Acylation, ionic liquids immobilized

The ability of iron(III) chloride genuinely to catalyze Friedel-Crafts acylation reactions has also been recognized by Holderich and co-workers [97]. By immobilizing the ionic liquid [BMIM]Cl/FeCl3 on a solid support, Holderich was able to acetylate mesitylene, anisole, and m-xylene with acetyl chloride in excellent yield. The performance of the iron-based ionic liquid was then compared with that of the corresponding chlorostannate(II) and chloroaluminate(III) ionic liquids. The results are given in Scheme 5.1-67 and Table 5.1-5. As can be seen, the iron catalyst gave superior results to the aluminium- or tin-based catalysts. The reactions were also carried out in the gas phase at between 200 and 300 °C. The acetylation reac-... 

In order to overcome these problems, attention was focused on the use of heterogeneous catalysis. We have found that functionalized solid materials, e.g., ionic liquids or tin triflates immobilized into mesoporous materials, can be used in N-acylation reactions as environmentally friendly replacements for traditional homogeneous acids which are useful but environmentally unacceptable catalysts [17, 18]. They had comparable activity to homogeneous reagents but can offer greater stability, safer and easier handling and can be... 

Biocatalysts also operate in ionic liquids [28]. The ones that have been most widely investigated are the lipase family of enzymes. For example, Candida Antarctica lipase B immobilized in [bmim][BF4] or [bmim][PFe] under anhydrous conditions is able to catalyse transesterifications at rates comparable to those observed in other solvents. Certain lipase mediated enantioselective acylations have even resulted in considerable improvements in enantiomeric excesses... 

In-situ IR-spectroscopic characterization of the Friedel-Crafts acylation of benzene in ionic liquids derived from AICI3 and FeCl3 showed that the mechanism of the reaction in ionic liquids was the same as that in 1,2-dichloroethane (128). The immobilization of ferric chloride-containing ionic liquid onto solid supports (e.g., silica and carbon) however failed to catalyze the acylation reaction, because leaching was a serious problem. When the reaction was carried out with gas-phase reactants, catalyst deactivation was observed. 

Pencillin G acylase from E. coli is functionally, but not structurally, related to lipases. The enzyme would find wider use if it could be rendered tolerant of low-water media, which is the kind of problem that ionic liquids were expected to solve. It was found, however, that a covalently immobilized penicillin acylase, PGA 450, required aw = 0.8, which also was the minimum in toluene, to stay active in the ionic liquids [BMIm][BF4], [OMIm][BF4], and [BMIm][PF6] [67]. In a simple amine acylation test reaction (Figure 10.4), PGA 450 was somewhat less active in ionic liquids than in toluene. 

Acylations of carbohydrate derivatives such as alkyl glucosides and galactosides have also been successfully performed in ionic liquids [63]. Similarly, the flavonoid glycosides naringin and rutin were acylated with vinyl butyrate in ionic liquid media in the presence of a number of lipases, e.g., CaLB (Novozym 435), immobilized TIL, and RmL [119]. The products are of interest for application as strong antioxidants in hydrophobic media. 

In order to investigate the efficiency and selectivity of the enzymahc acylation of various polyhydroxylated natural compounds in ionic liquid media the transesterification of sahcin, hehcin, escuHn, and naringin, as well as sUybin catalyzed by immobilized CALB was investigated using vinyl butyrate as the acyl donor. Two ionic Hquids were used, the water-miscible 1-butyl-3-methyHmidazolium tetrafluo-roborate ([bmim]BF4) and l-butyl-3-methylimidazoHum hexafluorophosphate ([bmimJPFs), in which the solubility of water is hmited. 

As can be seen, immobilized CALB efficiently catalyzes the acylation of aU polyhydroxylated compounds in the ionic liquid media used, leading to high conversion yields. The reaction rates for the enzymatic acylations are summarized in Table 9.3. Higher reaction rates were obtained for aU polyhydroxylated compounds tested when [bmim]BF4 was used as the reaction medium. It is interesting to note that the solubilities of esculin, salicin, helicin, naringin, and silybin at 60 C were approximately 68, 23,40,100, and 82 mM, respectively, in [bmim]BF4. On the other hand, in [bmim]PFs and acetone, in which the reactions rates were lower, the solubilities of the aforenamed compounds were 20, 5.5,19.5,1.5, and 7.2 mM and 6.5,... 

Figure 9.3 Effect of the molar ratio of vinyl butyrate to silybin (30mM) on the conversion yield for its enzymatic acylation catalyzed by immobilized CALB in the ionic liquids [bmim]BF4 and [bmimJPFs at 60°C after 72h of incubation.

