Alkylation using supported sulfonic acid

Furthermore, low-loading (e.g. 0.17 mmol/g) and alkylated polystyrene-supported sulfonic acid (LL-ALPSSO3H) such as 13 successfully calatyzed deprotection of tert-butyldimethylsilyl- (TBS) protected alcohols in water without using organic cosolvents (Scheme 3.38). 

Cationic polymerization of cyclosiloxanes is well known but used much less frequently than anionic reactions. The most widely used catalysts include sulfuric acid and its derivatives, alkyl and aryl sulfonic acids and trifluoroacetic acid1 2,1221. Due to their ease of removal, in industrial applications acid catalysts are generally employed on supports such as bentonite clay or Fuller s earth. 

In laboratory preparations, sulfuric acid and hydrochloric acid have classically been used as esterification catalysts. However, formation of alkyl chlorides or dehydration, isomerization, or polymerization side reactions may result. Sulfonic acids, such as benzenesulfonic acid, toluenesulfonic acid, or methanesulfonic acid, are widely used in plant operations because of their less corrosive nature. Phosphoric acid is sometimes employed, but it leads to rather slow reactions. Soluble or supported metal salts minimize side reactions but usually require higher temperatures than strong acids. 

Alkylation. Friedel-Crafts alkylation (qv) of benzene with ethylene or propylene to produce ethylbenzene [100-41 -4], CgH10, or isopropylbenzene [98-82-8], C9H12 (cumene) is readily accomplished in the liquid or vapor phase with various catalysts such as BF3 (22), aluminum chloride, or supported polyphosphoric acid. The oldest method of alkylation employs the liquid-phase reaction of benzene with anhydrous aluminum chloride and ethylene (23). Ethylbenzene is produced commercially almost entirely for styrene manufacture. Cumene [98-82-8] is catalytically oxidized to cumene hydroperoxide, which is used to manufacture phenol and acetone. Benzene is also alkylated with C1Q—C20 linear alkenes to produce linear alkyl aromatics. Sulfonation of these compounds produces linear alkane sulfonates (LAS) which are used as biodegradable deteigents. 

Only a few examples of solid-phase syntheses of phosphonic, phosphoric, and sulfonic acids have been reported (Figure 3.16). Benzyl esters of these strong acids can act as alkylating agents, and may therefore be too labile to serve as linkers for long synthetic sequences on solid phase. However, if cross-linked polystyrene is used as the support, the reactivity of, for example, benzyl sulfonates is strongly reduced, and even... 

The pioneer work in this field was carried out on polystyrene-supported acid catalysts [161]. Thereafter, several works on the use of sulfonic, strong acidic cation exchangers as acid catalysts were reported for alkylation, hydration, etherification, esterification, cleavage of ether bonds, dehydration, and aldol condensation [162,168-171], Besides, industrial applications of these materials were evaluated with reactions related to the chemistry of alkenes, that is, alkylation, isomerization, oligomerization, and acylation. [163,169], Also, Nation, an acid resin which has an acid strength equivalent to concentrated sulfuric acid, can be applied as an acid catalyst. It is used for the alkylation of aromatics with olefins in the liquid or gas phases and other reactions however, due to its low surface area, the Nation resin has relatively low catalytic activity in gas-phase reactions or liquid-phase processes where a nonpolar reactant or solvent is employed [166],... 

Cysteic acid is obtained in nearly quantitative yield from cysteine with aqueous hydrogen peroxide in the presence of iron(II) ions.397 Molybdates and tungstates have also been used as effective catalysts for similar transformations.398 An excellent route for the oxidation of 2-thioethanol to isothionic acid has been developed.399 Heteropolyoxometallates supported on alumina400 can also be used to oxidize a range of organo-sulfur compounds. For example, alkyl monosulfides to sulfoxides and sulfones, and thiols to sulfonic acids are a few possibilities (Figure 3.98). 

