Reaction of Phosphines with 1-Azido-(2-halogenomethyl)benzene Giving Aminophosphonium-Substituted Indazoles.

The reaction between a 1-azido-(2-halogenomethyl)benzene and a phosphine gives different products depending on the nature of the halogen, the phosphine itself, and the solvent employed. While PPh3 (2 equiv) reacts with the chloro reagent in toluene to give the expected iminophosphorane-phosphonium adduct, trialkylphosphines (PCy3 and PEt3) surprisingly furnish an aminophosphonium substituted by a zwitterionic indazole. The bicyclic product can also form from PPh3 using the bromo reagent in acetonitrile. A mechanism is proposed for this cyclization based on DFT calculations.