Friedel-Crafts acylation reactions catalytic homogeneous acylations

Ionic liquids in combination with supercritical fluids are a versatile tool for the immobilization and recycling of homogeneous catalysts, allowing continuous Friedel-Crafts acylation reactions to be realized. The acylation of anisole with acetic anhydride is carried out in a flow system using a metal triflate immobilized in the ionic liquid 1 -butyl-4-methylpyridinium bis(trifluoro-methylsulfonyl)imide as catalyst and scCO as continuous extraction phase [22]. Different metal triflates are utilized under continuous flow conditions using high pressure yttrium triflate possesses the best balance between sufficient acidity for catalytic activity and softness to release the product and so permits a good catalyst reuse (TONs up to 190). 

Friedel-Crafts acylation is widely used for the production of aromatic ketones applied as intermediates in both fine chemicals and pharmaceutical industries. The reaction is carried out by using conventional homogenous catalysts, which represents significant technical and environmental problems. The present work reports the results obtained in the Friedel-Crafts acylation of aromatic substrates (anisole and 2-methoxynaphthalene) catalyzed by Beta zeolite obtained by crystallization of silanized seeds. This material exhibits hierarchical porosity and enhanced textural properties. For the anisole acylation, the catalytic activity over the conventional Beta zeolite is slightly higher than with the modified Beta material, probably due to the relatively small size of this substrate and the weaker acidity of the last sample. However, the opposite occurred in the acylation of a bulky substrate (2-methoxynaphthalene), with the modified Beta showing a higher conversion. This result is interpreted due to the presence of a hierarchical porosity in this material, which favors the accessibility to the active sites. 

As already underlined in the introduction to this book, catalytic homogeneous acylation reactions represent a remarkable improvement in the preparation of aromatic ketones because, in the conventional Lewis-acid-promoted reactions, formation of a stable complex between the ketone product and the catalyst implies that at least a stoichiometric amount of catalyst must be utilized. This drawback prompted a great number of studies aimed at setting up the experimental conditions to make catalytic Friedel-Crafts acylation reactions. Some positive results from fhe homogeneous catalytic Friedel-Crafts acylations are described here, with special attention to crucial economic and environmental advantages such as the recycling of expensive catalysts and the development of solvenf-free and highly selective synthetic processes. 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

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