Friedel-Crafts acylation environmentally friendly

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. 

Among the new synthetic methodologies recently studied and exploited, the use of ionic liquids as solvent-catalysts to perform more environmentally friendly Friedel-Crafts acylations has been developed. In contrast to volatile organic solvents, ionic liquids have no measurable vapor pressure, and, therefore, fhere is no loss of solvent through evaporation. Moreover, ionic liquids can be easily recovered, cleaned, and reused for different runs. 

The use of HPAs and multicomponenf polyoxometalates as catalysts in liquid-phase reactions was reviewed by Kozhevnikov. Moreover, an interesting minireview was published concerning the Friedel-Crafts acylation of arenes and the Fries rearrangement catalyzed by HPA-based solid acids. The results show that HPA-based solid acids, including bulk and supported heteropoly acids as well as heteropoly acid salts, are efficient and environmentally friendly catalysis for all reacfions analyzed. 

The Friedel-Crafts acylation reaction, named after the French and American chemists who discovered it, used to prepare aryl ketones, is catalyzed by the Lewis acid aluminum chloride. Although aluminum chloride is a catalyst, it must be used (in Friedel-Crafts reactions) in stoichiometric amounts, as the portion of aluminum chloride that is catalytically inactive strongly binds to the product. Because aluminum chloride is corrosive and difficult to handle and must be destroyed when the reaction is complete, chemists continue to seek more environmentally friendly catalysts for this reaction. 

In comparison with molecular catalysts, solid catalysts can be isolated from the reaction mixtures by filtration or used in continuous processes this is both environmentally friendly and useful in laboratory-scale experiments. The most important reactions catalyzed by solid superbases are isomerization reactions and the alkylation of substituted arenes in the side chain (Scheme 2). They proceed at room temperature or below with high yield (typically >99%). The surperbase-cata-lyzed alkylation of aromatic compounds complements the acid-type Friedel-Crafts alkylation and acylation, because the latter results in ring alkylation, whereas the former results in side-chain alkylation. 

Especially, the eco-friendly ionic liquids have obtained extensive attention in organic synthesis with the merits provided as above. The ionic liquids as the unusual green solvents are applied extensively in various organic synthesis reactions, such as Friedel-Crafts reactions, oxidation reactions, reduction reactions, addition reactions, C-C formation reactions, nucleophilic substitution reactions, esterifications, rearrangements, hydroformylations, and nitration reactions [7-14]. Besides, the ionic liquids also have applications in the extraction separation, the electrochemistry, and preparation of nanostructured materials, the production of clean fuel, environmental science, and biocatalysis. This chapter would present in detail the application of the ionic liquids as the unusual green solvents (also as dual green solvent and catalyst) for the alkylation and acylation. 

In addition, this acylating reaction turns out to be very "environmentally friendly" process compared to the classical Friedel-Crafts reaction catalyzed by aluminium chloride, generating only byproducts such as acidic acid which can easily be recovered and recycled. 

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