A Brønsted‐Acidic Ionic Liquid Gel as an Efficient and Recyclable Heterogeneous Catalyst for the Synthesis of Bis(indolyl)methanes under Solvent‐Free Sonication

A Brønsted‐acidic ionic liquid gel was prepared from 1‐(4‐sulfonic acid)butyl‐3‐methylimidazolium hydrogen sulfate and tetraethoxysilane. Its structural characterization was accomplished by FTIR, TGA, SEM, EDS, and ICP‐OES. The ionic liquid gel promotes the synthesis of bis(indolyl)methanes. Interestingly, this catalyst allows the preparation of bis(indolyl)methanes in good to excellent yields within a short reaction time, through a clean and straightforward procedure. Moreover, the ionic liquid gel can be recovered by filtration and can be reused up to 5 times without any detectable loss of catalytic performance. Additionally, the method has a wide substrate scope and provides an accessible route for the large‐scale direct synthesis of bis(indolyl)methanes. More than simplicity: A clean, straightforward, and efficient ultrasound‐assisted synthesis of bis(indolyl)methanes using a Brønsted‐acidic ionic liquid gel as a heterogeneous catalyst has been developed. Not only is this protocol prominent owing to a simple work‐up step and an easily recovered catalyst, but it also exhibits remarkable versatility on various substrates to afford the desired products in excellent yields (up to 98 %).