Photoinduced Cobaloxime Catalysis for Oxidant-Free Radical Cyclization of 2-Methylthiolated Alkynones to Access Phosphorylated Thiochromones

An oxidant-free radical phosphinylation/cyclization of 2-methylthiolated phenylalkynones with secondary phosphine oxides by photoinduced cobaloxime catalysis is described. The reaction tolerates a wide range of substrates, particularly those that are sensitive to chemical oxidation, affording various phosphorylated thiochromones in good yields with methane evolution. The synthesis of new monophosphine and phosphine-olefin ligands as well as the construction of an iridium complex highlight the great potential applications of this approach. In addition, preliminary mechanistic studies were performed to elucidate the reaction mechanism.