Friedel–Crafts Alkylation of Indoles Exclusively in Water Catalyzed by Ionic Liquid Supported on a Polyacrylonitrile Fiber: A Simple “Release and Catch” Catalyst

Abstract

A series of ionic liquids supported on polyacrylonitrile fibers (FSILs) served as “release and catch” catalysts to mediate the Friedel–Crafts alkylation of indoles exclusively in water. Detailed characterization by elemental analysis, FTIR spectroscopy, and SEM confirmed that a significant amount of ionic liquid was immobilized on the surface layer of the fiber, and excellent results, in terms of the simple procedure, high yields (87–96 %), and superior catalytic recyclability (over 10 cycles), are reported for the metal and cosolvent‐free synthesis of a wide variety of bis(indolyl)methanes at room temperature. The reason why the reaction catalyzed with FSILs only proceeded in water is explained, and a “release and catch” catalytic mechanism is proposed. Moreover, the procedures can be scaled up efficiently, and the solvent can be recovered and reused without contamination. The prominent features of the fiber catalyst are very attractive for fixed‐bed reactors in the chemical industry.