Clays reactions catalyzed

Rearrangement reactions catalyzed by the clay surface were observed for par-athion (an organophosphate pesticide) when it was adsorbed on montmorillonite or kaolinite in the absence of a liquid phase. The rate of rearrangement reactions increased with the polarization of the hydration water of the exchangeable cation (Mingelgrin and Saltzman 1977). Table 14.1 summarizes a series of reactions catalyzed by clay surfaces, as reported in the literature. 

Table 14.1 Selected examples of reactions catalyzed by clay surfaces (Wolfe et al. 1990)...

Nonstereospecific aerobic epoxidations of alkenes in the presence of aldehydes catalyzed by nickel(II),298 iron(in),299 and cobalt(II)300 complexes, and clay-supported nickel acetylacetonate301,302 have been reported. A radical mechanism has been postulated. The involvement of active copper species and peracids were suggested in a similar reaction catalyzed by copper salts.303... 

Pyrroles and indoles are alkylated by electrophilic alkenes in reactions catalyzed by acidic clays, which are often more effective than Lewis acids (88TL2577). Indoles are alkylated as usual at C-3, and the alkylation of 3-substituted indoles has also been shown to take place at C-3 with subsequent migration occurring to C-2. 

Phenols react with cinnamyl compounds via an intermolecular condensation/cyclization providing a reliable method for the construction of dihydrocoumarins. Montmorillonite K-10 clay can catalyze the condensation/cyclization reaction of phloroglucinol and cinnamoyl chlorides to provide a high-yielding route to dihydrocoumarins (Equation 416) <2001S2247>. 

Molecular iodine-promoted Michael addition is a simple and efficient method for generating 2-pyrrolyl-2-phenyl-l-nitroalkanes in good yields (Scheme 67) [86]. Cr+3-Catsan (Cr+3 exchanged commercially available montmorillonite clay) and ZnCl2, which were first used as Lewis acids for Michael reactions of pyrrole, showed different selectivity under the same conditions [221], In general, while the reactions catalyzed by Cr+3-Catsan... 

Choi and Chung [16] were the first to prepare phenolic resin/layered sihcate nanocomposites with intercalated or exfoliated nanostructures by melt interaction using linear novolac and examined their mechanical properties and thermal stability. Lee and Giannelis [10] reported a melt interaction method for phenolic resin/clay nanocomposites, too. Although PF resin is a widely used polymer, there are not many research reports on PF resin/montmorillonite nanocomposites, and most of the research investigations have concentrated on linear novolac resins. Up to now, only limited research studies on resole-type phenolic resin/layered silicate nanocomposites have been published [17-19] and there is still no report on the influence of nano-montmorillonite on phenolic resin as wood adhesive. Normally H-montmorillonite (HMMT) has been used as an acid catalyst for the preparation of novolac/layered silicate nanocomposites. Resole resins can be prepared by condensation reaction catalyzed by alkaline NaMMT, just as what HMMT has done for novolac resins. 

Hydroxypyridone ring synthesis. 80 g. phenothiazine, diethyl n-butylmalonate, and unglazed clay plate fragments heated 22 hrs. at 277° crude l-keto-2-n-butyl-3-hydroxy-lH-pyrido[3,2,l-kl]phenothiazine. Y 73%.—Clay plate catalyzes this reaction and particularly prevents decreasing yields of larger batches. F. e., also with phenoxazine, s. M. Harfenist et al., J. Org. Chem. 27, 3977 (1962). 

Hydrodechlorination is a common reaction of chlorinated pesticides such as atrazine (eq. 15), alachlor, and metolachlor (2) (eq. 16). These reactions are catalyzed primarily by transition metals or by soil surfaces (clays or humic substances). 

Clay-catalyzed dimerization of unsaturated fatty acids appears to be a carbonium ion reaction, based on the observed double bond isomerization, acid catalysis, chain branching, and hydrogen transfer (8,9,11). 

Microwave chemistry has been found to be a useful method for accelerating reactions or catalyzing reactions that are difficult to carry out by other methods. A modification of the Hantzsch method to directly obtain pyridines has been communicated. A dry medium using ammonium nitrate bentonitic clay system with microwave irradiation affords pyridines 96 in a single pot within 5 minutes. When the pyridine is not the major product (> 75% yield), the dealkylated pyridine 97 becomes an... 

