Visible-light-driven Thio-Carboxylation of Alkynes with CO2: Facile Synthesis of Thiochromones

Difunctionalizing carboxylation of alkynes with CO2 is a sustainable and important strategy to generate valuable acrylate derivatives from both readily available starting materials. Such protocols, however, always suffer from the use of excess metallic reagents and transition metal residue. Herein, we report the first thio-carboxylation of alkynes with thiophenols and CO2, which is a visible-light-driven and transition metal-free process. In contrast to previous carboxylations of alkynes via two-electron activation of CO2, mechanistic and computational investigations suggest that the single-electron activation of CO2 is involved in the thio-carboxylation, rendering unique β-carboxylation. The following cyclizing acylation affords important thiochromones efficiently. Moreover, the one-pot method features mild reaction conditions (room temperature, 1 atmosphere of CO2), high chemo- and regio-selectivity, easy scalability and facile derivatization of products to bioactive compounds.