Friedel-Crafts acylation reactions clays

Unmodified acid clays have been little utilized as catalysts for Friedel-Crafts acylation reaction. Among the scanty information from the literature, the production of AQ 22 through cyclodehydration of BBA 21 is of some interest (Scheme 4.22). The reaction is performed in the presence of different clays at 350°C for 30 min. AQ 22 is directly recovered by sublimation from the reaction mixture. Some interesting results are achieved with commercially available catalysts such as Tonsil BW3, a mixture of mica, quartz, feldspar, and amorphous silica with bentonite and mont-morillonite KIO, product 22 being obtained in 78%-89% yield. Reusability studies show that Tonsil BW3 still gives product 22 in 60% yield after five... 

Thanks to their low cost and easy availability, many metal-exchanged clays have been patented as efficient solid catalysts in Friedel-Crafts acylation reactions. A great number of arylketones is prepared by electrophilic acylation of arenes with anhydrides in the presence of ion-exchanged clays at 150°C-250°C. Thus, for example, aluminum-enriched mica promotes the reaction of BAN with mcto-xylene at reflux for 4 h in 99% yield. 

The Friedel-Crafts acylation of aromatic compounds is an important synthesis route to aromatic ketones in the production of fine and specialty chemicals. Industrially this is performed by reaction of an aromatic compound with a carboxylic acid or derivative e.g. acid anhydride in the presence of an acid catalyst. Commonly, either Lewis acids e.g. AICI3, strong mineral acids or solid acids e.g. zeolites, clays are used as catalysts however, in many cases this gives rise to substantial waste and corrosion difficulties. High reaction temperatures are often required which may lead to diminished product yields as a result of byproduct formation. Several studies detail the use of zeolites for this reaction (1). 

Reactions can be performed in the solid state as well, by soaking the reagents on silica, alumina, or clays. Avoiding the use of organic solvents in the reactions leads to clean, efficient, and economical technologies. The Friedel-Crafts acylation shown in Figure 12.26 is an example. 

An impressive number of papers and books has been published and numerous patents have been registered on the aq lation of aromatic compounds over solid catalysts. Recently Sartori and Maggi [1] have written an excellent review with 267 references on the use of solid catalysts in Friedel-Crafts acylation. In one section of this review, namely acylation of aromatic ethers or thioethers, the authors report work on acylation by solid catalysts such as zeolites, clays, metal oxides, acid-treated metal oxides, heteropolyacids or Nafion. When examining in details these results, it appeared very difficult for us to build upon these experimental results as the reaction conditions differ drastically from one paper to the next. This prompted us to reinvestigate the scope and limitations of the Friedel-Crafts acylation using heterogeneous solids as catalysts, trying as much as we could to rationalize the observed effects. 

Maximum effort has been directed toward the use of solid acid catalysts. In fact, heterogeneous catalysts can be easily separated from the reaction mixture and reused they are generally not corrosive and do not produce problematic side products. Different classes of materials have been studied and utilized as heterogeneous catalysts for Friedel-Crafts acylations these include zeolites (acid treated), metal oxides, and heteropoly acids already utilized in hydrocarbon reactions. Moreover, the application of clays, perfluorinated resinsulfonic acids, and supported (fluoro) sulfonic acids, mainly exploited in the production of fine chemicals, are the subject of intensive studies in this area. 

BACs can be activated toward Friedel-Crafts acylation with commercially available catalyst Envirocat EPIC . The catalyst is constituted by a natural clay impregnated with polyphosphoric acid. The reaction occurs... 

D. Baudry-Barbier studied the catalytic properties of Sc, La and Nd modified KIO clays in Friedel-Crafts acylations of anisole with benzoyl chloride for yielding 4-methoxybenzophenone, and found La modified KIO clay exhibited the best catalytic activity for acylations of anisole. In comparison with the acid catalyzed reactions using liquid acids, the solid acid catalyzed process was non-polluting and also the final work-up didn t require any aqueous treatment, which exhibited wide prospect for the clean synthesis of building block and fine chemicals [195]. 

Zinc chloride exchanged clay catalysts have been reported to be highly active for the Friedel-Crafts alkylation and acylation reactions these are commercially sold by Contract Catalysts under the name Envirocats. These are montmorillonite catalysts modified by ZnCU and FeCli. Some of the reported examples of Friedel-Crafts reactions are given below there are claims that some of the processes are commercially practised. 

Thenaldehyde (thiophene-2-carbaldehyde) is readily available via the Vilsmeier-Haack reaction of DMF with thiophene catalyzed by phosphorus oxychloride. The Sommelet reaction with 2-chloromethylthiophene also gives reasonable yields (63AHC(l)l). Likewise, thiophene is readily acylated with acyl anhydrides or acid chlorides (equation 14), using mild Friedel-Crafts catalysts, such as tin(IV) chloride, zinc chloride, boron trifluoride, titanium tetrachloride, mercury(II) chloride, iodine and even silica-alumina gels or low-calcium-content montmorillonite clays (52HC(3)l). 

Choudhary, V. R., Jana, S. K., and Mandale, A. B. 2001. Highly active, reusable and moisture insensitive catalyst obtained from basic Ga-Mg-hydrotalcite anionic clay for Friedel-Crafts type benzylation and acylation reactions. Catal. Lett. 74 95-98. 

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