Lewis superacids Friedel-Crafts reaction

Friedel-Crafts aromatic substitution reactions have been widely explored in polymer chemistry [29,30] and generally proceed with Lewis acids such as AICI3 with elimination of hydrogen halides. In superacid solutions, however, the Friedel-Crafts reactions take place with dehydration from the oxygen of the carbonyl group and the proton of aromatics. The reactivity of the pro-tonated carbonyl group in the superacid can be further increased by the... 

Benzylation of toluene with benzyl chloride, which is a typical example of Friedel-Crafts alkylation, is known to be catalyzed by Lewis-type superacids such as A1C13 and BF3. This type of catalyst has been mostly used for the Friedel-Crafts reaction, which is one of the most studied of organic reactions. This reaction was performed over several metal oxides and sulfates, and iron sulfates showed an unexpected effectiveness for the reaction (102-104). The catalytic activities of FeS04 and Fe2(S04)3 for the reaction were examined in detail the activities were remarkably dependent on calcination temperature, the maximum activity being observed with calcination at 700°C (105-107). Catalytic actions analogous to the above case were also observed with other Friedel-Crafts reactions, the benzoyl-ation of toluene with benzoyl chloride (108), the isopropylation of toluene with isopropyl halides (109), and the polycondensation of benzyl chloride UIO). 

In 1877, Charles Friedel and James Mason Crafts [30a, b] corporately discovered that treatment of amyl chloride with aluminum strips in benzene led to the formation of amylben-zene. This type of transformation was found to be general for alkyl halides and aromatics under the catalysis of Lewis acid. Along with the discovery of the closely related acylation [30c, d], these two men are best remembered by Friedel-Crafts reaction that bears their names. With various modem modifications that appeared in the Uterature, including enan-tioselective variants [31], Friedel-Crafts alkylation and acylation have already become one of the most powerful C—C bond forming reactions in organic chemistry [32]. These methods are recognized to date as of fundamental importance not only in acadania but also in industry [33]. As shown in Scheme 10.18, some heteroaromatics, instead of the aryl component or alcohol, and alkenes instead of halides can be used as suitable substrates. Also, other common Lewis acids like BFj, TiCl, SnCl, ScfOTOj, etc., and Brpnsted acids snch as HF, H SO, and superacids (e.g., HF SbFj, HS03-SbFj) can also used as catalysts. 

This work incorporates all acid-catalyzed Friedel-Crafts-like acylation reactions. Thus, classic Lewis and Bronsted acid types are considered together with more innovative and advanced multicomponent superacid catalysts, ranging from rare earth triflates or triflimides and their... 

The halides NbF5, TaF5, NbCl5 and TaCl5 are useful starting materials in the chemistry of these metals. They are Friedel-Crafts catalysts and the Lewis acidity of NbFs and TaF5 is apparent in reaction 23.22 (which takes place in non-aqueous media, see Section 9.10), in the formation of related salts and other complexes (see later), and in the abihty of a TaF5/HF mixture to act as a superacid (see Section 9.9). 

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