In order to prevent S-alkylation in the repetitive acid-cleavage steps required in a multistep peptide synthesis, the protection of methionine residues as sulfoxides was proposed by Iselin.f This approach has since been widely used for peptide synthesis in solution and on solid supports.Generally, oxidation of L-methionine yields a mixture of S- and R-sulf-oxides and, depending on the conditions used, even a mixture of the related sulfone. Thereby, at least by oxidation with hydrogen peroxide the R-sulfoxide is formed in a preferred manner (80%), whereas oxidation of L-methionine by tetrachloroauric(III) acid has been reported to produce stereospecifically the 5-sulfoxide diastereoisomer. Isomeric pure Met(O) derivatives are obtained by isolation of the isomers from the 5,/ -sulfoxide mixture, taking advantage of the differences in solubility of the picrate salts.0 ... 

The alkyl sulfonic acid bearing polysiloxane is also used as a support for powder type precious metal catalysts 2 (particle size < 200 pm) that can be used in hydrogenolysis reactions. The conversion of 1 -phenylethanol to ethylbenzene proceeds quantitatively (>99%) with the same rate as achieved with the combination of H2SO4 and a Pd/C catalyst. Therefore,... 

Lewis acids supported on graphite as well as the perfluorinated resin sulfonic acids (Nafion-H) have been studied for their abilities to perform Friedel-Crafts alkylation of benzene and transalkylation of alkylbenzenes (Olah et aL, 1977). The graphite-supported materials were less stable than the perfluorinated resin sulfonic acid. Subsequent studies investigated the use of this latter resin in the methylation of phenols, benzene, and alkylbenzenes (Kaspi and Olah, 1978 Kaspi et aL, 1978), the nitration of aromatics (Olah et aL, 1978a), the rearrangement of allyl alcohols to aldehydes (Olah et al., 1978b), the pinacolone rearrangement (Olah and Meidar, 1978) and isomerizations of alkylbenzenes (Olah and Kaspi, 1978). 

The best solution appears to be the use of an almost insoluble liquid catalyst held within the pores of a suitable inert support. Supported liquid catalysts are well known and can be used with a continuous catalytic regeneration system similar to that developed for catalytic reforming processes. Haldor Topsoe has successfully tested trifluoromethane sulfonic acid in this way since 1993 with a variety of olefin feeds. " No formal regeneration was necessary apart from periodic removal of some catalyst for reimpregnation and the recovery of dissolved acid from the alkylate. Both catalyst and support are, therefore, recirculated. The small quantity of polymeric by-products formed (acid soluble oil) appears to be less tlm that formed in the sulfuric acid process, but slightly more than in the HF process. 

Because of the special structural requirements of the resin-bound substrate, this type of cleavage reaction lacks general applicability. Some of the few examples that have been reported are listed in Table 3.19. Lactones have also been obtained by acid-catalyzed lactonization of resin-bound 4-hydroxy or 3-oxiranyl carboxylic acids [399]. Treatment of polystyrene-bound cyclic acetals with Jones reagent also leads to the release of lactones into solution (Entry 5, Table 3.19). Resin-bound benzylic aryl or alkyl carbonates have been converted into esters by treatment with acyl halides and Lewis acids (Entry 6, Table 3.19). Similarly, alcohols bound to insoluble supports as benzyl ethers can be cleaved from the support and simultaneously converted into esters by treatment with acyl halides [400]. Esters have also been prepared by treatment of carboxylic acids with an excess of polystyrene-bound triazenes here, diazo-nium salts are released into solution, which serve to O-alkylate the acid (Entry 7, Table 3.19). This strategy can also be used to prepare sulfonates [401]. 

Only a few simple polymer-supported catalysts have found industrial use. All are strong acids, sulfonated poly(styrene-divinylbenzene), and are used in processes including phenol alkylation, the phenol-acetone condensation reaction to give bisphenol A, and the conversion of methanol and isobutylene into methyl-f-butyl ether, a high-octane gasoline component. 

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