo sulfonium salts constitute a unique subclass of diazo reagents that integrate diazo and sulfonium functionalities within a single molecular scaffold. While their reactivity via carbene, radical, or carbyne intermediates has been extensively investigated, the synthetic utility of their proton-activated dicationic species remains largely unexplored. Herein, we report a Brønsted acid-promoted, metal-free annulation of α-diazo sulfonium salts with binucleophilic α-vinylanilines. This transformation proceeds through a protonation-induced polarity inversion at the diazo carbon, generating a dicationic intermediate bearing two electrophilic centers. Sequential intramolecular nucleophilic substitution of this intermediate enables efficient access to mono-, di-, and trisubstituted quinolines under mild conditions. This study showcases a rare example of dielectrophilic activation in diazo chemistry and establishes a complementary strategy for constructing heterocycles beyond conventional carbene-based pathways.