Silica Chemisorbed Bis(Hydrogensulphato)Benzene (SiO2-BHSB) as a New, Environmentally Benign and Recyclable Catalyst for an Efficient Synthesis of Biscoumarin Scaffolds in Water Based Solvent

Abstract

The immobilization of homogeneous catalytic material over the inert heterogeneous support is a recent strategy to overcome the drawbacks and unite the merits associated with the homogeneous as well as heterogeneous catalysts. However the physisorption-induced immobilization does not serve the purpose because of its sensitive reversible nature, a tiny change in reaction parameters may revert the physisorption and so the immobilization. In this work, a new catalytic material silica chemisorbed bis(hydrogensulphato)benzene (SiO2-BHSB) was achieved through the chemisorption of bis(hydrogensulphato)benzene as an active catalytic part on the surface of porous silica. Structural features, purity, thermal stability, and acid strength of the synthesized SiO2-BHSB material were established by adequate analytical techniques, such as FT-IR, solid-state CP-MAS 13C NMR, solid-state CP-MAS 29Si NMR, EDX, DTG, TGA, and acid–base volumetric studies. An environmentally benign catalytic protocol for the synthesis of biscoumarin scaffolds through a tandem reaction between 4-Hydroxycoumarin and structurally diverse aldehydes was developed in which the synthesized material SiO2-BHSB was observed to work as an efficient and reusable catalyst. The structures of the synthesized biscoumarin derivatives were established from their physical and spectrometric data. The synthesized catalytic material was observed to show sustained catalytic activity even after five cycles of its recovery and reuse. In comparison with the earlier reported methods, a tiny amount (2.5 mol%) of catalyst is sufficient to bring out the transformation smoothly in an aqueous-based solvent, ease of recovery, and reusability of the catalyst are additional salient features of the present protocol.