Alkylation ionic liquids

Chan TH, Law MC, Wong KY. 2004. OrganometaUic reactions in ionic liquids. Alkylation of aldehydes with diethylzinc. Green Chem 6 241 244. 

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]. 

Liu ZC, Zhang R, Xu CM, Xia RG (2006) Ionic liquid alkylation process produces high-quality gasoline. Oil Gas J 104 52-56... 

Fig. 2. General formulas for ionic liquid alkylation catalysts prepared by mixing AICI3 and a hydrocarbyl-substituted pyridinium halide (A), a hydrocarbyl-substituted imida-zolium halide (B), trialkylammonium hydrohahde (C), and tetraalkylammonium halide (D)(59).

Is the liquid or the solid phase responsible for the low-melting points of ionic liquids Alkyl-chain-length dependence of thermodynamic properties of [C mim][NTf2]. 

The alkylation process possesses the advantages that (a) a wide range of cheap haloalkanes are available, and (b) the substitution reactions generally occur smoothly at reasonable temperatures. Furthermore, the halide salts formed can easily be converted into salts with other anions. Although this section will concentrate on the reactions between simple haloalkanes and the amine, more complex side chains may be added, as discussed later in this chapter. The quaternization of amines and phosphines with haloalkanes has been loiown for many years, but the development of ionic liquids has resulted in several recent developments in the experimental techniques used for the reaction. In general, the reaction may be carried out with chloroalkanes, bromoalkanes, and iodoalkanes, with the reaction conditions required becoming steadily more gentle in the order Cl Br I, as expected for nucleophilic substitution reactions. Fluoride salts cannot be formed in this manner. 

Many historical ways to make ionic liquids proved to be impractical on larger scales. Sometimes expensive starting materials are used (anion-exchange with silver salts, for example [2]), or very long reaction times are necessary for the alkylation steps, or filtration procedures are included in the synthesis, or hygroscopic solids have to be handled. All these have to be avoided for a good synthesis on larger scales. 

The second element of general importance in the synthesis of a task-specific ionic liquid is the source of the functional group that is to be incorporated. Key to success here is the identification of a substrate containing two functional groups with different reactivities, one of which allows the attachment of the substrate to the core, and the other of which either is the functional group of interest or is modifiable to the group of interest. Functionalized alkyl halides are commonly used in this capacity, although the triflate esters of functionalized alcohols work as well. 

Figure 3.1-4 shows the changes in liquefaction points (either melting points or glass transitions) for a series of l-allcyl-3-methylimida2olium tetrafluoroborate [26] and bis(trifyl)imide [45] ionic liquids with changing length of the linear alkyl-substituent on the N(3)-position. 

The reported transition temperatures for a range of [RMIM] ionic liquids [6, 23-26, 46] are shown in Figure 3.1-5, with varying anion and alkyl chain substituent length. 

Table 3.1-5 Melting points and heats of fusion for isomeric [BMIM][PFg] and [PMIM][PFs] ionic liquids, showing melting point and crystal stability increasing with the degree of branching in the alkyl substituent.

The size of the cation in the chloroaluminate ionic liquids also appears to have an impact on the viscosity. For ionic liquids with the same anion(s) and compositions, the trend is for greater viscosity with larger cation size (Table 3.2-2). An additional contributing factor to the effect of the cation on viscosity is the asymmetry of the alkyl substitution. Highly asymmetric substitution has been identified as important for obtaining low viscosities [17]. 

Ionic liquids are similar to dipolar, aprotic solvents and short-chain alcohols in their solvent characteristics. These vary with anion (from very ionic Cl to more covalent [BETI] ). IFs become more lipophilic with increasing alkyl substitution, resulting in increasing solubility of hydrocarbons and non-polar organics. 

The charged species were in all cases found to concentrate at the surface of the liquid under vacuum conditions. Little surface separation of the anions and cations was observed. For the [PFg] and [BFJ ions, the cation ring was found to prefer a perpendicular orientation to the surface, with the nitrogen atoms closest to the surface. An increase in the alkyl chain length caused the cation to rotate so that the alkyl chain moved into the bulk liquid, away from the surface, forcing the methyl group closer to the surface. For halide ionic liquids, the data were less clear and the cation could be fitted to a number of orientations. 

Singer and co-workers have shown that benzoyl chloride reacts with ethers to give alkyl benzoates [33] in chloroaluminate(III) ionic liquids. This reaction results in... 

In an attempt to study the behavior and chemistry of coal in ionic liquids, 1,2-diphenylethane was chosen as a model compound and its reaction in acidic pyri-dinium chloroaluminate(III) melts ([PyHjCl/AlCb was investigated [69]. At 40 °C, 1,2-diphenylethane undergoes a series of alkylation and dealkylation reactions to give a mixture of products. Some of the products are shown in Scheme 5.1-40. Newman also investigated the reactions of 1,2-diphenylethane with acylating agents such as acetyl chloride or acetic anhydride in the pyridinium ionic liquid [70] and with alcohols such as isopropanol [71]. 

Details of two related patents for the alkylation of aromatic compounds with chloroaluminate(III) ionic or chlorogallate(III) ionic liquid catalysts have become available. The first, by Seddon and co-workers [81], describes the reaction between ethene and benzene to give ethylbenzene (Scheme 5.1-51). This is carried out in an... 

Scheme 5.1-49 The alkylation of benzene with methyl chloride or n-propyl chloride in an ionic liquid.

Scheme 5.1-51 The alkylation of aromatic compounds in chloroaluminate(lll) or chlorogallate(lll) ionic liquids.

The production of linear alkyl benzenes (LABs) is carried out on a large scale for the production of surfactants. The reaction involves the reaction between benzene and a long-chain alkene such as dodec-l-ene and often gives a mixture of isomers. Greco et al. have used a chloroaluminate(III) ionic liquid as a catalyst in the preparation of LABs [83] (Scheme 5.1-53). 

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. 

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). 

Scheme 5.1-54 The sulfuric acid-catalyzed alkylation of benzene in a hydrogensulfate ionic liquid.

Scheme 5.1-56 The alkylation of benzene with dodecene with an ionic liquid on a solid support.

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]. 

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