Heterogeneous visible-light promoted dehydrogenative [4 + 2] annulation of benzothioamides and alkynes under aerobic conditions†
The dehydrogenative [4 + 2] annulation is a step- and atom-economical way to construct benzo-fused six-membered rings which are prevalent in natural products, pharmaceuticals and agrochemicals. 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 the metal-free heterogeneous photocatalyst graphitic carbon nitride (g-C3N4). The heterogeneous photocatalysis proceeded smoothly with simple operation and under 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 a single electron transfer process. The radical intermediate further mediated a [4 + 2] annulation with alkynes to produce isothiochromenes.