Supported Keggin Anion Catalyst: A Study of its Multifunctionality in Retro-Aldol Condensation of 2H-chromenes

Abstract

Retro-Aldol Condensation has emerged as a powerful strategy to produce high-value carbonyl derivatives from complex substrates. In this study, a heterogeneous catalyst was developed by immobilising a Keggin heteropoly anion on an ionic-liquid-functionalised mesocellular foam silica (HPW@MIM–MCF). The catalyst was synthesised through covalent grafting of methylimidazolium groups onto the silica surface, followed by anion metathesis with H_₃PW_₁₂O_₄₀. The material was characterised by N_₂ adsorption–desorption, FT-IR, ^²⁹Si and ^³¹P CP-MAS NMR, SEM-EDS, and XPS analyses. The catalytic activity was evaluated in the reaction between 2H-chromenes and amines, affording Schiff bases via retro-aldol condensation. No nucleophilic activation or recyclization typical products were detected. When o-phenylenediamine and related nucleophiles were employed, the reaction yielded benzimidazole and benzothiazole derivatives, demonstrating the system's dual catalytic behaviour. Finally, catalyst recovery and reuse were investigated over ten consecutive cycles, showing a gradual decrease in activity