Silica-supported reagents

Silica-supported reagents have been exploited as nontoxic, inexpensive, reusable, and environmentally acceptable catalysts for developing stoichiometric reaction methods in organic chemistry and specifically in carbohydrate chemistry. Apart from being easy to handle and to store, these systems allow facile workup, the catalyst being removed by simple filtration, and the reaction products isolated by chromatographic purification, if necessary. 

Bram, G. and Loupy, A. Silica-supported reagents reactions in dry media, in Preparative Chemistry using Supported Reagents, R Laszlo (Ed.), Academic Press Inc., London, 1987, p. 387-400. 

Polmer-snpported IBX Reagents. Two different phenoxide linked polymer-based IBX reagents have been developed. The silica supported reagent (Poly-IBX) is used in THE The oxidation rate increases relative to DMSO as solvent. The primary alcohol in the cyclohexanol may be oxidized preferentially, and thus avoid formation of the cyclohexanone expected from the secondary alcohol (eq 8). This selectively is retained even with a threefold... 

A silica-supported palladium reagent has been used to convert iodobenzene to butyl benzoate, in the presence of CO and butanol. Diaryl ketones can also be... 

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... 

Most of the parallel reactions described in Schemes 4.23 and 4.24 were performed as dry-media reactions, in the absence of any solvent. In many cases, the starting materials and/or reagents were supported on an inorganic solid support, such as silica gel, alumina, or clay, that absorbs microwave energy or acts as a catalyst for the reaction (see also Section 4.1). In this context, an interesting method for the optimization of silica-supported reactions has been described [83], The reagents were co-spotted neat or in solution onto a thin-layer chromatographic (TLC) plate. 

The quest for a solvent-free deprotection procedure has led to the use of relatively benign reagent, ammonium persulfate on silica, for regeneration of carbonyl compounds (Scheme 6.10) [48]. Neat oximes are simply mixed with solid supported reagent and the contents are irradiated in a MW oven to regenerate free aldehydes or ketones in a process that is applicable to both, aldoximes and ketoximes. The critical role of surface needs to be emphasized since the same reagent supported on clay surface delivers predominantly the Beckmann rearrangement products, the amides [49]. 

Bound reagents are an excellent alternative in cases where the reagent is used in excess and can be difficult to remove. Purification is now a simple process of filtration and evaporation. For example, silica-supported aluminium chloride is a Lewis acid and an effective catalyst for Friedel-Crafts alkylations (Figure 3.15). 

Figure 3.14 A supported reagent, in a manner similar to a scavenger, consists of a reactive functional group grafted onto insoluble silica. The spent reagent is easily removed by filtration.

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