Alkenes Friedel-Crafts alkylation with

C. Friedel-Crafts Alkylation with Alkenes and Alcohols... 

Friedel-Crafts alkylation using alkenes has important industrial appHcations. The ethylation of benzene with ethylene to ethylbenzene used in the manufacture of styrene, is one of the largest scale industrial processes. The reaction is done under the catalysis of AlCl in the presence of a proton source, ie, H2O, HCl, etc, although other catalysts have also gained significance. 

The Fiiedel-Ciafts alkylation of aiomatics with the lesonance-stabihzed ttichloiocyclopiopenium ttiflate offers a synthetic pathway to ttiaiyl cyclopiopenium salts (26). The ttichloiocyclopiopenium ion has also been shown to undergo Friedel-Crafts reaction with alkenes and alkynes to give trivinyl and tri(halovinyl) cyclopiopenium ions. 

Note This reaction involves a polar acidic mechanism, not a free-radical mechanism It is a Friedel-Crafts alkylation, with the slight variation that the requisite carbocation is made by protonation of an alkene instead of ionization of an alkyl halide. Protonation of C4 gives a C3 carbocation. Addition to Cl and fragmentation gives the product. 

Dibenzofuran undergoes Friedel-Crafts alkylation with alkyl ha-lides " " or alkenes in the presence of Lewis acids. The products... 

Friedel-Crafts reactions with alkenes are synthetically attractive but, as with alkyl halides, are most useful when stable cations are formed and when done intramolecularly. An internal Friedel-Crafts cyclization with... 

Biermann, U., Metzger, J.O., (1999) Friedel-Crafts Alkylation of Alkenes Ethylaluminum Sesquichloride Induced Alkylations with Alkyl Chloroformates, Angew. Chem. Int. Ed. 38, 3675-3677. 

We will show here the classification procedure with a specific dataset [28]. A reaction center, the addition of a C-H bond to a C=C double bond, was chosen that comprised a variety of different reaction types such as Michael additions, Friedel-Crafts alkylation of aromatic compounds by alkenes, or photochemical reactions. We wanted to see whether these different reaction types can be discerned by this... 

All lation of Phenols. The approach used to synthesize commercially available alkylphenols is Friedel-Crafts alkylation. The specific procedure typically uses an alkene as the alkylating agent and an acid catalyst, generally a sulfonic acid. Alkene and catalyst interact to form a carbocation and counter ion (5) which interacts with phenol to form a 7T complex (6). This complex is held together by the overlap of the filled TT-orbital of the aromatic... 

Alkylation of furan and thiophene has been effected with alkenes and catalysts such as phosphoric acid and boron trifluoride. In general, Friedel-Crafts alkylation of furans or thiophenes is not preparatively useful, partly because of polymerization by the catalyst and partly because of polyalkylation. 

The synthesis of an alkylated aromatic compound 3 by reaction of an aromatic substrate 1 with an alkyl halide 2, catalyzed by a Lewis acid, is called the Friedel-Crafts alkylation This method is closely related to the Friedel-Crafts acylation. Instead of the alkyl halide, an alcohol or alkene can be used as reactant for the aromatic substrate under Friedel-Crafts conditions. The general principle is the intermediate formation of a carbenium ion species, which is capable of reacting as the electrophile in an electrophilic aromatic substitution reaction. 

As catalysts Lewis acids such as AICI3, TiCU, SbFs, BF3, ZnCh or FeCl3 are used. Protic acids such as FI2SO4 or FIF are also used, especially for reaction with alkenes or alcohols. Recent developments include the use of acidic polymer resins, e.g. Nafion-Fl, as catalysts for Friedel-Crafts alkylations and the use of asymmetric catalysts. ... 

An important use of the Friedel-Crafts alkylation reaction is to effect ring closure. The most common method is to heat with aluminum chloride an aromatic compound having a halogen, hydroxy, or alkene group in the proper position, as, for example, in the preparation of tetralin ... 

Intermodular Alkylation by Carbocations. The formation of carbon-carbon bonds by electrophilic attack on the ir system is a very important reaction in aromatic chemistry, with both Friedel-Crafts alkylation and acylation following this pattern. These reactions are discussed in Chapter 11. There also are useful reactions in which carbon-carbon bond formation results from electrophilic attack by a carbocation on an alkene. The reaction of a carbocation with an alkene to form a new carbon-carbon bond is both kinetically accessible and thermodynamically favorable. 

Highly reactive mixed anhydrides can also promote acylation. Phenylacetic acid reacts with alkenes to give 2-tetralones in TFAA-H3P04.55 This reaction involves an intramolecular Friedel-Crafts alkylation subsequent to the acylation. 

In 2007, two groups independently described asymmetric phosphoric acid-catalyzed Friedel-Crafts alkylations of indoles. While You et al. chose the conventional approach and employed imines as substrates (Scheme 11), Terada and coworkers came up with a different concept and used electron-rich alkenes as precursors (Scheme 49) [73]. Enecarbamates 125 reacted with indoles 29 in the presence of BINOL phosphate (R)-io (5 mol%, R = bearing 2,4,6-triisopropyl-... 

Shortly after the discovery of the first asymmetric phosphoric acid-catalyzed transformation of enecarbamates, Zhou et al. expanded the scope of the Friedel-Crafts alkylation of indoles 29 with electron-rich alkenes to enamides 127 (Scheme 51) [74],... 

The Friedel-Crafts alkylation reaction usually involves the interaction of an alkylation agent such as an alkyl halide, alcohol, or alkene with an aromatic compound, to form an alkylated aromatic compound (Scheme 5.1-44). 

At higher temperatures, C—H and C—C bonds may be similarly broken. Thus, zeolite catalysts may be used for (i) alkylation of aromatic hydrocarbons (cf. the Friedel-Crafts reactions with AICI3 as the Lewis acid catalyst), (ii) cracking of hydrocarbons (i.e., loss of H2), and (Hi) isomerization of alkenes, alkanes, and alkyl aromatics. 

Besides aluminum chloride, the most often used and studied Friedel-Crafts catalyst, many other acid catalysts are effective in alkylation. Although Friedel-Crafts alkylation was discovered and explored mainly with alkyl halides, from a practical point of view, alkenes are the most important alkylating agents. Others include alcohols, ethers, and esters. 

Carbocations formed through protonation of alkenes by proton acids are usually assumed as intermediates in alkylation with alkenes. Metal halides, when free of protic impurities, do not catalyze alkylation with alkenes except when a cocatalyst is present. It was shown that no neat conjugate Friedel-Crafts acids such as HA1C14 or HBF4 are formed from 1 1 molar compositions in the absence of excess HC1 or HF, or another proton acceptor.163-166 In the presence of a proton acceptor (alkene), however, the Lewis acid halides—hydrogen halide systems are readily able to generate carbocations ... 

A binuclear Ir3+ complex used in the reaction of benzene with alkenes induces the formation of straight-chain alkylbenzenes with higher selectivity than in the branched isomers 407 The selectivity, unusual in conventional acid-catalyzed Friedel-Crafts alkylations, is suggested to result from the complex activating the C—H bond of benzene instead of the alkene. 

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