Acid-catalyzed reactions, various

For asymmetric acid-catalyzed reactions, various chiral boron compounds were developed that include 523-531,1042-10S1 (Scheme 40). 

As mentioned in the introductory remarks of Sect. 5.2, a particularly interesting question arose as to whether solid-state ion exchange would enable the convenient preparation of catalysts for acid-catalyzed reactions. Various experiments have indeed shown that solid-state ion exchange offers a new route for catalyst production. Some of the pertinent results are described below (cf. also Sects. 5.1.11,5.3 and 5.4). 

Little information is available about 5,6-dihydropteridines, of which various 6,7-diphenyl-5,6-dihydropterins (65HCA764, 69HCA306) and -lumazines (68HCA1029, 70HCA789) have been synthesized and characterized. As noticed already (51BSF521), this type of compound isomer-izes in an acid-catalyzed reaction to the 7,8-dihydro derivative (77HCA922) or oxidizes to... 

The term alkylation generally is applied to the acid catalyzed reaction hetween isohutane and various light olefins, and the product is known as the alkylate. Alkylates are the best of all possible motor fuels, having both excellent stability and a high octane number. 

Sulphoxides undergo various acid catalyzed reactions on the sulphinyl sulphur atom, which were reviewed elsewhere Both sulphoxides and sulphones have been found to be... 

Lewis-Acid Catalyzed. Recently, various Lewis acids have been examined as catalyst for the aldol reaction. In the presence of complexes of zinc with aminoesters or aminoalcohols, the dehydration can be avoided and the aldol addition becomes essentially quantitative (Eq. 8.97).245 A microporous coordination polymer obtained by treating anthracene- is (resorcinol) with La(0/Pr)3 possesses catalytic activity for ketone enolization and aldol reactions in pure water at neutral pH.246 The La network is stable against hydrolysis and maintains microporosity and reversible substrate binding that mimicked an enzyme. Zn complexes of proline, lysine, and arginine were found to be efficient catalysts for the aldol addition of p-nitrobenzaldehyde and acetone in an aqueous medium to give quantitative yields and the enantiomeric excesses were up to 56% with 5 mol% of the catalysts at room temperature.247... 

Indium trichloride349-351 is a mild Lewis acid that is effective for various kinds of Lewis-acid-catalyzed reactions such as Diels-Alder reactions (Scheme 85), aldol reactions, and Friedel Crafts reactions. Since indium trichloride is stable in water, several aqueous reactions have been investigated (Scheme 85) indium(III) triflate is also used as a Lewis acid. 

Table III provides a comparison of alkylate compositions for both the liquid acid-catalyzed reactions with various feed alkenes. The data show that H2SO4 produces a better alkylate with 1-butene, whereas HF gives better results with propene or isobutylene. The products from 2-butene and also from pentenes (not shown in Table III) are nearly the same with either acid.

Since our first paper181 on Lewis add catalysis in aqueous media appeared, many investigations and results in this area have been reported. Water-stable Lewis adds are now becoming common and useful catalysts in organic synthesis. These catalysts have been applied to various types of Lewis acid-catalyzed reactions. 

The various products obtained from acetylenic diols 54 in the presence of acids suggest the formation and interconversion of acetylene-allene-diene cationoid intermediates (equation 18)33. The allene intermediates can be sometimes isolated and they were reported as participants in the acid-catalyzed reactions of alkynylpyrylium salts 55, a driving force of which is an aromatization of the pyrane ring (equations 19-21)34,35. 

Various polymer acids are used as polymer catalysts. Sulfonated polystyrene (Eq. 9-39) has been used to catalyze a variety of acid-catalyzed reactions, including acetal and ketal... 

Transesterification Reactions. The heterogeneous acid-catalyzed transesterification of TGs has not been investigated as much as its counterpart, the base-catalyzed reaction. Various solids are available with sufficient acid strength to be effective catalysts for the named reaction. Among the solid acids available are functionalized polymers, such as the acid forms of some resins, as well as inorganic materials, such as zeolites, modified oxides, clays, and others. Some of these solids have already been found to be effective in transesterification reactions of simple esters and (3-ketoesters. 

