Ionic liquid-supported acid

Lapis, A. A. M. de Oliveira, L. F. Neto, B. A. D. Dupont, J. (2008). Ionic Liquid Supported Acid/Base-Catalyzed Production of Biodiesel. Chemsuschem., 1,759-762. 

Recently, iodobenzoates anchored onto an ionic liquid support (6.4) were coupled to various aryl boronic acids (6.5) in aqueous media using Pd(OAc)2 as the catalyst at 80°C to give the coupled product 6.6 (Scheme 6.3). Compounds 6.6 were purified simply by washing the reaction mixture with ether, which removed the unreacted starting materials and the side product 6.7 without the need of chromatography. Compounds 6.6 were then cleaved from the ionic liquid support... 

These supported cycloadducts were then treated with a base (LiOH, NaOH) in a mixture of water and alcohol to give the expected free acid derivatives. However, while the latter compounds were readily recovered, the same was not true for the ionic liquid 4b, which was obtained as a dark brown liquid impure by NMR analysis. Very likely, the basic hydrolysis of the ester function caused the deprotonation of the imidazolium ring leading to a series of undesired side-reactions. Therefore, milder reaction conditions were explored to cleave the Diels-Alder product from the ionic liquid support. Handy and Okello found that the best method was the cyanide-mediated transesterification that gave the corresponding methyl esters 9-11 and allowed recover of 4b in at least 90% yield. It was also demonstrated that the recovered 4b could be used for further supported syntheses. In fact, in two subsequent mns the yields of the final ester compound were similar, indicating that the ionic liquid 4b could be efficiently recycled. 

Kawano and Togo introduced an ionic liquid group into iodoarenes, to form ionic liquid-supported iodoarenes, and used them for the promotion of the synthesis of oxazoles [24]. The results of the reactions of acetonitrile, m-chloroperbenzoic acid (mCPBA), trifluoromethanesulfonic acid (TfOH), and acetophenone are shown in Table 12.1, using various IL-supported iodoarenes (IL-supported Phis). The reactivities of IL-supported iodoarenes (Phis) 17-25 are shown in entries 1-9, and IL-supported Phi 20 showed the best reactivity. Instead of acetonitrile as solvent, room temperature ILs, such as [emim][OTs], [bmim][PFg], and [bmpyjlNTf ], were used in the presence of IL-supported Phi 20 (entries 10-12). However, [emim][OTs] did not promote the oxazole formation at all, while [bmimJPF and [bmpy][NTfJ provided the oxazole in moderate to low yields. Thus, use of acetonitrile as solvent yielded the best reactivity as compared with these ILs. 

Mecerreyes et al. [91] used ionic liquid-supported HRP in water to conduct enzyme-catalyzed free radical polymerization. In this reaction procedure, the HRP was immobilized in BMIM NTf2 by simple dissolution. This was then added to an aqueous solution of aniline, dodecylbenzenesulfonic acid (DBSA) and H202 at pH 4.3. Under these conditions, polymerization occurred immediately (as evidenced by the green color of the solution), but yield was low due to precipitation and association of polyaniline at the surface of the ionic liquid. To overcome this problem, a less hydrophobic ionic liquid, BMIM PF6 was used. In this case, the solution separated into two liquid phases after 0.5 hours one contained the polymer in water and the other was ionic liquid-immobilized HRP. The IL-HRP could be recycled and polymerization successfully repeated up to five times. The resulting polymer had similar electric conductivity properties to conventionally prepared polymer. A schematic of this process is presented in Figure 13.10. 

The same authors have also reported 1,3-dipolar cycloadditions using 2-hydroxy and 3-hydroxybenzaldehydes grafted on a soluble ionic liquid support [62]. New benzaldehyde-supported ionic liquids were prepared via two different routes. In the first approach the synthesis started from an N-alkylimidazole and 2-chloroethanol, thermolysis of which, followed by anion exchange to form the BF4 or PF ionic liquid, gave the desired supports. After esterification with an acid-functionalized 2-hydroxybenzaldehyde, excellent yields of the benzaldehyde-supported ionic liquids were obtained. The synthetic approach is shown in Scheme 7.13. 

Synthesis of thiazolidinones [PEG -RMIM]X ionic liquids have been used for rapid synthesis of a small library of amido 4-thiazolidinones from amine, aldehyde, and mercaptoacid components (Scheme 7.22) [74]. In an initial feasibility study, acid-functionalized benzaldehydes were first coupled to the [PEG -RMIM]X ionic liquids. Imines were formed by reaction of the supported aldehydes with primary amines. The reactions were run in open vessels. Optimum results were obtained by irradiating the reaction mixture with low power at 100 °C for 20 min. The imines were then condensed with mercaptoacids to give the desired thiazolidinones which were then cleaved from the ionic liquid support by amide formation. Microwave irradiation was again used in this cleavage step. The procedure entailed addition of a small amount of solid potassium tert-butoxide to a premixed mixture of the amine and supported thiazolidinone and microwave exposure for 10-20 min at 100 or 150 °G depending on the amine used. In another study, a series of one-... 

Recently, iodobenzoates anchored onto an ionic liquid support (6.4) were coupled to various aryl boronic acids (6.5) in aqueous media using Pd(OAc)2 as the catalyst at 80°C to give the coupled product... 

