Iodine-Catalyzed Oxidative [3+2] Annulation/Dehydrogenation Cascade: Synthesis of Unexpected Mesoionic [1,2,4]Triazolo[5,1‑a]isoquinoliniums

A metal-free, iodine-catalyzed protocol has been established for constructing rarely reported mesoionic [1,2,4]triazolo[5,1-a]isoquinoliniums, using CHP as an oxidant. This synthesis is achieved via a formal [3+2] annulation between 3,4-dihydroisoquinolin-2(1H)-amines and isocyanates. The operationally simple, one-pot process proceeds under mild conditions through a cascade sequence of hydroamination, intramolecular cross-dehydrogenative coupling (CDC), and oxidative dehydrogenation. It features broad substrate tolerance and high atom and step economy. Furthermore, preliminary mechanistic studies indicate that the transformation likely proceeds via a hypoiodite-mediated pathway.