Ionic dialkylimidazolium, 1,3-, salt

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. 

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]. 

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. 

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... 

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... 

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]. 

The real breakthrough in the use of ionic liquids as solvents came in 1992, when Wilkes and Zaworotko studied the formation of dialkylimidazolium salts associated with water-stable anions. This led to the discovery of the ionic liquids [emim][BF4] 5 (Figure 2.2) and [emim][N03], with melting points of 15°C and 38" C, respectively. These ionic liquids are hygroscopic, but they are not degraded in the presence of moisture. Shortly afterward, the synthesis of the corresponding hexafluorophosphate derivative 6 was described. Today, most ionic liquids are based on imidazolium derivatives associated with non-covalent anions. 

Similar results have been recently reported with iV-alkylp3rridinium oriV,iV-dialkylimidazolium salts as supports of the Wilkinson catalyst in the 1-alkene hydrosilylation by triethoxysilane. The catalytic system [RhCl(PPh3)2]/ionic liquid (molten salts) can be recycled at least 10 times without noticeable decreases in the activity and selectivity (122). 

Ionic liquids are salts that form a stable fluid at or near room temperature and are expected to replace hazardous and volatile organic solvents because they have low vapor pressure, non flammability, high polarity and relative inertness. Most ionic liquids consist of bulky organic cations, for example Af,Af-dialkylimidazolium, A-alkylpyridinium, quaternary ammonium, quaternary phosphonium, and common weakly coordinating anions such as AlCU", Bp4 , PP6 , CFsSOs", TfO , (CP3S03)2N and some of them, such as Af,Af-dialkylimidazolium salts, show excellent conductivity [222]. Typical chemical structures of cations and anions are reported in Fig. 1.19. 

Recently, a so-called ion-supported TEMPO was synthesized bybuilding a TEMPO moiety into the side chain of a dialkylimidazolium salt (Fig. 7.19). The resulting material catalyzed the oxidation of alcohols with NaOCi or I2 in water or an ionic liquid/water mixture. ... 

Wasserscheid P, Welton T (2003) Ionic liquids in synthesis. Wiley-VCH, Weinheim Visser AE, Swatloski RP, Reichert WM et al (2001) Task-specific ionic liquids for the extraction of metal ions from aqueous solutions. Chem Commun 135-136 Visser AE, Swatloski RP, Reichert WM (2002) Task-specific ionic liquids incorporating novel cations for the coordination and extraction of Hg and Cd " " synthesis, characterization, and extraction studies. Environ Sci Technol 36 2523-2529 Howarth J, Hanlon K, Fayne D et al (1997) Moisture-stable dialkylimidazolium salts as heterogeneous and homogeneous Lewis acids in the Diels-Alder reaction. Tetrahedron Lett 38 3097-3100... 

Among ionic liquids [1-3] (ILs), 1,3-dialkylimidazolium salts (Im-ILs) are undoubtedly one of the most popular and most investigated classes. They are widely used in a great variety of applications, such as solvents for many synthetic chemical processes [1-3]. One of the reasons for their success as solvents involves the fact that these compounds are often considered inert, since their electrochemical window is wide, and they neither possess easily dissociating protons nor energetically easily available lone pairs. 

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

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 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. 

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