Ruthenium-catalyzed heteroannulation of anilines with alkanolammonium chlorides leading to indoles

Anilines react with alkanolammonium chlorides in an aqueous medium (H₂O-dioxane) at 180°C in the presence of a catalytic amount of a ruthenium catalyst together with SnCl₂·2H₂O to afford the corresponding indoles in moderate to good yields. Especially, when triisopropanolammonium chloride is employed to react with anilines, 2-methylindoles are formed regioselectively. The presence of SnCl₂·2H₂O is necessary for the effective formation of indoles. A reaction pathway involving alkanol group transfer from alkanolamines to anilines, N-alkylation of anilines by anilinoalkanols and heteroannulation of 1,2-dianilinoalkanes is proposed for this catalytic process.