Acidic ionic liquids

When [EMIMJCl is present in a molar excess over AICI3, only equilibrium (2.1-1) need be considered, and the ionic liquid is basic. When a molar excess of AICI3 over [EMIMJCl is present on the other hand, an acidic ionic liquid is formed, and equilibria (2.1-2) and (2.1-3) predominate, further details of the anion species present may be found elsewhere [23]. The chloroaluminates are not the only ionic liquids prepared in this manner. Other Eewis acids employed have included AlEtCl2 [24], BCI3 [25], CuCl [26], and SnCl2 [27]. In general, the preparative methods employed for all of these salts are similar to those indicated for AlCl3-based ionic liquids as outlined below. 

Wilkes and co-workers have investigated the chlorination of benzene in both acidic and basic chloroaluminate(III) ionic liquids [66]. In the acidic ionic liquid [EMIM]C1/A1C13 (X(A1C13) > 0.5), the chlorination reaction initially gave chlorobenzene, which in turn reacted with a second molecule of chlorine to give dichlorobenzenes. In the basic ionic liquid, the reaction was more complex. In addition to the... 

Keim and co-workers have carried out various alkylation reactions of aromatic compounds in ionic liquids substantially free of Lewis acidity [84]. An example is the reaction between benzene and decene in [BMIM][HS04], which was used together with sulfuric acid as the catalyst (Scheme 5.1-54). These authors have also claimed that these acid-ionic liquids systems can be used for esterification reactions. 

In cases in which 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 still 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) will interact with the solvent in a way that will usually result in a lower electron density at the catalytic center (for more details see Section 5.2.3). 

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. 

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 transition metal complex. The opportunity to optimize the ionic medium used, by variation of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for optimization. However, the choice of these parameters may be restricted by some possible incompatibilities with the feedstock used. Undesired side reactions caused by the Lewis acidity of the ionic liquid or by strong interaction between the Lewis acidic ionic liquid and, for example, some oxygen functionalities in the substrate have to be considered. 

Moreover, these experiments reveal some unique properties of the chlorostan-nate ionic liquids. In contrast to other known ionic liquids, the chlorostannate system combine a certain Lewis acidity with high compatibility to functional groups. The first resulted, in the hydroformylation of 1-octene, in the activation of (PPli3)2PtCl2 by a Lewis acid-base reaction with the acidic ionic liquid medium. The high compatibility to functional groups was demonstrated by the catalytic reaction in the presence of CO and hydroformylation products. 

Table 7.2-1 Polymerization of isobutene in the acidic ionic liquid [EMIMJCI/AICL (XjAICL)...

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]. 

The presence of Lewis acidic species in chloroaluminate ionic liquids has also been used to bring about various acid catalysed transformations that do not require additional catalysts. For example, acidic ionic liquids are ideally suited to Friedel-Crafts acylation reactions. In a traditional Friedel-Crafts acylation an acylium ion is generated by reaction between acyl chloride and A1C13 or FeCL as shown in Scheme 10.7. 

Gui JZ, Liu D, Wang C, Lu F, Lian JZ, Jiang H, Sun ZL (2009) One-Pot Synthesis of 3,4-Dihydropyrimidin-2(lH)-ones Catalyzed by Acidic Ionic Liquids Under Solvent-Free Conditions. Synth Commun 39 3436-3443... 

An area of broad interest in catalysis is the search for viable replacements for the widely used Bronsted liquid acids such as HF and H2SO4 and solid Lewis acids such as AICI3 and MgCl2. The liquid acids are corrosive and also costly, because of the need to work up the products by neutralization and repeated washing. In many cases, the contamination of the products by these acids induces degradation over time and limits the application of the products. The cost of multi-step washing can be quite high. Acidic ionic liquids therefore offer potential alternatives for such reactions (//). 

The equilibrium (1) dominates in basic ionic liquids, where x AlCl3<0.5 the equilibrium (2) dominates in acidic ionic liquids, where x AICI3 >0.5. This means that over much of the liquid range the anions present in significant quantities are Cl, AICI4, and AI2CI7, and their relative amounts are controlled by the equilibrium ... 

