1,3-dialkylimidazolium salts

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. 

Scheme 5.3-2 Formation of carbene complexes by dialkylimidazolium salt deprotonation.

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. 

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]. 

Although such catalyst systems are known to have rather high productivities for the reaction, the addition of several equivalents of 1,3-dialkylimidazolium salts per equivalent of palladium leads to complete deactivation of the catalyst, which was attributed to the formation of highly stable palladium imidazolylidene complexes (Scheme 18). 

Values of molar volumes can be calculated from densities measured for the liquid salt, or can be calculated as for hypothetical subcooled liquid at 298.15 K using the group contribution method [47]. As expected, the molar volumes of 1,3-dialkylimidazolium salts and quaternary ammonium salts increase progressively as the length of alkyl chain of the substituent increases. Some molar volumes values at 298.15 K are listed in Table 1.3. 

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. 

It is well-known that many ILs are not really inert solvents, but can be quite reactive. Dialkylimidazolium salts in particular are prone to deprotonation in the 2-position [69-71]. In addition to this, exchange with D2O was found for all three imidazolium ring protons for halide salts [29,32], but not for BF4 and PF5 anions [32]. Our own work demonstrates that highly pure, neat... 

Ionic liquids (ILs), previously known as molten salts, were mainly used in electrochemistry studies due to their ionic nature. The most important step in the chemistry of the ILs occurred when Osteryoung described a mixture of 1-(1-butylpyridinium)-chloride and aluminium chloride which was liquid at room temperature30. Later on, Wilkes discovered other ionic liquids based on dialkylimidazolium salts that featured even more convenient physical and electrochemical properties than the butylpyridinium salts3i. 

Suitable 1,2-diimino species can replace 1,2-diaminoalkanes or -alkenes in cyclizations of this general type. Thus, diazadiencs (4), which are easily prepared from glyoxal and primary amines, can be converted into 2-substituted 1,3-dialkylimidazolium salts by treatment with dry HCl (Scheme 3.1.6)... 

Hydrosilylation reactions with ionic liquids derived from 1-alkylimidazole, i.e. in 2-position unsubstituted 1,3-dialkylimidazolium salts, did not give the desired polyethersiloxanes. We assume that the proton in 2-position of the 1,3-dialkylimidazolium salts react with the SiH group of the... 

Scheme 5 Synthesis of 1,3-dialkylimidazolium salts from the dialkylimidazolium carboxylate...

Enantioselective chemo- and bio-catalysis in ionic liquids (1,3-dialkylimidazolium salts) 04CC1033. 

Holbrey JD, Reichert WM, Swatloski RP et al (2002) Efficient, halide free synthesis of new low cost ionic liquids 1,3-dialkylimidazolium salts containing methyl- and ethyl-sulfate anions. Green Chem 4 407- 13... 

Consort CS, Suarez PAZ, de Souza RA et al (2005) Identification of 1,3-dialkylimidazolium salt supramolecular aggregates in solution. J Phys Chem B 109 4341-4349... 

Chu Y, Deng H, Cheng JP (2007) An acidity scale of 1,3-dialkylimidazolium salts in dimethyl sulfoxide solution. J Otg Chem 72 7790-7793... 

Imidazoles react with haloalkanes in the absence of strong bases because the pyridine-like N-atom effects a nucleophilic substitution of halogen. The quaternary salts that are formed initially usually undergo rapid deprotonation to 1-alkylimidazoles. These can react with a second mole of haloalkane to give 1,3-dialkylimidazolium salts ... 

Not only platinum forms carbene complexes by oxidative addition of 1,3-dialkylimidazolium salts. CaveU and coworkers also reported the formation of stable carbene complexes of nickel and palladium by reaction with imidazolium ionic liquids [53]. Even in cases where the imidazolium was protected with a methyl group in the 2-position of the imidazolium ring, carbene formation has been observed in the 4- or 5-position in some cases [54]. 

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