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Ureas have been traditionally synthesised by methodologies mainly based on the use of dangerous reagents such as phosgene and isocyanates. In the last few years, however, these reagents have been increasingly substituted by cleaner alternative and inherently safer compounds since the goal of the modern environmentally friendly synthetic chemistry is the development and optimisation of reaction conditions to reduce or eliminate the use and production of hazardous materials while maximising energy usage. Thus different and sometimes sophisticated phosgene substitutes such as bis(4-nitrophenyl)carbonate, triphosgene, di-tert-butyl dicarbonate, 1,1-carbonylbisimidazole, 1,1-carbonylbisbenzotriazole, S,S-dimethyldithiocarbonate and trihaloacetylchlorides have been utilised as safer reagents which can be stored and handled without special precautions. Moreover, the large scale production of urea derivatives has been more conveniently performed by using simpler and cheaper raw materials such as CO and CO 2 in catalytic processes, avoiding production of large amounts of saline by-products which represent the main constituent of chemical waste.

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|Chem. Rev. 2011, 111, PR181-PR214 Chemical Reviews REVIEWEven though these results could suggest some practical developments, they have not led to any very important industrial application. However, 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 FriedelÀCrafts acylations: these include zeolites, (acid treated) metal oxides, and heteropoly acids already utilized in the hydrocarbon reactions. 13 Moreover, the application of clays, perfluorinated resinsulfonic acids, and supported (fluoro) sulfonic acids, mainly exploited in the production of fine chemicals, is the subject of intensive studies in this area.Owing to the great interest in the argument, minireviews can be found on the use of solid catalysts in FriedelÀCrafts acylation. Kouwenhoven and van Bekkum in a chapter of the Handbook of Heterogeneous Catalysis faced the basic problem of the use of zeolites in the reaction. 14 A further essential overview of the same argument was reported by M etivier in Fine Chemicals through Heterogeneous Catalysis. 15 Furthermore, Bezouhanova described the synthetic aspects of the zeolite-catalyzed aromatic ketones preparation. 16a Very recently, Cejka et al. reported the development in the acylation reactions of arenes, aromatic ethers, and phenols catalyzed by zeolites and mesoporous materials. 16b The present review deals with the developments, mainly during the past decade, in the use of heterogeneous catalysts for FriedelÀCrafts acylation of aromatic compounds. The whole argument is organized in five sections, namely, acylation of arenes, aromatic ethers, aromatic thioethers, phenolic substrates (split up into Fries rearrangement and direct acylation), and heterocycles. It is structured according to a mechanistic point of view, taking into special account the role played by the surface active sites in the activation of reagents as well as in the different modes of regioselectivity encountered in the acylation of arenes, aromatic ethers, and phenols.Even if the great majority of the papers reviewed concern reactions carried out in batch conditions, where the blocking of pores and active sites can have a negative effect on catalyst efficiency, we particularly stressed results obtained in the gas phase or under continuous fixed-bed reaction conditions, where the continuous removal of products and poisons from the catalyst surface enhances performance.When we decided to face the problem of writing a review on the FriedelÀCrafts acylation reaction, immediately our minds went back to the fruitful discussions on this topic with our mentor Prof. Giuseppe Casnati and to the passionate studies performed under his guidance about the physicochemical factors that control the C-regioselectivity in the acylation of ambidental phenol substrates. ...

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