Friedel-Crafts alkylations aromatic systems, aluminum chloride

A 1 2 mixture of l-methyl-3-ethylimidazolium chloride and aluminum trichloride, an ionic liquid that melts below room temperature, has been recommended recently as solvent and catalyst for Friedel-Crafts alkylation and acylation reactions of aromatics (Boon et al., 1986), and as solvent for UV/Vis- and IR-spectroscopic investigations of transition metal halide complexes (Appleby et al., 1986). The corresponding 1-methyl-3-ethylimidazolium tetrachloroborate (as well as -butylpyridinium tetrachlo-roborate) represent new molten salt solvent systems, stable and liquid at room temperature (Williams et al., 1986). 

The behavior of such activated halides as alkylating agents under Friedel-Crafts conditions expands the scope of the synthesis. Aluminum chloride enhances the electrophilic character of the a,/S-unsaturated carbonyl system and permits the nucleophilic attachment of the aromatic addendum (Y ) to the carbon bearing the positive charge, with displacement of halogen [Eq. (5)]. Thus,... 

The carbon-carbon bond may be formed when a carbon nucleophile is used in the combination system. The most representative examples include the Friedel-Crafts-type alkylation of aromatics (Scheme XI, equation 1) (18) and the acid-catalyzed Diels-Alder reaction (Scheme XI, equation 2). The reaction of a combination system consisting of aluminum chloride and 1,3-dienes leading to regio- and stereoselective functionalization of 1,3-dienes via the thienium cation Diels-Alder reaction (19) (Scheme XI, equation 3) is described here. 

The most general method for direct introduction of alkyl groups on an aromatic ring system is the Friedel-Crafts reaction. It involves generation of a carbonium ion intermediate or a related electrophilic carbon species. The most general method for generating these electrophiles involves reaction between an alkyl halide and a Lewis acid. The most common Lewis acid for preparative work is aluminum chloride. Alternative routes to the alkylating species include protonation (followed by dehydration) of alcohols and protonation of alkenes. 

The generation of the appropriate electrophile (carbocation, carbocation complex, or acylium ion) in the presence of an aromatic ring system (nucleophile) can lead to alkylation or acylation of the aromatic ring. This set of reactions, discovered by Charles Friedel and James Crafts in 1877, originally used aluminum chloride as the catalyst. The reaction is now known to be cat-al) ed by a wide range of Lewis acids, including ferric chloride, zinc chloride, boron trifluoride, and strong acids, such as sulfuric, phosphoric, and hydrofluoric acids. 

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