Solid-acid Catalysts-Aromatic Substitution

With this purpose, several different types of solid acid catalysts have been investigated for the acylation of aromatics, but the best performances have been obtained with medium-pore and large-pore zeolites (3-9). In general, however, the use of acylating agents other then halides, e.g., anhydrides or acids, is limited to the transformation of aromatic substrates highly activated towards electrophilic substitution. In a previous work (10), we investigated the benzoylation of resorcinol (1,3-dihydroxybenzene), catalyzed by acid clays. It was found that the reaction mechanism consists of the direct 0-benzoylation with formation of resorcinol monobenzoate, while no primary formation of the product of C-benzoylation (2,4-dihydroxybenzophenone) occurred. The latter product formed exclusively by... 

Aromatics. The application of solid acid catalysts provides excellent possibilities to carry out aromatic electrophilic substitutions in an environmentally friendly way. Various zeolites were found by Smith and coworkers to exhibit high activities and selectivities.250 Acetyl nitrate generated in situ from acetic anhydride and HNO3 transforms alkylbenzenes to the corresponding para-nitro derivatives in high yield (92-99%) and with excellent selectivity (79-92%) when applied in the presence of large-pore H-Beta zeolites.251 Lattice flexibility and the coordination of acetyl... 

Aromatic ketones are important intermediates in the production of fine chemicals and pharmaceuticals1,2. Thus, the anti-rheumatic Naproxen is produced by the Friedel-Crafts acetylation of 2-methoxynaphthalene into 2-acetyl-6-methoxynaphthalene and subsequent Willgerodt-Kindler reaction. Commercial acylation processes involve over-stoechiometric amounts of metal chlorides (e g. AICI3) as catalysts and acid chlorides as acylating agents, which results in a substantial formation of by-products and in corrosion problems. This is why the substitution of these corrosive catalysts by solid acid catalysts and of acid chlorides by anhydrides or acids is particularly desirable. 

Solid acid catalysts are, in principle, applicable to a plethora of acid-promoted processes in organic synthesis [27-29]. These include various electrophilic aromatic substitutions, e.g. nitrations, and Friedel-Crafts alkylations and acylations, and numerous rearrangement reactions such as the Beckmann and Fries rearrangements. 

In aromatic alkylation with olefins, the solid acid catalyst based process has instead largely substituted the homogeneous acid catalysis process. This evidences that the change of substrate (isobutane vs. aromatic) could change completely the applicability of one technology with respect to another. 

Smith, K., El-Hiti, G. A., Jayne, A. J., and Butters, M. 2003. Acetylation of aromatic ethers using acetic anhydride over solid acid catalysts in a solvent-free system scope of the reaction for substituted ethers. Org. Biomol. Chem. 1 1560-1564. 

The carbon atom of alkyl halides, R—Hal, is electrophilic, but rarely is it sufficiently so to effect the substitution of aromatic species the presence of a Lewis acid catalyst, e.g. AlHalj is also required. That alkyl halides do react with Lewis acids has been demonstrated by the exchange of radioactive bromine into EtBr from AlBr on mixing and re-isolation also the actual isolation of solid 1 1 complexes, e.g. CHjBr-AlBrj, at low temperatures (-78°). These complexes, though polar, are only faintly conducting. Where R is capable of forming a particularly stable carbocation, e.g. with McsC—Br, it is probable that the attacking electrophile in alkylation is then the actual carbocation, Me3C , as part of an ion pair ... 

Substituted aromatics are essential chemical feedstocks. Among the xylenes, for example, p-xylene is in great demand as a precursor to terephthalic acid, a polyester building block. The pura-isomer is therefore more valuable than the o- and m-xylenes, so there is a powerful incentive for conversion of o- and m-xylene to p-xylene. Isomerisation over solid acids occurs readily as a result of alkyl shift reactions of the carbenium-ion-like transition state. The initial protonation occurs by interaction of the Bronsted acid site with the aromatic 71 system, by an electrophilic addition. Over non-microporous solid acids, at high conversion, xylenes are produced at their thermodynamically determined ratios, which favour the meta rather than the ortho or para isomers. In addition, unwanted transalkylation reactions occur, giving rise, for example, to toluene and trimethylbenzenes. Zeolite catalysts can be much more selective. 

A small selection of major reaction types has been studied, but the list is by no means complete. Enzymes, solid acids such as zeolites and Envirocats (see Table 16.3), and new oxidation catalysts are obvious cases where the new technology can bring genuine improvements to chemistry on an industrial scale. Metallocenes are now causing a revolution in polymer production, replacing conventional Ziegler-Natta catalysts for as many as sixty monomers [28]. Further catalyst improvements are still desirable, particularly in the areas of oxidation and aromatic substitution. 

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