The purpose of the present work is to incorporate aluminum into the framework of SBA-15 during the synthesis in order to create acid sites on the surface of the material directly and to enhance its activity in acid-catalyzed reactions and to study the stability of SBA and AlSBA molecular sieves under various treatments. The influence of these treatments on the pore size, wall thickness and the environment of Al in these materials are investigated in detail. X-ray diffraction (XRD), Electron Microscopy (TEM) and N2 adsorption were used to characterize the structure, the porosity and the stability of these materials. 27Al MAS NMR was used to ascertain the nature and environment of Al, cumene cracking to test the catalytic activity of parent materials and ammonia chemisorption to probe their surface acidity. 

A wide variety of acid-catalyzed reactions besides those described above have been investigated with heteropoly compounds as catalysts. Al203-supported H3PW12O40 (probably decomposed) catalyzed propylene-ethylene codimerization at 573 K to form pentenes with a selectivity of 56% (butenes 17%, hexenes 27%) (224). Propylene oligomerization proceeded on various kinds of salts of H3PWl204o (225). The activities of the salts decrease in the order A1 > Co > Ni, NH4 > H, Cu > Fe, Ce > K. The A1 salt gave trimers with 90% conversion at 503 K. The selectivities to trimer are about 40% for Al, Ce, Co, and Cu, while that of the acid form is 25%. 

Reaction with Nitrogen Nucleophiles. The acid-catalyzed reaction of primary, secondary, and tertiary amines with ethyleneimine yields asymmetrically substituted ethylenediamines (71). Steric effects dominate basicity in the relative reactivity of various amines in the ring-opening reaction with ethyleneimine (72). The use of carbon dioxide as catalyst in the aminoethylation of aliphatic amines, for which a patent application has been filed (73), has two advantages. First, the corrosive salts produced when mineral acids are used as catalysts (74,75) are no longer formed, and second, the reaction proceeds with good yields under atmospheric pressure. 

The success of carbocation chemistry lies in the stabilization of carbocations in a medium of low nucleophilicity. Superelectrophiles, in turn, are reactive intermediates generated by further protonation (protosolvation). This second protonation increases electron deficiency, induces destabilization, and, consequently, results in a profound increase in reactivity. In particular, charge-charge repulsive interactions6 play a crucial role in the enhanced reactivity of dicationic and tricationic superelectrophilic intermediates. As various examples of acidity dependence studies show, without an appropriate acidity level, transformations may occur at much lower rate or even do not take place at all. In addition to numerous examples of superacid catalyzed reactions, various organic transformations, in which the involvement of superelectrophilic intermediates is invoked or superelectrophiles are de facto observed in the condensed state, are also included in this chapter. 

A wide range of variously substituted thiochromans has been obtained from the readily accessible ot-(benzotriazo-lyl)methyl thioethers by their Lewis acid-catalyzed reaction with styrenes. Initial loss of the benzotriazole unit generates a thionium cation which undergoes an efficient cationic cycloaddition to the alkene. The reaction, which generally proceeds with high diastereoselectivity, is considered to occur in a stepwise manner rather than as a concerted [4++2] process (Scheme 174) <2001JOC5595>. 

In the pulse radiolysis of aqueous solutions of poly(styrenesulfonate), various radical species have been identified [83]. OH radicals reacted with this polymer to produce a mixtures of OH adducts. When several OH radical adducts were produced on the same polymer molecule, the intramolecular radical-radical reactions occurred. A radical cation was also produced from the OH adduct by the following acid catalyzed reaction. 

Access to compounds of the calixresorcinarene class [63] can be achieved by an acid-catalyzed reaction between resorcinol and various aldehydes. The ease of synthesis of many calixarenes and calixresorcinarenes. along with the ready availability of the starting materials, renders them of great potential for studying both academically and industrially motivated problems. These span many areas [58] from adhesives to the selective metal ion response of calixarene monolayers [64]. 

Consequently, the acid-catalyzed reaction between ketones and nitriles via intermediate formation of Af-acyliminium ions and enamides provides various modes to connect the reactants as well as different ways of assembling end products, depending on the conditions and the structure of the substrate. The reaction is therefore very promising as a synthetic tool for making polyfunctional and heterocyclic compounds. 

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