Ionic liquids, Friedel-Crafts alkylation

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). 

Acidic chloroaluminate ionic liquids have already been described as both solvents and catalysts for reactions conventionally catalyzed by AICI3, such as catalytic Friedel-Crafts alkylation [35] or stoichiometric Friedel-Crafts acylation [36], in Section 5.1. In a very similar manner, Lewis-acidic transition metal complexes can form complex anions by reaction with organic halide salts. Seddon and co-workers, for example, patented a Friedel-Crafts acylation process based on an acidic chloro-ferrate ionic liquid catalyst [37]. 

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. 

It was reported that Sc(OTf)3 catalyzes Diels-Alder reactions in [bmim][X] (X = BF4, SbFg or OTf), in this case at much lower catalyst loadings (0.2 m%). In contrast to the Friedel-Crafts alkylation (see below) the product did not form a separate phase and was recovered by extraction with ether. It was shown, however, that the ionic liquid containing the catalyst could be recycled eleven times without loss of activity, Scheme 4. Furthermore, improved endojexo selectivities were observed with cyclic dienes. 

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). 

Silica-alumina particles coated with a permselective silicalite membrane is almost completely selective in the formation of p-xylene in the disproportionation of toluene.402 Friedel-Crafts alkylations were performed in ionic liquids. The strong polarity and high electrostatic fields of these materials usually bring about enhanced activity.403 404 Easy recycling is an additional benefit. Good characteristics in the alkylation of benzene with dodecene were reported for catalysts immobilized on silica or MCM-41 405... 

Acidic chloroaluminate ionic liquids were used as reaction media for Friedel-Crafts reactions as early as 1976 [34], Systematic investigations into Friedel-Crafts alkylations of benzene with the same acidic systems followed in 1986 by Wilkes et al. [35]. The alkylation of benzene with alkenes in acidic imidazolium chloroaluminate melts was disclosed in a patent by BP Chemicals in 1994 [36]. Here, as advantages over the reaction with aluminum trichloride in organic solvents, claims are made regarding the easy isolation of the product, the practically total reusability of the liquid catalyst and the better selectivity to the desired products. 

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. 

Friedel Crafts Alkylation Reactions With Immobilised Ionic Liquids... 

The third case shows the immobilisation of Lewis-acidic ionic liquids. The resulting catalysts, named Novel Lewis-Acidic Catalysts (NLACs), are highly active in the Friedel-Crafts alkylation of aromatic compounds with dodecene. Conversions and selectivities to the desired monoalkylated products were excellent. No leaching of the catalytically active component could be observed. The isomer distribution of the monoalkyated products is very similar to that obtained over pure aluminum(III)chloride. The main drawback of the NLACs is that thy are very sensitive towards water, which leads to irreversible deactivation. A second problem is the deactivation after long reaction times. The most likely cause is olefin oligomerisation. 

Electron-rich phenols react with isoprene in ionic liquids and with added Sc(OTf)3 to give 2,2-dimethylchromans <07SL3050>. Ionic liquids also promote the one-pot synthesis of 4-arylchromans from benzaldehydes, phenols and allyl bromide which involves sequential Barbier allylation, Friedel-Crafts alkylation and an intramolecular hydroalkoxylation <07SL1357>. 

The first example of a Friedel-Crafts type reaction in an ionic liquid medium dates back to 1976 when the electrochemical oxidation of hexamethylbenzene in [C2py]Br-AlCl3 afforded a mixture of alkylated polyphenyl compounds.[69] Other early examples include the alkylation of benzene in C2Ciim C1-A1C137 and the acylation of ferrocene in [C2Ciim]I-AlClJ71 There are now numerous examples of Lewis- or Bronsted acid-catalysed Friedel-Crafts type reactions in ionic liquids. These include alkylation,[72 76] acylation,[71,77"83] arylation,[77 841 sulfonylation,[851 sulfoamylation[86] and O-acetylation of alcohols.[87,881... 

