Catalyst- and oxidant-free electrooxidative site-selective [3/4 + 2] annulation to fused polycyclic heteroaromatics

Abstract

Annulation π-extension of arenes with alkynes is a straightforward and practical strategy for the rapid construction of polycyclic aromatic compounds. Recently, some intriguing transition-metal-catalyzed polycyclic arene syntheses were described. However, metal catalysts and exogenous oxidants were essential, which led to undesired by-products, low atom-economy and potential explosion risk. Herein, a straightforward electrochemical oxidation [3 + 2] or [4 + 2] annulation of imidazo[1,2-a]pyridines with alkynes to heterocyclic aromatic compounds under metal- and exogenous oxidant-free conditions has been accomplished. This green and sustainable methodology features excellent regioselectivity, wide substrate adaptability and good functional group compatibility. Moreover, a possible mechanism has been proposed on the basis of radical-trapping, isotope-labelling, KIE and cyclic voltammetry experiments.