Halogen Bond-Catalyzed Oxidative Annulation of N-Alkyl Pyridinium Salts and Alkenes with Air as a Sole Oxidant: Metal-free Synthesis of Indolizines

The utilization of ambient air as a sole oxidant is highly desired for the sustainability of chemical industry. Herein, we reported a mild and metal-free synthesis of indolizines using pyridinium salts and alkenes with ambient air as a sole oxidant. In this reaction, a halogen bond donor (perfluoro-1-iodooctane, 10 mol%) was successfully applied to catalyze this desired oxidative [3+2] annulation. The C-I bond of halogen bond donor is crucial for this transformation, while the corresponding C-Br bond and C-F bonds gave much lower yields. Moreover, the phenolic hydroxyl increases the amount of side-product, probably due to the strong interaction of phenolate anion and the halogen bond donor. Finally, with the rapid development of halogen bond catalysis, this halogen bond-catalyzed strategy could continuously promote the synthesis of indolizines in a sustainable manner.