Reactions on Clay

It has been a long time since mineral clays like montmorillonite were discovered to have strong acidity in the solid state. Moreover, clays had been utilized as catalysts for catalytic cracking to produce gasoline before amorphous silica-alumina and zeolites were invented. At the present time, besides use as catalysts, clay is being utilized in a variety of fields medicine, paint, cosmetics, detergent, and casting. 

Aldol Condensation of Enolsilanes with Aldehydes and Acetals 

The aldol process constitutes one of the fundamental bond constructions in both organic synthesis and biosynthesis. This reaction is well recognized as the most accessible bond forming reaction for the creation of 1,3-0,0 heteroatom-heteroatom relationships in carbon chain molecules 63-66). 

In recent organic synthesis, stereoselective aldol condensations has been performed under two different conditions. Under the influence of acid, stabilized enol derivatives, enolsilanes (M = SiMe3), can condense with aldehydes or acetals in a stereoselective fashion [Eq. (12)]. In this reaction the role of the acid is to activate aldehydes or acetals. Alternatively, under basic conditions, the same process can be carried out directly with aldehydes and reactive, preformed metal enolates (M = Li, MgL, ZnL, AIL2, BL2, etc.) of defined geometry. 

A number of methods that utilize enolsilanes directly in the aldol process with either aldehydes or acetals have been developed recently. These reactions are usually performed either in the presence of Lewis acids such as titanium tetrachloride (67) or with fluoride ion (68). Recently trimethylsilyl triflate (CF3S03SiMe3) was found to be an efficient acid catalyst for condensation 

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Scheme 6.47 Synthesis of 2-aryl-l, 2,3,4-tetrahydro-4-quinolones via cyclization reaction on clay.

Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids (Sect. 6.3), accepting electron pairs. Catalytic reactions on clay surfaces involve surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which is affected by the type of clay and the clay-saturating cation involved in the reaction. Dissociation of water molecules coordinated to surface, clay-bound cations contributes to the formation active protons, which is expressed as a Bronsted acidity. This process is affected by the clay hydration status, the polarizing power of the surface bond, and structural cations on mineral colloids (Mortland 1970, 1986). On the other hand, ions such as A1 and Fe, which are exposed at the edge of mineral clay coUoids, induce the formation of Lewis acidity (McBride 1994). 

Chaussidon J, Calvet R (1974) Catalytic reactions on clay surfaces. Third int congr of pesticides chemistry (lUPAC), Helsinki In Coulton E, Albaky NY, Konle E (eds) Environmental quaUty and safety, vol 3. Gerg Thiem. Stuttgart... 

Sparks et al. (1980b) introduced a continuous flow method (next subsection) that is quite similar in principle to liquid-phase column chromatography. This method was used to study potassium adsorption dynamics on soils and clay minerals (Sparks and Jardine, 1981 Sparks and Rechcigl, 1982 Jardine and Sparks, 1984 Ogwada and Sparks, 1986a,b,c), silicate sorption on soils (Miller et al., 1986), S04 sorption and desorption on soils (Hodges and Johnson, 1987), and Al reactions on clay minerals and peat (Jardine etal., 1985a). 

Laszlo, P. 1986a. Chemical reactions on clays. Science 235 1473-1477. 

Reactions on clay catalysts show some selectivity by size and shape. Aluminum montmorillonite was used to rearrange phenyl ethers (6.45).246 The bulkier 7ert-butyl group led to only the para product. Allylation of aromatic compounds with octenols (6.46) gave only the unbranched isomer.247 Aniline can be ethylated with ethanol to give 86% TV-ethylaniline and 14% TVW-diethylaniline in 52% conversion with K10 montmorillonite with, or without, vanadium oxide at 400°C.248... 

Scheme 2.2-36 Beckmann rearrangement reaction on clay surface.

Finally, the experimental results of electrically enhanced transformations on clay surfaces were presented and discussed in the framework of an electrochemical model. The data appeared to support the hypothesis that faradic current passage orthogonal to the planes in the electric double layer of clay particles may drive forth redox reactions on clay surfaces. 

Metal sorption reactions on clay minerals, (hydr)oxides, humic substances and soils is usually characterized by a rapid, followed by a slow, reaction (Figure... 

Microwave activation of organic reactions on clay-adsorbed compounds Rearrangement of pinacol to pinacolone intercalated in montmoriUonite Gutierrez et al. (137)... 

Dabbagh, H. A. Teimouri, A. Chermahini, A. N. Green and efficient diazotization and diazo coupling reactions on clays. Dyes Pigments2007,73,239-244. 

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