1.3- Dialkylimidazolium hydrogen

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... 

Chauvin, Y., Mussmann, L., and Olivier, H., A novel class of versatile solvents for two-phase catalysis hydrogenation, isomerization, and hydroformylation of alkenes catalyzed by rhodium complexes in liquid 1,3-dialkylimidazolium salts, Angew. Chem. Int. Ed., 34, 2698-2700,1996. 

Stereochemical and kinetic studies have confirmed the enhancement of the hydrogen bond ability of the imidazolium cation on going from l,2-dimethyl-3-alkylimidazolium salts to 1,3-dialkylimidazolium cations. However, deprotonation of the site between nitrogens is not particularly simple, it requires strong bases and depends on the ionic liquid counter-anion (Scheme 4.1). ... 

The cationic rhodium complex [Rh(nbd)(PPh3)2]+ (nbd = norbornadiene), previously described by Osborn as a catalyst precursor for the hydrogenation of alkenes and dialkenes when dissolved, e.g., in acetone [28], has proved to be active in dialkylimidazolium PF, CuCI, or SbFf, salts [29]. Due to the very low solubility of alkanes in ionic liquids, the reaction is truly biphasic. The rhodium loss in the organic phase is under the limit of detection. The catalyst can be used repeatedly. In this case, the presence of coordinating anions, such as traces of chloride, inhibits the catalyst activity. Halide contaminants may arise from unreacted starting material used in the preparation of NAILs or from the possible decomposition of the halogenated anion (e.g., PF ). Purification and quality control of ionic liquids are then crucial [30]. 

Multinuclear NMR spectroscopy experiments of various 1,3-dialkylimidazolium ILs dissolved in organic solvents have also pointed to the formation of floating aggregates through hydrogen bonds [81-84]. In particular, it has been demonstrated by heteronuclear NMR experiments on [C4CjIm]BF4 that contact ion pairs exist in the presence of small amounts of water and even in dimethyl sulfoxide (DMSO) solution [85]. 

Fig. 5.M Ability ofthe 1,3-dialkylimidazolium cation to hydrogen bond methyl acrylate in the course of its reaction with cyclopentadiene.

These data, although they give important information about the hydrogen donor ability of 1,3-dialkylimidazolium ionic hquids, do not exclude the possibility that other factors can influence the rate and selectivity of Diels-Alder reactions. Recently, the kinetic study of reaction between the first excited state of molecular oxygen and... 

ILs can act as both hydrogen bond acceptors (anions) and donors (cations), and as expected, they interact with substances with both acceptor and donor sites. In particular, 1,3-dialkylimidazolium ILs can dissolve a plethora of classical polar and non-polar compounds. All 1,3-dialkylimidazolium ILs reported to date are hygroscopic, and their miscibility with water is largely controlled by the nature of the anion. While salts containing nitrate, chloride, and perchlorate anions are usually miscible with water in all compositions, those associated with hexafluorophosphate and bis(trifluoromethane) sulfonylamidate anions are almost completely immiscible with water.It is also known that an increase in the A-alkyl chain length increases the hydrophobicity for a series of l-alkyl-3-methylimidazolium hexafluorophosphate ILs." The miscibility of water in ILs can be increased by the addition of short-chain alcohols or lowered by the addition of salts (salting-out effect). ... 

The 1,3-azoles are quatemised easily at the imine nitrogen with alkyl halides the relative rates are 1-methylimidazole thiazole oxazole, 900 15 1." In the case of imidazoles which have an A -hydrogen, the immediate product is a pro-tonated N-alkylimidazole this can lose its proton to unreacted imidazole and react a second time, meaning that reactions with alkyl halides give a mixture of imidazolium, 1-alkylimidazolium and 1,3-dialkylimidazolium salts. Furthermore, an unsymmetrically-substituted imidazole can give two isomeric 1-alkyl derivatives. The use of a limited amount of the alkylating agent, or reaction in basic solution, when it is the imidazolyl anion (section 21.4.1) which is alkylated, can minimise these complications. 

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. 

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