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

Clay-catalyzed asymmetric Diels-Alder reactions were investigated by using chiral acrylates [10]. Zn(II)- and Ti(IV)-K-10 montmorillonite, calcined at 55 °C, did not efficiently catalyze the cycloadditions of cyclopentadiene (1) with acrylates that incorporate large-size chiral auxiliaries such as cA-3-neopentoxyisobornyl acrylate (2) and (-)-menthyl acrylate (3, R = H) (Figure 4.1). This result was probably due to diffusion problems. 

Good results were obtained with (R)-0-acryloylpantolactone (4) in which the dienophile was incorporated with a smaller chiral auxiliary. Some results are reported in Table 4.4, where the cycloadditions catalyzed by Zn(II)-, Fe(II)- and Ti(IV)-K-10 exchanged montmorillonite calcined at 120 °C and 550 °C are compared with those that were not catalyzed and with TiCU- and EtAlCh-catalyzed reactions. Among the metal-clays activated, the Ti(IV)-K-10 was the best catalyst with high conversion and acceptable enantioselectivity obtained after 2 h. 

Keywords clay catalyze(i reactions, montmorillonite catalyst, kaolinite catalyst... 

A solvent-free strategy for the synthesis of thiazoles involved mixing of thioamides with a-tosyloxy ketones in a clay-catalyzed reaction (Scheme 7). The typical procedure entailed mixing of thioamides and in situ produced a-tosyloxy ketones with montmorillonite K-10 clay in an open glass container. The reaction mixture was irradiated in a microwave oven for 2-5 min with intermittent irradiation and the product was extracted into ethyl acetate to afford 2-substituted thiazoles in 88-96% yields [8]. 

Lewis acids such as SnCl4 also catalyze the reaction, in which case the species that adds to the alkenes is H2C —O— SnC. Montmorillonite KIO clay containing zinc(IV) has been used to promote the reaction. The reaction can also be catalyzed by peroxides, in which case the mechanism is probably a free-radical one. Other transition metal complexes can be used to form allylic alcohols. A typical example is. ... 

Selected clay stabilizers are shown in Table 1-10. Thermal-treated carbohydrates are suitable as shale stabilizers [1609-1611]. They may be formed by heating an alkaline solution of the carbohydrate, and the browning reaction product may be reacted with a cationic base. The inversion of nonreducing sugars may be first effected on selected carbohydrates, with the inversion catalyzing the browning reaction. 

The positively charged phosphonium ligand was intercalated in the hectorite and was used to catalyze olefin hydroformylation.156 Cu(II)-exchanged clays were tested as catalysts in the cyclopropanation reaction of styrene with... 

Aldehydes and ketones have been protected as acetals and dioxolanes using orthoformates, 1,2-ethanedithiol or 2,2-dimethyl-l,3-dioxolane by Hamelin and coworkers. This acid-catalyzed reaction proceeds in the presence of p-toluenesulfonic acid (p-TsOH) or KSF clay under solvent-free conditions (Scheme 6.2). The yields ob-... 

The preparation of imines, enamines, nitroalkenes and N-sulfonylimines proceeds via the azeotropic removal of water from the intermediate in reactions that are normally catalyzed by p-toluenesulfonic acid, titanium(IV) chloride, or montmorillonite K 10 clay. A Dean-Stark apparatus is traditionally used which requires a large excess of aromatic hydrocarbons such as benzene or toluene for azeotropic water elimination. 

Alumina, silica, clays, and zeolites are increasingly used as acidic or basic supports [26], Cycloaddition reactions often require Lewis-acid catalysts if good yields are to be obtained. Clay and doped silica gel catalysts have emerged as useful alternatives to the use of Lewis acids. Cycloaddition offuran (5) under solvent-free conditions, catalyzed by K10 montmorillonite, results in a decrease in the reaction time the endo-exo relationship is no different that obtained by use of classical heating (Scheme 9.2) [27]. 

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