In addition to free fatly acids and their esters as previously described, compounds with antioxidant potency were also used as acyl donors. In order to investigate the synthesis of hybrid antioxidants, the enzymatic acylation of escuhn catalyzed by immobilized CALB in the ionic liquids [bmtm]BF4 and [bmim]PF6 was investigated using several phenolic acids as well as their corresponding vinyl esters. 

The ionic liquids, as the green solvents, have been extensively applied to the C-, 0-, N-, and 5 -acylations. It is obvious that the use of ionic liquids as replacement to the conventional organic solvents has been eco-friendly to the environment. Besides, the ionic liquids as the unusual reaction medium could promote the reaction and increase the conversion and selectivity as well. Moreover, the immobilization of homogeneous catalysts in the ionic liquids to form composite catalytic systems could make the catalysts to be more easily separation and reuse. 

In the alkylation section, it was stated that the immobilized ionic liquids can combine the advantage of green media with solid support materials, which may enable the wide apphcation of precious ionic hquids by the reduchon of usage and also realize the sustainability of the chemical reaction process. The supported ionic hquids (SlLs) catalysts commonly employ the supports such as the macroporous polymer, metal oxide (SiO, AI2O3, etc.), zeohte, clay, and achve carbon, and after the immobilization of the ionic hquids, the ionic hquids still maintain their special solvent effect. Presently, the immobilized ionic hquids have been applied extensively to the alkylation, acylation, hydroformylation, oxidation, esterihcation, hydrolyza-tion, hydrogenahon, and other unit reactions this part of the chapter only discusses the application of immobilized ionic hquids to the acylahon. 

In addition, immobilized ionic liquids have been employed for the 0-acylation and A-acylation. Kara et al. have investigated the reactions that supported ionic liquids in Upase-catalyzed asymmetric acylation of 7-phenylethanol with vinyl acetate in the reaction temperature range of 25-60°C and found that the immobilized [EMlM][NTfJ can stabilize the lipase against inactivation and maintain good enantioselectivity [152]. Furthermore, Paun s group via grafting 7,5-dimethyl-5-(5-triethoxysilylpropyl)-imidazolium tetrafluoroborate or bis (trifluoromethyl)... 

For the application of the immobilized ionic liquids to the acylation (although the advantages are obvious, such as the simplification of the product isolation), the realization of the continuous reactions, the leaching of the ionic liquids, and the inactivation of the catalysts because of carbon distribution are still the problems urgently needed to be resolved. Therefore, the interaction of the ionic liquids with supports, the recovery of the supported catalysts, and so on need the in-depth and intensive research. 

Li J, Su W, Lin J et al (2005) Friedel-Crafts acylation of ferrocene catalyzed by immobilized ytterbium (III) tiiflate in ionic liquid. Syn Commun 35 1929-1937... 

Coman SM, Elorea M, Parvulescu VI et al (2007) Metril-triflate ionic liquid systems immobilized onto mesoporous MS41 materials as new and efficient catrilysts for 7V-acylation. J Catal 249 359-369... 

Supported ionic liquid compositions are also a vivid field of research in which many companies tried to make their claims [88-90]. By immobilizing ionic liquids onto silica- or alumina-based carriers it is possible to obtain new Lewis acid catalysts with interesting characteristics. These are presently preferably used for alkylation and acylation reactions of aromatic compounds [91, 92] or isomerizations. Even the co-immobilization of ionic liquids with transition metal complexes [93] or Lewis acids [94] has been described, and it can be anticipated that this particular field offers many options for future catalyst development. 

Comparison of results obtained from the alkylation of benzene with dodecene with the resulting catalysts made by the two immobilization methods revealed, in particular, excellent activity and selectivity for the materials obtained by cation grafting. Furthermore, these initial - but highly promising - results have been accompanied by an increased industrial interest for commercialization of these grafted ionic liquid catalyst systems. Hence, Johnson Matthey Catalysts aimounced (ICC Paris, July 2004) the commercialization of such grafted ionic liquid alkylation/ acylation catalysts in late 2004, through their associated chemical company Alfa Aesar [74]. 

Ionic liquids in combination with supercritical fluids are a versatile tool for the immobilization and recycling of homogeneous catalysts, allowing continuous Friedel-Crafts acylation reactions to be realized. The acylation of anisole with acetic anhydride is carried out in a flow system using a metal triflate immobilized in the ionic liquid 1 -butyl-4-methylpyridinium bis(trifluoro-methylsulfonyl)imide as catalyst and scCO as continuous extraction phase [22]. Different metal triflates are utilized under continuous flow conditions using high pressure yttrium triflate possesses the best balance between sufficient acidity for catalytic activity and softness to release the product and so permits a good catalyst reuse (TONs up to 190). 

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