Heterogeneous acid catalysts Friedel-Crafts acylation

Our pioneering work in 1986[1] has shown that acid zeolites are efficient catalysts in the Friedel-Crafts acylation of toluene and xylene with carboxylic acids and constitutes a breakthrough in environmentally friendly fine chemistry replacing the conventional AICI3 method by a heterogeneous catalysts. Since this initial study, a tremendous amount of work has been performed in this area[2] and particularly, in recent years, the acetylation reaction, which is a field of research with large potential for the production of fine chemicals, has been intensively investigated. 

Step a involves a Friedel-Crafts acylation where HF serves as the catalyst and the solvent for the reaction. Although HF is highly toxic, it can be contained on an industrial scale and completely recycled. The acetic acid by-product can also be recycled. The second step (b) involves heterogeneously-catalyzed hydrogenation of a carbonyl group to give the corresponding alcohol 80 with an atom economy of 100%. Either palladium on charcoal or Raney-nickel can be used as a catalyst. Step c is an excellent example of an alcohol carbonylation that we saw earlier in Section 9-5 this process, like carbonylation of methanol, has an atom economy of 100%. The overall atom economy of the new process, which... 

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. 

Benzoylation under Friedel-Crafts acylation is an important process for the preparation of many industrially valuable chemicals. However, like the acetylation, the use of conventional Lewis acids, for example, AICI3, ZnCl2, and so on, in the homogeneous Friedel-Crafts acylation of arenes entails problems of materials due to corrosion work up and effluent issues. Thus, for benzoylation, as with acetylation, the use of heterogeneous catalysts seems to be an interesting alternative technique to the homogeneous reaction. 

Significant advances resulting from the use of aluminosilicate solids were made during the last few years [3-6] and the first industrial application of zeolites in large scale Friedel-Crafts acylations was reported very recently [7]. However, most of the efforts devoted so far focused on the acylation of aromatic compounds. To the best of our knowledge, recourse to heterogeneous aluminosilicate catalysts for the acylation of alkenes has not yet been reported. Conventional methods for alkene acylation [8] involve the use of Br0nsted or Lewis acids such as sulfuric acid [9], boron trifluoride [10], zinc chloride [11], or... 

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. 

Sarvari, M. H. and Sharghi, H. 2005. Solvent-free catalytic Friedel-Crafts acylation of aromatic compounds with carboxylic acids by using a novel heterogeneous catalyst system p-toluenesulfonic acid/graphite. Helv. Chim. Acta 88 2282-2287. 

Chalcones (123) are commonly prepared by Claisen-Schmidt or aldol condensation. An important way to synthesize chalcones (123) is the Friedel-Crafts acylation involving treatment of acid chlorides (121) with arenes (122). Such a protocol for the synthesis of chalcones (123) was developed by More et al. (2012) using nano-ZnO heterogeneous catalyst under solvent-free conditions at room temperature (Scheme 9.38). Arenes (122) of all sorts, activated as well as unactivated, reacted smoothly to afford the chalcones (123) in excellent yield. High regioselectivity was observed during the course of reaction, which occurred selectively at the para-position of OMe, Br, Me, and Cl. 

Examples here are Friedel-Crafts alkylations and acylations, in which the zeolite catalyst replaces the homogeneous Lewis acid. Zeolite catalysts can also be used in place of mineral acids and organic acids. However, these catalyst substitutions also involve changing the process from homogeneous to heterogeneous catalysis. 

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