Acylation, ionic liquids aromatic compounds

Supported ionic liquid compositions are also a vivid field of research in which many companies tried to make their claims [88-90]. By immobilizing ionic liquids onto silica- or alumina-based carriers it is possible to obtain new Lewis acid catalysts with interesting characteristics. These are presently preferably used for alkylation and acylation reactions of aromatic compounds [91, 92] or isomerizations. Even the co-immobilization of ionic liquids with transition metal complexes [93] or Lewis acids [94] has been described, and it can be anticipated that this particular field offers many options for future catalyst development. 

Friedel-Crafts acylation reactions usually involve the interaction of an aromatic compound with an acyl halide or anhydride in the presence of a catalyst, to form a carbon-carbon bond [74, 75]. As the product of an acylation reaction is less reactive than its starting material, monoacylation usually occurs. The catalyst in the reaction is not a true catalyst, as it is often (but not always) required in stoichiometric quantities. For Friedel-Crafts acylation reactions in chloroaluminate(III) ionic liquids or molten salts, the ketone product of an acylation reaction forms a strong complex with the ionic liquid, and separation of the product from the ionic liquid can be extremely difficult. The products are usually isolated by quenching the ionic liquid in water. Current research is moving towards finding genuine catalysts for this reaction, some of which are described in this section. 

The first example of SILP-catalysis was the fixation of an acidic chloroaluminate ionic liquid on an inorganic support. The acidic anions of the ionic liquid, [AI2CI7] and [AI3CI10], react with free OH-groups of the surface to create an anionic solid surface with the ionic liquid cations attached [72]. The catalyst obtained was applied in the Friedel-Crafts acylation of aromatic compounds. Later, the immobilisation of acidic ionic liquids by covalent bonding of the ionic liquid cation to the surface was developed and applied again in Friedel-Crafts chemistry [73]. 

Acidic chloroaluminate ionic liquids are able to generate acylium ions and are therefore ideally suited to Friedel-Crafts reactions. Acylation of mono-substituted aromatic compounds in acidic chloroaluminate ionic liquids leads almost exclusively to substitution at the 4-position on the ring [9] (Scheme 10.8). 

Ionic liquids can provide an ideal medium for reactions that involve reactive ionic intermediates due to their ability to stabilize charged intermediates such as acyl cations in Friedel-Crafts acylation. As shown in the previous chapter, examples of electrophilic aromatic acylation are reported utilizing ionic liquid-catalysts based on classic Lewis acids, namely, [emim] Cl-aluminum chloride and [emim]Cl-iron trichloride, which can give acylation of both activated and deactivated aromatic compounds. ... 

Gmouh, S., Yang, H., and Vaultier, M. 2003. Activation of bismuth(III) derivatives in ionic liquids novel and recyclable catalytic systems for Friedel-Crafts acylation of aromatic compounds. Org. Lett. 5 2219-2222. 

The Friedel-Crafts reaction was one of the first to be attempted in ionic liquids (for typical examples, see Scheme 20. Friedel-Crafts acylation, which allows easy functionalization of aromatic compounds to ketones, is of significant commercial importance. The electrophilic substitution with an acylating agent is catalyzed by an acid, typically AICI3. Since this catalyst can form a stable adduct with the carbonyl of the product, an excess of AICI3 is required, which gives rise to a copious amount of inorganic waste. 

Aluminum chloride is noteworthy as a Lewis acid and is extensively used as such in the Friedel-Crafts alkylation and acylation of aromatic hydrocarbons. This property is evidenced in the structure of the compound. While the crystal consists of an ionic lattice, with each aluminum ion surrounded by six chloride ions in an octahedral array, the compound becomes a dimeric molecule in the nonconducting liquid phase and in the gas phase. This molecule, AI2CI6, consists of tetrahedrally coordinated aluminum with two chlorine atoms bridging the two aluminum atoms, effectively a Lewis acid-base adduct of one monomer to another. 

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