1.3- Dialkylimidazolium-based ionic liquids

With respect to ILs as cosolvents, the interest was centred on two types of ILs aUcylammonium and 1,3-dialkylimidazolium-based ionic liquids. For the first type, we selected ethylammonium nitrate, considering that it is a protic ionic liquid (PIL) and that it can act as a potential acid catalyst. With respect to the second type of ILs, we selected those based on l-methyl-3-n-butylimidazolium cation, characterised by a slighter HBD acidity than that of the alkylammonium type (Figs. 13.2, 13.3). These ILs exhibit a wide spectrum of physicochemical properties. It was demonstrated that their water content, density, viscosity, surface tension, melting point and thermal stability are affected by the length of the alkyl chain and the nature of the anion. Several anions were incorporated in this class of ILs. 

The nature of both the cation and the anion determine the melting point of the ionic liquid. For example, if 1,3-dialkylimidazolium-based ionic liquids are to be liquids at room temperature the cation should be unsymmetrical [11]. Of all the ionic liquids, 1,3-dialkylimidazolium-based ionic liquids are most widely used in synthesis and catalysis. 

As ionic liquids are stiU expensive in comparison with conventional molecular solvents, their efficient recycling is an important issue that addresses the economics of their use, especially in large-scale applications. The most studied examples are the 1,3-dialkylimidazolium-based ionic liquids where the ionic solvent is usually recycled through several cycles of the reaction. Numerous examples describe the immobilization of a transition metal catalyst in the ionic liquid phase of a biphasic system. A variety of palladium coupling reactions, such as the Heck [27], Suzuki... 

Deuteration of the ionic liquid s cation has not only been applied to obtain proton-free ionic liquids for H-NMR experiments but also as direct probe for the reactivity of 1,3-dialkylimidazolium based ionic liquids vs. Ir(0) nanoclusters [70]. After addition ofD2 to [BMIM][(CF3S02)2N] in the presence of an lr(0) nanocluster Finkeand coworkers found deuterium incorporation at the 2-H, 4-H, 5-H and 8-H positions of the imidazolium cation while the control experiment in the absence of the lr(0) cluster showed no D-incorportion. The authors concluded from their H-NMR experiments that a sequence of N-heterocylic carbene formation by oxidative addition (see Section 3.1.2 for more details) of the imidazolium cation, H/D scrambling atop the nanocluster surface, followed by the reductive elimination of a C-D bond takes place. From carefiil kinetic investigations the authors concluded that the co-ordinatively unsaturated nanocluster surface acts indeed as the true catalyst for the H/D-ejadiange reaction. 

For an example of continuous Radziszewski reaction in a microreactor setup, to prepare 1,3-dialkylimidazolium-based ionic liquids, see J. Zimmermann, B. Ondruschka, A. Stark, Org. Process Res. Dev. 2010,14,1102-1109. Efficient synthesis of 1,3-dialkylimidazolium-based ionic liquids the modified continuous Radziszewski reaction in a microreactor setup. 

The following are routes to 1,3-dialkylimidazolium-based ionic liquids. What is the product in each case Comment on the roles of the reagents. 

More interestingly, Ru3(CO)i2 dissolved in 1,3-dialkylimidazolium-based ionic liquids, in particular those associated with chloride anion [49], effectively catalyzed the hydroformylation of various kinds of alkenes with carbon dioxide to give the corresponding alcohols (Table 6.2). Compared to the conventional reaction, this reaction proceeded in the biphasic system, where the chemoselectivity in the... 

An interesting novel and cheaper method for ionic liquid synthesis by modified Radziszewski reaction was recently published by Zimmermann et al. (2010). They demonstrated that water-soluble 1,3-dialkylimidazolium-based ionic liquids could be produced in high yields and purities, starting from available and relatively cheap precursors - monoalkylamines, glyoxal, formaldehyde, and mineral or organic acids (Figure 5). The synthesis of 1,3-dibutylimidazolium acetate ([dBIM][OAc]) and 1,3-dibutylimidazolium chloride ([dBIM]Cl) was achieved by continuous production in a microreactor. The solution... 

Zimmermann, J., Ondruschka, B., Stark, A. (2010). Efficient synthesis of 1,3-dialkylimidazolium-based ionic liquids The modified continuous Radziszewski... 

Dialkylimidazolium-based ionic liquids can act as sources of imidazolylidenes, often referred to as N-heterocyclic carbenes (NHCs), which can act as nucleophilic catalysts [93]. Hedrick et al. have used ionic hquids as both the source of the NHC... 

Imidazolium-based ionic liquids (ILs) have been used extensively as media for the formation and stabilization of transition-metal nanoparticles [14—17]. These 1,3-dialkylimidazolium salts (Figure 15.3) possess very interesting properhes they have a very low vapor pressure, they are nonflammable, have high thermal and electrochemical stabilities, and display different solubilities in organic solvents [18-20]. 

Ionic liquids usually have good thermal stability, although decomposition can occur at high temperatures. Vacuum pyrolysis studies of 1,3-dialkylimidazolium halide-based ionic liquids showed that the primary decomposition pathway was dealkylation (Scheme 7.3) [13]. This can be prevented by using non-nucleophilic... 

Scheme 7.3. Decomposition of 1,3-dialkylimidazolium halide-based ionic liquids.

