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

A Bronsted-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.