Silica-Gel Supported Heteropoly Acid

P205/Si02 was used for the solvent-free nitration of aromatic compounds. Activated aromatic compounds were converted into the corresponding nitro-aromatic compounds at room temperature under solvent-free conditions in excellent yields in 2-7 min, and deactivated aromatic compounds were converted into nitroben-zenes in moderate to good yields in 15-20 min [125], 

NaHS04/Si02, which has been used as a catalyst for the selective acetylation of diols [126], has been used more recently for the synthesis of heterocyclic compounds. For example, a three-component reaction for the synthesis of quinazoli-nones was catalyzed with NaHS04/Si02 [127]. 

Phosphoric acid immobilized on silica (H3P04/Si02) catalyst has been widely exploited over many years [128-130]. It has also been used as a reusable catalyst for solvent-free imino-aldol three-component coupling reaction in recent years [131]. 

Silica-gel supported polyphosphoric acid (PPA/Si02) has been developed as a reusable catalyst-oxathio- and dithio- acetals were synthesized by using PPA/Si02. The recovered catalyst can be reused nine successive times, after drying each time at 100 °C for 2 h under reduced pressure [60], 

Many reactions in organic synthesis are reactions of nucleophiles and electrophiles, in other words they are acid-base reactions. Usually, the nucleophilicity or the electrophilicity of a substrate is enhanced by strong acid or base to promote a reaction. However, a reaction can be also promoted when weak acid and base activate both a nucleophile and an electrophile at the same time. In a homogeneous medium, acid and base could not coexist because of neutralization. Therefore, it is not expected to promote a reaction by acid and base cooperation. However, acid and base supported on an inorganic solid make it possible to promote a reaction by acid and base cooperation. 

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