Alkylation reactions using ionic liquids

Many ionic liquids with other anions are produced in related one-pot methylation reactions that do not give rise to by-products [13]. For example, methyl triflate, CF3S03Me, can be used to methylate alkylimidazoles to give 1,3-alkyl-methylimidazolium triflate ionic liquids as shown in Scheme 4.4. 

The Heck reaction is a C-C coupling reaction where an unsaturated hydrocarbon or arene halide/triflate/sulfonate reacts with an alkene in presence of a base and Pd(0) catalyst so as to form a substituted alkene. Kaufmann et al. showed that the Heck reaction carried out in presence of ILs such as tetra-alkyl ammonium and phosphonium salts without the phosphine ligands, resulted in high yields of product. They attributed the activity to the stabilizing effect of ammonium and phosphonium salts on Pd(0) species. Carmichael et al. used ionic liquids containing either A,A -dialkylimidazolium and A-alkylpyridinium cations with anions such as halide, hexafluorophosphate or tetrafiuoroborate to carry out reactions of aryl halide and benzoic anhydride with ethyl and butyl acrylates in presence of Pd catalyst. An example of iodobenzene reacting with ethyl acrylate to give trans-et vy cinnamate is shown in Scheme 14. 

A summary of metathesis reactions in ionic liquids is presented in Table 7.1. From the data available it appears that these reactions can easily be performed in neat ionic liquids and increased reaction rates are sometimes observed relative to molecular solvents. Co-solvents are used in some cases, mainly to obtain a biphasic system and thereby facilitate product isolation. So far, only imidazolium-type solvents have been employed with variations in the alkyl substitution pattern and the nature of the anion. Apart from the common perfluorinated anions, chloroaluminates have also been used,... 

Water can be used as the solvent in the presence of added surfactants. Reactions in ionic liquids and supercritical fluids are also feasible. A variety of reagents promote cychzation, which can be achieved at room temperature. Examples of compounds that promote and accelerate the reaction include A-methyhnorphohne A-oxide, trimethylamine A-oxide, phosphine oxides, dimethylsulfoxide, alkyl methyl snlfides, molecular sieves, and lithium perchlorate. A comparison of a few promoters is seen in Scheme 246. Promoters... 

Finally, the data related to the application of ionic liquids for the cleavage of ethers are also notable. Chloroaluminates [44], anhydrous hydrobromic acid in 1-methylimidazole [45] and concentrated hydrobromic acid in [BMIM][BF4] or p-TsOH/[BMIM][Br] in [BMIM][BF4] [46] have been used to regenerate phenols from the corresponding aryl alkyl ethers. In ionic liquids, in the presence of an efficient proton donor (HBr or TsOH), the bromide anion behaves as a strong nucleophile able to cleave ethers. These latter reactions seem, therefore, to indicate that, as opposed to the situation in the absence of adds, the nudeophihcity of the bromide anion in ionic liquids in the presence of proton donors is higher than in molecular solvents (both pro tic and aprotic). This behavior may be a consequence of a... 

Reaction of sodium azide with alcohols using DEAD/PPhs, PPhs, BF3-Et20, or Zeolites can be used to form azides. Similarly, alkyl halides in ionic liquids or under other conditions, " " chlorocyclodextrin, and certain amines can be reacted with sodium azide to form organic azides as outlined in eqs 35-40. 

A matter of concern when using ionic liquids as alkylation catalysts (and in general in any other acid-catalyzed reaction) is the gradual loss of activity. Deactivation during alkylation in the presence of ILs is believed to take place by the formation of conjunct polymers (polyunsaturated cyclic, polycyclic, and acyclic molecules) which bound to the AICI3 in the IL through acid-base interactions (the... 

Modified graphene oxide was prepared using alkyl imidazol-ium ionic liquids (l-butyl-3-methylimidazolium tetrafluoroborate, l-butyl-3-methylimidazolium hexafluorophosphate and l-hexyl-3-methyhmidazolium bis(trifluoromethylsulfonyl) amide via an epoxide ring-opening reaction, cation-p stacking, or van der Waals interactions, with modified graphene exfoliated from a graphite rod by a moderate electrochemical method as a a comparative material (3). 

Friedel-Crafts reaction of pyrrole can be achieved without Lewis acid/base catalysts when the reaction is performed using ionic liquids as cosolvent. Regioselective alkylation of pyrrole with simple alkyl halides and mesylates using [bmimJlSbFg] (1-n-butyl-3-methylimidazolium hexafluoroantimonate) as cosolvent proceeds to deliver the monoalkylated pyrrole as the major product when excess pyrrole (10 equiv) is used (eq 8). 

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 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). 

The rhodium catalyzed carbonylation of ethylene and methanol can be conducted in the absence of added alkyl halide if the reactions are conducted in iodide based ionic liquids or molten salts. In the case of ethylene carbonylation, the imidazolium iodides appeared to perform best and operating in the absence of ethyl iodide gave improved selectivities relative to processes using ethyl iodide and ionic hquids. In the case of... 

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