Silica gel-supported reagents

The procedure is commendable for its sinq>licity, reduced toxicity (chromium in all its oxidation states is carcinogenic) and achieves good yields of ketones from alcohol, for example, octan-2-ol is oxidized into octan-2-one (92%), cyclohexanol into cyclohexanone (90%) and menthol into menthone (98%). Pyridinium chromate is also a well-known oxidant for allylic oxidations. As a silica gel supported reagent, this is turned into an efficient alcohol oxidant that will leave acid-labile functions unscathed. Another advantage of the reagent is the long shelf-life of more than a year. These solid-supported oxidants also greatly facilitate pr uct work-up, when compared with their solution counterparts. 

Simple and rapid deacetalation was accomplished by Couri et al. [26] by irradiation in a domestic oven under solvent-free conditions with silica gel-supported reagents. 

The efficient solvent-free microwave-assisted procedure for C-alkyl pyrroles synthesis using silica gel-supported reagent has been reported. Thus, a general procedure for the Michael addition of pyrroles with electron-deficient olefin has been developed. In the case of the moderately bulky electron-deficient olefins, a catalytic amount of BiCls has to be employed to promote the reaction. The experimental results indicated that BiCla exhibited a high catalytic activity and did not induce polymerization and polyalkylation under microwave (Equation 5) [26a]. 

The oxidation of benzyl alcohols to benzaldehydes, which is difficult with Jones reagent, can be achieved by using silica gel-supported Jones reagent in CH2CI2. 

All, M. H., Wiggin, C. J. Silica gel supported Jones reagent (SJR) a simple, versatile, and efficient reagent for oxidation of alcohols in non-aqueous media. Synth. Common. 2001, 31, 3383-3393. 

Salehi, P., Motlagh, A. R. Silica gel supported ferric perchlorate a new and efficient reagent for one pot synthesis of amides from benzylic alcohols. Synth. Common. 2000, 30, 671-675. 

The synthesis of imidazoles is another reaction where the assistance of microwaves has been intensely investigated. Apart from the first synthesis described since 1995 [40-42], recently a combinatorial synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles has been described on inorganic solid support imder solvent-free conditions [43]. Different aldehydes and 1,2 dicarbonyl compounds 42 (mainly benzil and analogues) were reacted in the presence of ammonium acetate to give the trisubstituted ring 43. When a primary amine was added to the mixture, the tetrasubstituted imidazoles were obtained (Scheme 13). The reaction was done by adsorption of the reagent on a solid support, such as silica gel, alumina, montmorillonite KIO, bentonite or alumina followed by microwave irradiation for 20 min in an open vial (multimode reactor). The authors observed that when a non-acid support was used, addition of acetic acid was necessary to obtain good yields of the products. 

In this chapter, we demonstrate the potential of such agents as catalysts/promoters in key steps for the derivatization of sugars. The most significant catalytic approaches in carbohydrate chemistry that use aluminosilicate porous materials, namely zeolites and montmorillonite clays, are reviewed and discussed. Silica gel is a porous solid silicate that has also been used for heterogeneous catalysis of organic reactions in general. We include here its usefulness as promoter and reagent support for the reactions under consideration. 

The silica gel network readily retains water molecules. Therefore, the presence of hydrates or silicic acid is ascribed to be the support reagents responsible for mediating a chemical transformation.27... 

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