Heterogeneous visible-light promoted dehydrogenative [4+2] annulation of benzothioamides and alkynes under aerobic conditions

Abstract

The dehydrogenative [4+2] annulation is a step and atom-economical way to construct benzo-fused six-membered ring which are prevalent in natural products, pharmaceuticals and agrochemicals. Traditional protocols often require C-H activation by transition metal catalyst, excessive amounts of metal oxidants and high reaction temperature. Herein, we disclosed a visible-light induced dehydrogenative [4+2] annulation of benzothioamides and alkynes, which was performed under mild aerobic conditions and catalyzed by a metal-free heterogeneous photocatalyst g-C3N4. The heterogeneous photocatalysis proceeded smoothly under simple operation and mild conditions and gave a variety of isothiochromenes in good yields. g-C3N4 displays excellent stability and recyclability for the photochemical reaction. The detailed mechanistic studies revealed that g-C3N4 promoted the formation of a sulfur centered radical under visible-light irradiation via single electron transfer process. The radical intermediate further mediated a [4+2] annulation with alkynes to produce isothiochromenes.