The heptachloroaluminate ion is a strong Lewis acid, and chloride ion is the conjugate Lewis base. The basic ionic liquids, in which equilibrium (1) dominates, are light green in color, and the acidic ones, in which equilibrium (2) dominates, are darker and brownish in color (23). The acidic ionic liquids in this family are less viscous than the basic ones. 

In the BASF BASIL process that utilizes A-methylimidazole to scavenge HCl byproduct, the acidic ionic liquid A-methylimidazolium chloride [HMIMJCl was formed, with a melting point of 75°C (13,102). Recently, the group of Bronsted acidic ionic liquids with the same cation was extended to include other anions, such as BFF, TfO , and TsO . The melting point of the salt is between 30 and 109°C. Strong hydrogen bonding in the tosylate salt was characterized by IR spectroscopy. 

Several authors reported the use of ionic liquids containing protonic acid in catalysis (118-120). For example, strong Bronsted acidity in ionic liquids has been reported to successfully catalyze tetrahydropyranylation of alcohols (120). Tetra-hydropyranylation is one of the most widely used processes for the protection of alcohols and phenols in multi-step syntheses. Although the control experiments with the ionic liquids showed negligible activity in the absence of the added acids, high yields of product were obtained with the ionic liquid catalysts TPPTS or TPP.HBr-[BMIM]PF6. By rapid extraction of the product from the acidic ionic liquid phase by diethyl ether, the reaction medium was successfully reused for 22 cycles without an appreciable activity loss. A gradual loss of the catalyst and a reduced volume of the ionic liquid were noted, however, as a consequence of transfer to the extraction solvent. 

Chloride-free ionic liquid was essential for the high activity the addition of only 5 mol% [BMIM]C1 led to a significant decrease in the yield because of the basicity of [BMIM]C1 (722). Remarkably, this acid-ionic liquid combination could be reused many times for the glycosidations without any loss in efficiency. 

Bronsted acidic ionic liquids [HMIM]BF4, [HMIM]TfO, and [HMIM]TsO are effective for proton- and metal-assisted catalytic processes such as the dimerization of methyl acrylate and ring-closure metathesis (Scheme 3) (72J). For example, the acidic [HBIM]BF4 in association with the [Ru( = C = C = CPh2)(/ -cymene) (PCy3)Cl][OTf catalyst afforded a dramatic increase in the activity (100%) and selectivity (100%) for the ring-closure metathesis of A,A-diallyltosylamide. 

The acylative cleavage of the cyclic ether tetrahydrofuran showed an excellent yield (95%) to 4-iodobutylbenzoate when fully acidic ionic liquid [EMIM]Al2Cl7 was used, but the yield to di-functionalized product (61%) suffered when the mildly acidic halogenoaluminate ionic liquid (which has a mole fraction of AICI3 (x) at a value of 0.52) was used as the solvent. In contrast, for 1,5-dimethyltetrahydrofuran and tetrahydropyran, a good yield to the cleavage products was obtained when the mildly acidic chloroaluminate was used. 

An excellent demonstration of the tunability of ionic liquids for catalysis is provided by an investigation of the dimerization of 1-butene (235). A Ni(cod)(hfacac) catalyst (Scheme 23) was evaluated for the selective dimerization of 1-butene after it was dissolved in various chloroaluminate ionic liquids. Earlier work on this reaction with the same catalyst in toluene led to the observations of low activity and difficult catalyst separation. In ionic liquids of varying acidity, little catalytic activity was found. However, a remarkable activity was achieved by adding a weak buffer base to an acidic ionic liquid. The reaction took place in a biphasic reaction mode with facile catalyst separation and catalyst recycling. A high selectivity to the dimer product was obtained because of a fast extraction of the Cg product from the ionic liquid phase, with the minimization of consecutive reaction to give trimers. Among a number of weak base buffers, a chinoline was chosen. The catalyst performance was compared with that in toluene. The catalyitc TOF at 90°C in toluene was... 

Ionic liquids containing chloroaluminate ions are strong Lewis, Franklin and Bronsted acids. Protons present in [emim][AlCl4] have been shown to be superacidic with Hammett acidities up to —18. Such highly acidic ionic liquids are, nonetheless, easily handled and offer potential as non-volatile replacements for hazardous acids such as HF in several acid-catalyzed reactions. 

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