Nitriles are versatile and important components of a range of dyes, natural products, and pharmaceuticals. Aryl nitriles can be synthesized from aryl halides by direct reaction between aryl halides and copper cyanide, known as the Rosemund von Braun reaction [36]. These cyanation reactions can have several disadvantages, in particular the long reaction times required. Ren and coworkers showed that 1,3-dialkylimidazolium halide-based ionic liquids can be used as solvents in the Rosemund von Braun reaction [37]. Complete conversion, based on GC-MS analysis, was achieved after 24 h at 90 °C. When using microwave irradiation and ionic liquid as a solvent, Leadbeater and coworkers showed the reaction times could be reduced to between 3 and 10 min [38]. Under the optimized reaction conditions, 2 equiv. CuCN and 1 equiv. aryl halide were rapidly heated to 200 °C in [i-PrMIM]Br as solvent. Representative results are collected in Table 7.3. The microwave method works as well as the conventional method for a range of aryl iodide and aryl bro-... 

Ren and Wu showed that alcohols can be converted to alkyl halides by using 1,3-dialkylimidazolium halide-based ionic liquids as both solvent and nucleophile in... 

We had no good way to predict if they would be liquid, but we were lucky that many were. The class of cations that were the most attractive candidates was that of the dialkylimidazolium salts, and our particular favorite was l-ethyl-3-methylimid-azolium [EMIM]. [EMIMJCl mixed with AICI3 made ionic liquids with melting temperatures below room temperature over a wide range of compositions [8]. We determined chemical and physical properties once again, and demonstrated some new battery concepts based on this well behaved new electrolyte. We and others also tried some organic reactions, such as Eriedel-Crafts chemistry, and found the ionic liquids to be excellent both as solvents and as catalysts [9]. It appeared to act like acetonitrile, except that is was totally ionic and nonvolatile. 

This chapter will concentrate on the preparation of ionic liquids based on 1,3-dialkylimidazolium cations, as these have dominated the area over the last twenty... 

The cation in an ionic liquid typically is an organic nitrogen-based ion such as alkyl ammonium, alkyl pyridinium, or dialkylimidazolium, examples of which appear below. 

Room temperature ionic liquids (RTILs), such as those based on A,A-dialkylimidazolium ions, are gaining importance (Bradley, 1999). The ionic liquids do not evaporate easily and thus there are no noxious fumes. They are also non-inflammable. Ionic liquids dissolve catalysts that are insoluble in conventional organic chemicals. IFP France has developed these solvents for dimerization, hydrogenation, isomerization, and hydroformylation reactions without conventional solvents. For butene dimerization a commercial process exists. RTILs form biphasic systems with the catalyst in the RTIL phase, which is immiscible with the reactants and products. This system is capable of being extended to a list of organometallic catalysts. Industrial Friedel-Crafts reactions, such as acylations, have been conducted and a fragrance molecule tra.seolide has been produced in 99% yield (Bradley, 1999). 

Many ionic liquids are based on N,N-dialkylimidazolium cations (BMI) which form salts that exist as liquids at, or below, room temperature. Their properties are also influenced by the nature of the anion e. g. BF T PFg. The C-2(H) in imidazole is fairly labile but the C-4(H) and the C-5(H) are less so. Under microwave-enhanced conditions it is therefore possible to introduce three deuterium atoms (Scheme 13.4). As hydrogen isotope exchange is a reversible reaction this means that the three deuterium atoms can be readily exchanged under microwave irradiation. For storage purpose it might be best to back-exchange the C-2(D) so that the 4,5-[2H2] isotopomer can be safely stored as the solid without any dangers of deuterium loss. The recently... 

The physical properties of ionic liquids have been extensively studied and some trends are beginning to emerge. In particular, ionic liquids based on 1,3-dialkylimidazolium cations have been investigated in detail, partly due the their wide use as solvents to conduct synthesis and catalysis. The attraction of the imidazolium cation in synthetic applications is because the two substituent groups can be varied to modify the properties of the solvent. For example, Table 4.1... 

The stability of dialkylimidazolium cation-containing ionic liquids can be a problem even at moderate temperatures in the presence of some reagents or catalysts. For example, when CsF and KF were used in the ionic liquid [BMIM]PFg to perform a halogen exchange reaction in an attempt to replace Br from bromo-carbons with F , it was found that alkyl elimination from the [BMIM] cation took place, forming methyl imidazole, 1-butene, 1-fluorobutane, and other unidentified products at 150°C overnight 69). The fluoride ion acted as a base that promotes elimination or substitution processes. 

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. 

Beckmann rearrangements of several ketoximes were performed in room-temperature ionic liquids based on l,3-dialkylimidazolium or alkylpyridinium salts containing phosphoras compounds (such as PCI5) by Deng and Peng [59] (Scheme 5.1-31, BP = 1-butylpyridinium). Turnover numbers of up to 6.6 were observed, but the authors did not mention whether the ionic liquid could be reused. 

Qin, W., Wei, H., and Li, S. F. Y, 1,3-Dialkylimidazolium-based room-temperature ionic liquids as background electrolyte and coating material in aqueous capillary electrophoresis, ]. Chromatogr. A, 985, 447-454, 2003. 

---