Facile Synthesis of 1H-Indoliums via Silica Gel-Assisted Silver-Catalyzed Intramolecular Hydroammoniumation of ortho-Alkynylanilines

Abstract 1H-Indoliums, a class of cationic heterocycles, with potential industrial applications have been used as components in capacitors. In this study, we describe a facile method for the synthesis of indoliums using silica gel-assisted silver-catalyzed intramolecular hydroammoniumation of ortho-alkynylanilines. The cyclization of alkynylanilines in the presence of AgOTf (5 mol%), LiOTf (1 equiv.), and SiO2 [SiO2/AgOTf (w/w: 199/1)] in CH2Cl2 at room temperature under aerobic conditions afforded indoliums in good-to-excellent yields. This reaction is an effective and novel method, in which the 5-endo-dig cyclization reaction to obtain indoliums is promoted only by the addition of SiO2 in the presence of a silver catalyst. This novel methodology offers a practical and effective route for the synthesis of indoliums under mild conditions.