Light Mediated Sulfonylation/C–H Activation/Annulation of Benzimidazoles Derivatives Catalyzed by Diarylmethylium Tetrafluoroborate Salts: An Experimental and Computational Study

In this article, diarylmethylium tetrafluoroborates are exploited as catalysts in a visible light promoted domino sulfonylation/C–H activation/annulation of N-alkynylbenzimidazoles producing polycyclic sulfonyl-benzimidazoles. Annulated benzimidazoles are important structures in pharmaceutical research and drug discovery known for their biological and medicinal properties. More than 20 polycyclic sulfonyl-benzimidazoles, including azepines, are obtained in good to excellent yields, in mild and metal free conditions. A mechanism proposal, based on both experimental and computational studies, establishes the role of the photoactive ion couple formed between the cation catalyst and the sulfinate in producing the key sulfonyl radical. The latter adds to the triple bond thus promoting a domino process in which the acid additive has a fundamental role in the hydrogen transfer. In addition, an acid-catalyzed proton-coupled electron-transfer closes the catalytic cycle regenerating the diarylcarbenium salt thus yielding the desired product.