Brønsted Acidic Ionic Liquid: An Efficient Organocatalyst for the Synthesis of Pyrrolo[1,2‐a]indoles under Neat Conditions

Abstract

A new synthetic approach has emerged for constructing the 9H-pyrrolo[1,2-a]indole scaffolds by the reactions between indoles and chalcones under metal and solvent-free conditions at 80 °C. The reaction occurs smoothly in presence of Brønsted acidic ionic liquid named 1-butane sulfonic acid-3-methylimidazolium tosylate, [BSMIM]OTs (BAIL) as a catalyst, enabling the synthesis of the desired products with satisfactory yields. The developed protocol is applicable for the construction of biologically important pyrrolo[1,2-a]indole derivatives from easily accessible chalcones having various substituents. The process commenced with Michael addition to chalcones, followed by annulations induced by the elimination of a water molecule yielded the 9H-pyrrolo[1,2-a]indole scaffolds. Few control experiments were carried out for better understanding the reaction pathway. The feasibility of catalyst recycling was also demonstrated. This method produces only water as the byproduct and represents a green synthetic protocol. Clean reaction, easily accessible reactants, and metal and solvent-free and environmentally friendly reaction conditions are the notable advantages of this procedure.