1.3- dialkylimidazolium ionic liquid

Synthesis of 1,3-dialkylimidazolium ionic liquids does not require very much effort. One can start from commercially available 1-methylimidazole for a one-step or imidazole for a two-step synthesis (Scheme 3). 

Dialkylimidazolium ionic liquids with BF4 and IT), anions are quite stable and while being polar (see below) are also non-nucleophilic. They can be prepared in a variety of ways, most commonly by metathesis reactions21-23 and also by a methylation reaction.24 These reactions are illustrated in Schemes 2 and 3, respectively. The chloride produced in the metathesis reactions (Scheme 2) is very difficult to remove completely and it effects the physical properties of the liquid and can also effect certain reactions carried out in the liquid. Figure 6 shows how the viscosity of l-butyl-3-methylimidazolium tetrafluoroborate varies with chloride concentration.25... 

Mancini PME, Fortunate G, Adam C, Vottero LR (2008) Solvent effects on chemical processes. New solvents designed on the basis of the molecular-microscopic properties of binary mixtures molecular solvent + 1,3-dialkylimidazolium ionic liquids. J Phys Org Chem... 

Dupont J, Spencer J (2004) On the noninnocent nature of 1,3-dialkylimidazolium ionic liquids. Angew Chem Int Ed 43 5296-5297... 

Chemical Stability and Toxicity of 1,3-Dialkylimidazolium Ionic Liquids... 

This observation was supported by subsequent evaluations from the same group [22] who conducted a comprehensive study on the biological effects of 1,3-dialkylimidazolium ionic liquids on luminescent bacteria (Vibriofisdwri) and two... 

Scheme 5.1-17 Addition of ICI2 to alkenes in a 1,3-dialkylimidazolium ionic liquid.

In summary it can be stated that low melting salts and ionic liquids are very interesting reaction media for Pd-catalyzed coupling reactions. However, the role of the ionic liquid is variable and may be very complex. Ionic liquids have been found to act as solvents, ligands, ligand precursors and dispergents. In many cases more than one of these functions was taken by the ionic reaction medium. 1,3-dialkylimidazolium ionic liquids, especially, are often not inert under the reaction conditions. Moreover, the formation of Pd nanoparticles has always to be taken into account as a possible explanation for the observed results. These particles seem to be stabilized in the ionic liquids (see Sections 5.3.2.7 and 6.3 for more details) and the resulting mixtures have shown excellent catalytic performance in at least some cases. 

By using competitive experiments, the relative efficiency of different co-catalysts wasdeterminedtobeBMICF3C02>BMI BF4>BMIPFG, which was the opposite of that observed by Afonso ef al. [8] in solution. Based on the interception of these unprecedented supramolecular species, Eberlin and coworkers propose (Scheme 3.4) that 1,3-dialkylimidazolium ionic liquids function as efficient co-catalysts for the MBH reaction by (i) activating the aldehyde toward nucleophilic attack by BMI coordination (species 8 ) and (ii) stabilizing the zwitterionic species that act as the main MBH intermediates through supramolecular coordination (species 9", lO, 11 +, and 13 + ). 

In COlL-1, all of the main aspects of the liquid-state structure of room-temperature ionic liquids were already laid out in contributions by several groups, although sometimes still in an incipient way. Hardacre and co-workers reported neutron diffraction studies of short-chain dialkylimidazolium ionic liquids, perfectly illustrating the charge ordering and n-interactions [17]. They also used different spectroscopic and simulation methods to study how the solvation of aromatic and polar molecules (benzene and ethanenitrile, respectively) in the ionic liquids modifies the structure of the media [46] and the balance between the different terms in the interactions coulombic, van der Waals (dispersive), hydrogen bonds and multipolar. 

Remsing RC, Liu Z, Sergeyev I, Moyna G (2008) N -Dialkylimidazolium ionic liquids a multinuclear NMR spectroscopy and molecular dynamics simulation study. J Phys Chem B 112 7363-7369... 

Every HA, Bishop AG, MacFarlane DR, Oradd G, Forsyth M (2004) Transport properties in a family of dialkylimidazolium ionic liquids. Phys Chem Chem Phys 6 1758-1765... 

