Mechanism of Copper(I)-Catalyzed 5‑Iodo-1,2,3-triazole Formation from Azide and Terminal Alkyne

5-Iodo-1,2,3-triazole (iodotriazole) can be prepared from a copper­(I)-catalyzed reaction between azide and terminal alkyne in the presence of an iodinating agent, with 5-protio-1,2,3-triazole (protiotriazole) as the side product. The increasing utilities of iodotriazoles in synthetic and supramolecular chemistry drive the efforts in improving their selective syntheses based on a sound mechanistic understanding. A routinely proposed mechanism takes the cue from the copper­(I)-catalyzed azide–alkyne cycloaddition, which includes copper­(I) acetylide and triazolide as the early and the late intermediates, respectively. Instead of being protonated to afford protiotriazole, an iodinating agent presumably intercepts the copper­(I) triazolide to give iodotriazole. The current work shows that copper­(I) triazolide can be iodinated to afford iodotriazoles. However, when the reaction starts from a terminal alkyne as under the practical circumstances, 1-iodoalkyne (iodoalkyne) is an intermediate while copper­(I) triazolide is bypassed on the reaction coordinate. The production of protiotriazole commences after almost all of the iodoalkyne is consumed. Using ¹H NMR to follow a homogeneous iodotriazole forming reaction, the rapid formation of an iodoalkyne is shown to dictate the selectivity of an iodotriazole over a protiotriazole. To ensure the exclusive production of iodotriazole, the complete conversion of an alkyne to an iodoalkyne has to, and can be, achieved at the early stage of the reaction.