Silica-Supported Perchloric Acid (HClO₄-SiO₂)

Introduction

Heterogeneous catalysts have gained much importance in recent years due to economic and environmental benefits. [¹] These catalysts make synthetic processes clean, safe, high-yielding, and inexpensive. [²] A tremendous interest has sparked in various chemical transformations promoted by catalysts under heterogeneous conditions. Recently, silica-supported perchloric acid (HClO₄-SiO₂), synthesized by Chakraborthi et al. [³] has been found to be an efficient and recyclable heterogeneous catalyst for various organic transformations, such as synthesis of bis-indolylmethanes, bis-indolylglycoconjugates, 1,4-dihydropyridines, coumarins via Pechmann condensation, 1,3-dithiolane/dithiane, transformation of thioglycosides to their corresponding 1-O-acetates, Ferrier rearrangement of glycols, acetal/ketal formation and chemoselective ­carbon-sulfur bond formation. It is also used in carbon-carbon formation between aldehydes and cyanides. Further, it is easy to handle, stable, and environmentally safe. Thus, silica-supported perchloric acid is a versatile organic reagent used as an acid catalyst.

For the preparation of silica-supported perchloric acid HClO₄ (as a 70% aqueous solution) was added to the suspension of silica gel in diethyl ether. The mixture was concentrated and the residue heated at 100 ˚C for 72 h ­under vacuum to afford HClO₄-SiO₂ as a free flowing powder. [³]

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