For Friedel-Crafts alkylation

Synthesis This route has been carried out successfully (Rec. Trav. Chem., 1958, 77, 854). Note that no AICI3 is needed for Friedel-Crafts alkylation with easily formed t-alkyl compounds. 

Neither Friedel-Crafts acylation nor alkylation reactions can be earned out on mtroben zene The presence of a strongly deactivating substituent such as a nitro group on an aromatic ring so depresses its reactivity that Friedel-Crafts reactions do not take place Nitrobenzene is so unreactive that it is sometimes used as a solvent m Friedel-Crafts reactions The practical limit for Friedel-Crafts alkylation and acylation reactions is effectively a monohalobenzene An aromatic ring more deactivated than a mono halobenzene cannot be alkylated or acylated under Friedel-Crafts conditions... 

In addition, boron, aluminum, and gallium tris(triduoromethanesulfonates) (tridates), M(OTf)2 and related perduoroalkanesulfonates were found effective for Friedel-Crafts alkylations under mild conditions (200). These Lewis acids behave as pseudo haUdes. Boron tris(tridate) shows the highest catalytic activity among these catalysts. A systematic study of these catalysts in the alkylation of aromatics such as benzene and toluene has been reported (201). 

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. ... 

The methodology of a Lewis acid dissolved in an ionic liquid has been used for Friedel-Crafts alkylation reactions. Song [85] has reported that scandium(III) tri-flate in [BMIM][PFg] acts as an alkylation catalyst in the reaction between benzene and hex-l-ene (Scheme 5.1-55). 

Friedel-Crafts alkylations are among the most important reactions in organic synthesis. Solid acid catalysts have advantages in ease of product recovery, reduced waste streams, and reduction in corrosion and toxicity. In the past, people have used (pillared) clays (18), heteropolyacids (19) and zeohtes (20) for Friedel-Craft alkylations, with mixed success. Problems included poor catalyst stabihty and low activity. Benzylation of benzene using benzyl chloride is interesting for the preparation of substitutes of polychlorobenzene in the apphcation of dielectrics. The performance of Si-TUD-1 with different heteroatoms (Fe, Ga, Sn and Ti) was evaluated, and different levels of Fe inside Si-TUD-1 (denoted Fei, Fe2, Fes and Feio) were evaluated (21). The synthesis procedure of these materials was described in detail elsewhere (22). 

Bismuth tra-tri lluoromcthancsulfonate, Bi(OTf)3, and BiCh were found to be effective catalysts for the Friedel-Crafts acylation of both activated and deactivated benzene derivatives such as fluorobenzene.19 Ga(III) triflate is also effective for Friedel-Crafts alkylation and acylation in alcohols and can tolerate water.20 This catalyst is water-stable... 

Bound reagents are an excellent alternative in cases where the reagent is used in excess and can be difficult to remove. Purification is now a simple process of filtration and evaporation. For example, silica-supported aluminium chloride is a Lewis acid and an effective catalyst for Friedel-Crafts alkylations (Figure 3.15). 

Non-chlorinated Lewis acids, such as scandium triflate, were found to be good catalysts for Friedel-Crafts alkylation reactions (167). Although no aromatic hydrocarbon alkylation occurred in CH2CI2, [BMIMJPFg, Sc(OTf)3 catalyzed the alkylation of benzene with high yields of the monoalkylated product. The lower acidity of the ionic liquid led to fewer byproducts and therefore higher yields. The products were separated by simple decantation and the catalyst was reused. 

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 importance of aluminum trichloride (and AlBr3 which is more soluble in hydrocarbons) as a catalyst, particularly for Friedel-Crafts alkylation and acylation of aromatic compounds,... 

Friedel—Crafts Alkylation Using Other Carbocation Sources We have seen several ways of generating carbocations, and most of these can be used for Friedel-Crafts alkylations. Two common methods are protonation of alkenes and treatment of alcohols with BF3. 

Alcohols are another source of carbocations for Friedel-Crafts alkylations. Alcohols commonly form carbocations when treated with Lewis acids such as boron trifluoride (BF3). If benzene (or an activated benzene derivative) is present, substitution may occur. 

The use of immobilised ILs as catalysts would result in the easy separation of the catalyst from the reaction mixture, allowing its fast reuse and avoiding the generation of contaminated waste and its subsequent treatment. Ionic liquids have already been proposed as catalysts for Friedel-Crafts alkylation of benzene with olefins in order to produce LABs32-35. They show Lewis acidic properties when a Lewis acid (e.g. 

The alkylation of toluene with methanol over HZSM-5 proceeds at low temperatures via a protonated methanol species in the transition state [107] and weakly coadsoibed toluene as classically predicted for Friedel Crafts alkylation. The reaction rate is directly proportional to the concentration of the chemisorbed methanol (in the presence of excess toluene) as shown in Figure 6 [108]. Alkylation leads preferentially to ortho- and para- substituted products which rapidly isomerise in the zeolite pores. Specific reaction conditions and tailoring of the catalyst pore structure can be employed so that para- substituted products are preferentially... 

In Sec. 11.10 we said that two mechanisms are possible for Friedel-Crafts alkylation. Both involve electrophilic aromatic substitution, but they differ as to the nature of the electrophile. 

One mechanism for Friedel-Crafts alkylation involves the following steps,... 

On the other hand, there is additional evidence (of a kind we cannot go into here) that makes it very likely that there is a second mechanism for Friedel-Crafts alkylation. In this mechanism, the electrophile is not an alkyl carbonium ion, but an acid-base complex of alkyl halide and Lewis acid, from which the alkyl group is transferred in one step from halogen to the aromatic ring. 

The most likely mechanism for Friedel-Crafts acylation is analogous to the carbonium ion mechanism for Friedel-Crafts alkylation (Sec. 11.10), and involves... 

Ertel, T. S., Bertagnolli, H. EXAFS spectroscopy and MNDO/AM1/PM3 calculations a structural study of a model system for Friedel-Crafts alkylation. J. Mol. Struct. 1993, 301, 143-154. 

Electron acceptors, such as NO2 group are usually enough to prevent the reaction, so it is possible to use solvents like nitrobenzene for Friedel-Crafts alkylation. Some cations undergo isomerisation in the process, so not all derivatives may be obtained this way. 

Song s group [25] has developed a novel and recyclable catalytic system for Friedel-Crafts alkylation of aromatic compounds with alkenes. The Sc(OTf)j was immobilized in the ionic Uquids, to obtain quantitatively the desired alkylated products in the reaction of Ihe allqrlation of benzene with hex-l-ene, in which the ionic liquids containing 20 mol% ScCOTl) such as [EMIM][SbFg] ([EMIM] is denoted as... 

Lin JH, Zhang CP, Zhu ZQ et al (2009) A novel pyrrohdinium ionic liquid with 1,1,2,2-tetra-fluoro-2-(l,l,2,2-tetrafluoroethoxy)ethanesulfonate anion as a recyclable reaction medium and efficient catalyst for Friedel-Crafts alkylations of indoles with nitroalkenes. J Fluorine Chem 130 394-398... 

Figure 6.7 Microflow system for Friedel-Crafts alkylation of aromatic compounds with an N-acyliminium ion (M, micromixer R, microtube reactor)...

The two mechanisms for Friedel-Crafts alkylation are not dissimilar to the two mechanisms for nucleophilic aliphatic substitution. In an S,j1 mechanism, a carbocation is generated from an alkyl halide before the nucleophile attacks, but in an S 2 reaction the halide departs simultaneously with the nucleophile attacking the R group. In the Friedel-Crafts reaction, benzene behaves as the nucleophile. 

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