Clay-supported reagents

Synthesis of aziridines, benzimidazoles, flavones, quinolines thiazoles, etc. using clay and clay-supported reagents 02T1235. 

Several clay-supported reagents have been prepared by treating KIO mont-morillonite with acetone solvate of metal salts and subsequent removal of the solvent under reduced pressure. 

Cornelius, A., and P. Laszlo, Clay Supported Reagents II. Quaternary Ammonium Exchanged Montmorillonite as Catalyst in the Phase Transfer Preparation of Symmetric Formaldehyde Acetals, Synthesis, 162 (1982). 

Varma, R. S. 2002. Clay and clay-supported reagents in organic synthesis. Tetrahedron 52 1235-1255. 

Clay-supported reagent. Thallium(III) nitrate adsorbed on K-10, a mont-inorillonite clay, is a remarkably effective reagent for oxidation. Generally oxidations proceed more cleanly than with TTN in methanol. The thallium(I) nitrate formed is also bound to the support. TTN itself is a trihydrate, but (he. supported reagent appears to be nonhydrated. 

Glycals can be transformed into 1,6-anhydro sugar derivatives by intramolecular cyclization in the presence of Lewis and Brpnsted acids, a reaction that has been termed the intramolecular Ferrier glycosylation.168 Sharma el al.169 showed that a montmorillonite clay-supported silver reagent can be an efficient catalyst for this transformation. The 1,6-anhydro-2,3-dehydro sugars obtained were then selectively dihydroxylated to furnish 1,6-anhydro saccharides. 

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]. 

Solvent-free reactions nsing supported reagents (adsorbed on the snpport, noncovalently linked to it), snch as clays, silica-gel, zeolites and graphite... 

Reaction on solid mineral support. Reaction between supported reagents on solid mineral supports in dry media by impregnation of compounds on alumina, silica or clay takes place. The reactants are impregnated on solid... 

Novel reagents consisting of metallic nitrates impregnated on montmorillonite have been introduced in aromatic nitration. With clay-supported copper(II) nitrate in the presence of acetic anhydride it is possible to nitrate toluene quantitatively with a high para preference (79% para, 20% ortho, 1% meta). Also, good para selectivities have been found on nitration of some other aromatic hydrocarbons with copper(II) nitrate on clay. Halobenzenes are mononitrated with clay-supported copper(II) nitrate in the presence of acetic anhydride. Compared to other methods high para ortho ratios are obtained under these conditions. ... 

The scope of the Ritter reaction can also be extended by avoiding strong acid as the carbenium ion initiator. In at least one case it is possible to omit this reagent entirely because of the facility of 5 1 reaction. Thus, reflux of r-butyl bromide in acetonitrile or propionitrile for 24 h led to formation of A -f-butylamide products. Less commonly used initiators have included cation exchange resins and clayfen. In this latter case, the clay-supported iron(III) nitrate is believed to give rise to nitrosonium ions, which react with the alkyl halide substrate to produce a carbenium ion. Yields of amide from this technique are generally modest (21-35%). 

Recent developments in the design and application of supported reagent catalysts offer considerable potential for clean synthesis. Synergistic effects between the support material and the reagent can lead to unexpectedly high activity. Thus "clayzic" is a potent solid acid catalyst for certain Friedel-Crafts reactions despite the low activity of the individual components. This and other similar clay and silica-based solid acid catalysts have been shown to have many useful applications in liquid phase reactions and some of them have been commercialised as environmentally friendly replacements for conventional acidic reagents. ... 

The use of clay-based supported reagent catalysts in Friedel-Crafts reactions adds a new dimension to this area and has resulted in successful application on an industrial scale. This is discussed in Chapter 4. 

Clays and clay-based supported reagents (see Chapter 4) have become established catalysts in organic synthesis and it is not suprising that they have also become commonly associated with the clean synthesis of organic compounds. Some interesting recent examples are described below. 

The actual chemical nature of the support material may be and often is of direct importance to its usefulness as a support material.15 Silicas can react with small nucleophiles such as F-, OH- and CN. Thus, silica-supported fluorides are inactive, both as nucleophilic fluorinating agents and as bases. Similarly, silicas are not effective support materials for cyanides due to the formation of strong Si-CN bonds. For different reasons, an acidic clay would not be a suitable support for cyanides, due to the possible formation of toxic HCN. Charcoal is the most effective support material for stabilising Cu(I), probably due to its aromatic character.16 For many chemisorbed supported reagent catalysts, silicas are preferred since they give relatively strong surface bonds. However, Si-O-C bonds are hydrolytically vunerable and direct Si-C bonds are preferred.17... 

Inorganic supported reagents such as clayzic (add-treated clay supported zinc chloride)42,93-95 have a limited but valuable range of applications, notably in Friedel-Crafts benzylations (Figure 4.8), where a remarkable synergism between the individually weakly active components results in a very active catalyst. 

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