Synthesis of indole derivatives from the S3•−-mediated intramolecular cyclization of o-alkynylanilines

A new method for the synthesis of 2-(phenyl)indole via the cyclization of 2-(phenylethynyl)aniline in the presence of K₂S was established, which was used to prepare 2-(phenyl)indole derivatives in high yields (66%-93%). Mechanistic studies have revealed that a trisulfur radical anion (S₃^•−) is generated from K₂S, which acts as an initiator for the intramolecular addition of the alkyne and the amino group in o-alkynylaniline, ultimately leading to the formation of the indole derivative. This method provides a new way to prepare 2-(phenyl)indoles and a novel application of S₃^•− in organic synthesis.