Ionic liquids containing chloroaluminate anions are strong Lewis acids. This characteristic, coupled with the fact that they are relatively easy to handle, makes them attractive alternatives to standard Lewis and Bronsted acids such as HF. The IL can function both as a catalyst and a solvent for acid-catalyzed processes. Some of the first reactions studied in ILs were Friedel-Crafts alkylations and arylations. The [emim][Al2Cl7] affords complete conversion of benzene and acetyl chloride to acetophenone in less than 5 minutes at room temperature. 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

Despite the early use of phosphonium salt melts as reaction media [12, 18, 25], the use of standard ionic liquids of type 1 and 2 as solvents for homogeneous transition metal catalysts was described for the first time in the case of chloroaluminate melts for the Ni-catalyzed dimerization of propene [5] and for the titanium-catalyzed polymerization of ethylene [6]. These inherently Lewis-acidic systems were also used for Friedel-Crafts chemistry with no added catalyst in homogeneous [7] as well as heterogeneous fashion [8], but ionic liquids which exhibit an enhanced stability toward hydrolysis, i. e., most non-chloroaluminate systems, have been shown to be of advantage in handling and for many homogeneously catalyzed reactions [la]. The Friedel-Crafts alkylation is possible in the latter media if Sc(OTf)3 is added as the catalyst [9]. 

It is not surprising that electrophilic aromatic substitutions were the first organic reactions investigated using acidic room-temperature chloro-almninate(III) ionic liquids. Indeed, chloroaluminate(ni) species combine their properties of good solvents for simple arenes to their role as Lewis acid catalysts. In Friedel-Craft alkylations, polyalkylation is common as well as the isomerisation of primary halides to secondary carbonium ions. 

The Friedel-Crafts alkylation of aromatic compounds is of great importance in laboratory synthesis and industrial production. For example, the industrial processes for ethylbenzene, cumene and linear alkylbenzenes, etc., are on the base of this kind of reaction. It is well known that the drawbacks of the traditional acid catalysts such as A1Q3, H SO, and HF do great harm to the equipment and the environment, and these catalysts cannot be reused after the usual aqueous work-up besides, most of the reactions are carried out in the harmful and volatile organic solvents which can cause the environmental pollution aU of these problems need the replacement of the solvents or the acid catalysts. In this context, room-temperature ionic liquids have been iuCTeasingly employed as green solvents. 

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... 

In the Xiao s laboratory [27], the pyridinium-based ionic liquids as the suitable media were used for the Friedel-Crafts alkylation of benzene, which was carried out at a better rate but at relatively lower temperature with high product conversion. They found that the [EtPyKCFjCOO] (i-ethyl-pyridinium trifluoroacetatel-FeClj was an enviromnental friendly, recyclable, and reusable solvent-catalyst system to replace the traditional aluminum catalysis system. During the process, the alkylation... 

Table 2.1 The ionic liquids [EtPy][CF3COO]-FeCl3/AlCl3 as catalytic system for the Friedel-Crafts alkylation...

Not only in Friedel-Crafts alkylation or C-alkylation but also in the A-alkylation, ionic liquids have obtained mnch attention for obvious advantages including... 

Table 2.4 Friedel-Crafts alkylations using ionic liquids... 

Xiao Y, Malhotra SV (2005) Friedel-Crafts alkylation reactions in pyridinium-based ionic liquids. J Mol Catal A Chem 230 129-133... 

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... 

Zhao ZK, Li ZS, Wang GR et al (2004) Friedel-Crafts alkylation of 2-methylnaphthalene in room temperature ionic liquids. Appl Catal A Gen 262 69-73... 

Friedel-Crafts alkylation. Secondary alkyl mesylates are adequate alkyl donors in this reaction." Both Sc(OTf)3 and TfOH can be used as the catalyst. It has also been reported that Sc(OTf js immobilized in ionic liquid forms a recyclable system for arene alkylation with alkenes. ... 

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