Additive free electrodeposition of nanocrystalline aluminium in a water and air stable ionic liquid. Electrochem. Commun., 7,1111-1116 Zhong, C. Sasaki, T. Jimbo-Kobayashi, A. Fujiwara, E. Kobayashi, A. Tada, M. Iwasawa, Y. (2007). Syntheses, structures, and properties of a series of metal ion-containing dialkylimidazolium ionic liquids. Bull. Chem. Soc. fpn., 80, 2365-2374... 

Commonly used ionic liquids are N-alkylpyridinium, N,N -dialkylimidazolium, alkylammonium and alkylphosphonium salts. 

Room-temperature ionic liquids, salts with A,A-dialkylimidazolium cations in synthesis and catalysis 99CRV2071. 

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... 

Despite all the advantages of this process, one main limitation is the continuous catalyst carry-over by the products, with the need to deactivate it and to dispose of wastes. One way to optimize catalyst consumption and waste disposal was to operate the reaction in a biphasic system. The first difficulty was to choose a good solvent. N,N -Dialkylimidazolium chloroaluminate ionic liquids proved to be the best candidates. These can easily be prepared on an industrial scale, are liquid at the reaction temperature, and are very poorly miscible with the products. They play the roles both of the catalyst solvent and of the co-catalyst, and their Lewis acidities can be adjusted to obtain the best performances. The solubility of butene in these solvents is high enough to stabilize the active nickel species (Table 5.3-3), the nickel... 

In recent years ionic liquids have also been employed as media for reactions catalyzed both by isolated enzymes and by whole cells, and excellent reviews on this topic are already available [47]. Biocatalysis has been mainly conducted in those room-temperature ionic liquids that are composed of a 1,3-dialkylimidazolium or N-alkylpyridinium cation and a noncoordinating anion [47aj. 

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). 

Ionic liquids, being polar and ionic in character, couple to the MW irradiation very efficiently and therefore are ideal microwave absorbing candidates for expediting chemical reactions. An efficient preparation of the 1,3-dialkylimidazolium halides via microwave heating has been described by Varma et al. that reduces the reaction time from several hours to minutes and avoids the use of a large excess of alkyl ha-lides/organic solvents as the reaction medium (Scheme 6.56) [26-28]. 

The preparation of 1,3-dialkylimidazolium halides by conventional heating in solvent under reflux requires several hours to afford reasonable yields and also uses a large excess of alkyl halides and/or organic solvents as the reaction medium. To circumvent these problems Varma and coworkers [106] investigated the preparation of a series of ionic liquids 72 (Scheme 8.74), using microwave irradiation as the energy source, by simple exposure of neat reactants, in open containers, to microwaves by use of an unmodified household MW oven (240 W). 

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 possibility of adjusting acidity/coordination properties opens up a wide range of possible interactions between the ionic liquid solvent and the dissolved transition metal complex. Depending on the acidity/coordination properties of the anion and on the reactivity of the cation (the possibility of carbene ligand formation from 1,3-dialkylimidazolium salts is of particular importance here [37]), the ionic liquid can be regarded as an innocent solvent, as a ligand precursor, as a co-catalyst or as the catalyst itself. 

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... 

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]. 

Heretofore, ionic liquids incorporating the 1,3-dialkylimidazolium cation have been preferred as they interact weakly with the anions and are more thermally stable than the quaternary ammonium cations. Recently, the physical properties of 1,2,3,4-tetraalkylimidazolium- and 1,3-dialkylimidazolium-containing ionic liquids in combination with various hydrophobic and hydrophilic anions have been systematically investigated (36,41). The melting point, thermal stability, density, viscosity, and other physical properties have been correlated with alkyl chain length of the imidazolium cation and the nature of the anion. The anion mainly determines water miscibility and has the most dramatic effect on the properties. An increase in the alkyl chain length of the cations from butyl to octyl, for example, increases the hydrophobicity and viscosity of the ionic liquid, whereas densities and surface tension values decrease, as expected. 

The cations in ionic liquids are generally bulky monovalent organics. The typical cations of ionic liquids, not including the familiar alkylammonium and alkylphosphonium ions, are shown in Fig. 2. It is primarily the cations, which account for the low melting points of ionic liquids. The dialkylimidazolium ions, such as 1-butyl-3-methyl imidazolium [BMIM], have been widely investigated because low-melting ionic liquids can be made readily from such cations and because of their thermal and chemical stability. 

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