Visible-Light Photocatalyzed C3–H Alkylation of 2H-Indazoles/Indoles with Sulfoxonium Ylides via Diversified Mechanistic Pathways

Herein, the C3–H alkylation of 2H-indazoles and indoles with sulfoxonium ylides is developed under visible-light photocatalysis. This protocol employs easily accessible reagents, and a wide range of 2H-indazoles, indoles, and sulfoxonium ylides are suitable for this reaction to afford the desired products under benign conditions. Synergistic experimental and computational studies suggest that the sulfoxonium ylides involving C3–H alkylation of 2H-indazoles and indoles under visible-light photocatalysis could proceed via different mechanistic pathways. For the C3-alkylation of 2H-indazoles, a triplet energy transfer mechanistic pathway of 2H-indazoles is proposed for quenching the excited photocatalyst. Subsequently, the formed excited triplet state of 2H-indazoles could undergo radical attack on the C═S moiety of sulfoxonium ylides. After the dissociation of DMSO and 1,2-H migration, the final product of C3-alkylation of 2H-indazoles could be yielded. However, such a mechanistic pathway is not applicable for indoles. Instead, sulfoxonium ylides could be converted to a C-centered radical in the presence of KH2PO4 under visible-light photoredox conditions. The formed C-centered radical can attack the C3-site of indoles and thus lead to the C3-alkylation product of indoles.