Likewise, efficient recyclable bifunctional catalysts 112 and 113 (Eigure 4.3) bearing ionic liquid-supported TEMPO and iodoarene moieties have been developed and used for the environmentally benign catalytic oxidation of alcohols [92]. Compounds 112 and 113 have been tested as catalysts for the oxidation of alcohols to the corresponding carbonyl compounds using peracetic acid as a green and practical oxidant. Ion-supported catalysts 112 and 113 can be conveniently recovered from the reaction mixture and reused without any loss of catalytic activity (Section 5.5). 

Figure 5.4 shows several examples of ionic-liquid-supported (also called ion-supported) hypervalent iodlne(III) reagents [93], Ion-supported (diacetoxyiodo)arenes 98-100 are prepared by the peracetic oxidation of appropriate ion-supported aryl iodides [93-95], The ion-supported tosylates 101 and 102 are made by treatment of the appropriate acetates with toluenesulfonic acid in acetonitrile [95,96] and the ion-supported iodosylbenzenes 103 and 104 are prepared by treatment of appropriate diacetates with sodium hydroxide in water [93], Ion-supported derivatives 98-104 are thermally stable solids or viscous liquids that can be used as efficient recyclable hypervalent iodine(III) reagents. Reactions of these reagents with organic substrates produce the corresponding ion-supported aryl iodides as by-products, which can be easily recovered from the reaction mixture either by extraction into an ionic liquid phase or by simple filtration. 

Ren J, Wu L, Li B Preparation and CO2 sorption/desorption of N-(3-aminopropyl) aminoethyl tributylphosphonium amino acid salt ionic liquids supported into porous silica particles, Ind Eng Chem Res 51 7901—7909, 2012. 

Utilized in the synthesis of diaryl ethers with sodium phenoxides in DMF at 100 °C [54]. Yields were moderate, which might be because of poor chemoselectivity resulting in oxygenation of the polystyrene backbone. Ionic liquid-supported diaryliodonium salts were recently utilized in chemoselective 0-arylations of phenols and carboxylic acids using literature conditions in good yields without need for chromatography [55]. 

This type of co-catalytic influence is well loiown in heterogeneous catalysis, in which for some reactions an acidic support will activate a metal catalyst more efficiently than a neutral support. In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transition metal catalysts dissolved in it. 

Recently, there has been considerable interest in developing molten salts that are less air and moisture sensitive. Melts such as l-methyl-3-butylimidazolium hexa-fluorophosphate [211], l-ethyl-3-methylimidazolium trifluoromethanesulfonate [212], and l-ethyl-3-methylimidazolium tetrafluoroborate [213] are reported to be hydro-phobic and stable under environmental conditions. In some cases, metal deposition from these electrolytes has been explored [214]. They possess a wide potential window and sufficient ionic conductivity to be considered for many electrochemical applications. Of course if one wishes to take advantage of their potential air stability, one loses the opportunity to work with the alkali and reactive metals. Further, since these ionic liquids are neutral and lack the adjustable Lewis acidity common to the chloroaluminates, the solubility of transition metal salts into these electrolytes may be limited. On a positive note, these electrolytes are significantly different from the chloroaluminates in that the supporting electrolyte is not intended to be electroactive. 

Even if the ionic liquid is not directly involved in creating the active catalytic species, a co-catalytic interaction between the ionic liquid solvent and the dissolved transition metal complex often takes place and can result in significant catalyst activation. When a catalyst complex is, for example, dissolved in a slightly acidic ionic liquid some electron-rich parts of the complex (e.g., lone pairs of electrons in the ligand) may interact with the solvent, providing increased activity to the resulting catalytic centre. Acidic ionic liquids can be considered as liquid acid supports for transition metal catalysts dissolved therein. 

The first example of SILP-catalysis was the fixation of an acidic chloroaluminate ionic liquid on an inorganic support. The acidic anions of the ionic liquid, [AI2CI7] and [AI3CI10], react with free OH-groups of the surface to create an anionic solid surface with the ionic liquid cations attached [72]. The catalyst obtained was applied in the Friedel-Crafts acylation of aromatic compounds. Later, the immobilisation of acidic ionic liquids by covalent bonding of the ionic liquid cation to the surface was developed and applied again in Friedel-Crafts chemistry [73]. 

This section describes catalytic systems made by a heterogeneous catalyst (e.g., a supported metal, dispersed metals, immobilized organometaUic complexes, supported acid-base catalysts, modified zeolites) that is immobilized in a hydrophilic or ionic liquid catalyst-philic phase, and in the presence of a second liquid phase—immiscible in the first phase—made, for example, by an organic solvent. The rationale for this multiphasic system is usually ease in product separation, since it can be removed with the organic phase, and ease in catalyst recovery and reuse because the latter remains immobilized in the catalyst-philic phase, it can be filtered away, and it does not contaminate the product. These systems often show improved rates as well as selectivities, along with catalyst stabilization. 

This type of co-catalyhc influence is well known in heterogeneous catalysis, in which for some reachons an acidic support will achvate a metal catalyst more efficiently than a neutral support In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transihon metal catalysts dissolved in it As one would expect, in those cases in which the ionic liquid acts as a co-catalyst, the nature of the ionic liquid becomes very important for the reactivity of the transihon metal complex. The opportunity to opdmize the ionic medium used, by vari-ahon of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for ophmizahon. However, the choice of these parameters may be restricted by some possible incompahbilihes with the feedstock used. Undesired side reachons caused by the Lewis acidity of the ionic Hquid or by strong interaction between the Lewis acidic ionic Hquid and, for example, some oxygen funchonalihes in the substrate have to be considered. 

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