Direct Access to π-Extended Phosphindolium Salts by Simple Proton-Induced Cyclization of (o-Alkynylphenyl)phosphanes

A detailed synthetic and mechanistic study for the synthesis of phosphindolium salts from easy accessible (o-alkynylphenyl)phosphanes is reported. Mechanistic investigation indicates a fast protonation at phosphorus as evidenced by the isolation of the phosphonium intermediate, followed by a protophosphonylation reaction across the alkyne moiety. DFT calculations support our mechanistic proposal and indicate a reaction highly exergonic compared to our recently reported phosphinoauration. Photophysical measurements recorded fluorescence quantum yields up to 97 % in solution for the phosphindolium core and fluorescence was observed in the solid state.