Imidazolium cation derivatives

Though various modifications of cations such as ammonium, pyridinium, oxazolinium, and thiazolium have occurred, the imidazolium-cation-derived chiral ionic liquids have gained widespread usage due to their facile preparation, low melting points, and relatively favorable viscosity. The best-known imidazole-derived salts that have been reported so far contain the chiral moieties bonded to one or both of the nitrogen atoms on positions 1 and 3 of the imidazolium cation. Also, there are several chiral ionic Uqnids in which the chiral moiety is contained in the anion. 

ILs are not limited to those based on imidazolium cation derivatives. In particular, quaternary ammonium salts and phosphonium salts, which are appreciably more resistant to reduction as compared with azoles, have considerably extended the variety of... 

Finally, we examined replacement of the quaternary ammonium centre of clofilium with a positive centre derived from an imidazolium cation, for example, (56) [156]. Although these types of compound possessed Class III... 

The broader subject of the interaction of stable carbenes with main-group compounds has recently been reviewed. Accordingly, the following discussion focuses on metallic elements of the s and p blocks. Dimeric NHC-alkali adducts have been characterized for lithium, sodium, and potassium. For imidazolin-2-ylidenes, alkoxy-bridged lithium dimer 20 and a lithium-cyclopentadienyl derivative 21 have been reported. For tetrahydropyrimid-2-ylidenes, amido-bridged dimers 22 have been characterized for lithium, sodium, and potassium. Since one of the synthetic approaches to stable NHCs involves the deprotonation of imidazolium cations with alkali metal bases, the interactions of alkali metal cations with NHCs are considered to be important for understanding the solution behavior of NHCs. 

ImH+ imidazolium cation amine (chiral auxiliary reagent derived... 

Formylation of the less reactive phenol and anisole with CO in HF-BF3 was found to require at least stoichiometric amount of the acid for effective transformation (50 equiv. of HF, 2 equiv. of BF3, 50 bar CO, 45°C).445 Conversion increases with increasing reaction time but results in decreasing paralortho ratios suggesting a change from kinetic control to thermodynamic control and the reversibility of formylation. Furthermore, the amount of byproducts (mainly diphenylmethane derivatives) originating from reactions between substrates and products also increases. Additional studies in ionic liquids showed that imidazolium cations with increased chain lengths—for example, l-octyl-3-methylimidazolium salts—are effective in the formylation process. This was attributed to the enhanced solubility of CO in the ionic liquid medium. Tris(dichloromethyl)amine, triformamide, and tris (diformylamino)methane have recently been applied in the formylation of activated aromatic compounds in the presence of triflic acid at low temperature (— 10 to 20°C) albeit yields are moderate.446... 

Chiral ionic liquids could become an important class of solvents,194 100 1011 which may even be relatively cheap when prepared from the natural chiral pool with anions such as lactate. However, as yet, their ability to induce enantiomeric excess in catalysed reactions has been only met with limited success.1102 1041 With increasing evidence to suggest that carbene ligands derived from ionic liquid imidazolium cations are involved in certain catalysed reactions it is likely that chiral ionic liquids that act as both solvent and ligand could prove highly useful in stereoselective reactions. Some examples of chiral cations that may lead to ionic liquids are shown in Figure 2.10. 

QC m] [various] [C4C Ciim][BF4] Pd(PPh3)4, Pd(OAc)2 and many others Na2C03 110 °C. Broad study used to identify the active catalyst species both imidazoylidenes (carbenes derived from the imidazolium cation) and halide ligands are implicated in the catalytic cycle products extracted with Et20 catalyst stable for at least 4 runs. [105]... 

In this chapter the neutral DA charge transfer complex that exhibits a rapid increase of conductivity during the melt, the cation radical salts of TTF derivatives with low melting point, the anion radical salts of TCNQ with imidazolium cations with low melting point, and l-ethyl-3-methylimidazolium (EMI) salts containing complexes with paramagnetic metals will be described. 

Figure 13.28 X-ray crystal structure of the liquid-clathrate-derived inclusion compound MiV -dimethylimidazolinm+(PFg ) 0.5C6Hg showing the stacking of the benzene guest with the imidazolium cations. ...

Until recently the primary interest in the electronic stmcture calculations has been to derive partial charges on the imidazohum cation for use in classical simulations [11,13,14,16,62,83]. The charge distribution however depends strongly on the method of analysis. The Natural Atomic Orbital (NAO) [84-88] and MuUiken population analysis methods have been apphed to the imidazolium cations [16,64]. A number of methods have been used to determine partial charges from the electrostatic potential these also show significant variation. There is an especially large... 

In contrast to sodium, a residual Lewis base such as the starting N-alkylimidazole can severely poison organometallic catalysts. These compounds are coordinating bases and have been proven difficult to remove from ILs due to their high boiling points. A simple colorimetric method of detection was first developed, which is based on the complexation of N-methylimidazole with copper(II) chloride [13]. ft enables monitoring of N-methyhmidazole content in [EM1][C1] to better than 0.2 mol%. More recently, capillary electrophoresis was applied for the detection of imidazole derivatives in l-alkyl-3-methylimidazolium salts [14]. Using this technique, it is then possible to separate and identify the different N-alkylimidazole and imidazolium cations in the ILs. 

Rather than starting from 1-methylimidazole, imidazole can be converted to 1-alkylimidazole following deprotonation with a strong base. Quaternization leads to the formation of imidazolium cations that can bear a variety of alkyl substituents. Instead of halide derivatives, alkyl mesylate, tosylate or triflate can also be used, resulting in the direct formation of ionic liquids with these corresponding anions (Scheme A one-step preparation of hydrophobic... 

ILs used in self-assembly systems can be divided in two categories protic ionic liquids (PILs, solvent properties are similar to water) and aprotic ionic liquids (AILs, largest group most of them derived from disubstituted imidazolium cations). Many surfactant ILs have thermotropic properties, forming liquid crystal phases with no additional solvent present. The solvophobic interaction between PILs and... 

Reaction of 3.40, 3.41, 3.43 or 3.51 with 1-methylimidazole, pyridine, amine " and tributylphosphine afforded cationic pillar[5]- and pil-lar[6]arenes (3.52-3.56). A pillar[5]arene with 10 ionic liquids (imidazolium cations 3.53) was produced in the liquid state at 25 °C by choosing an appropriate counter anion. A pillar[5]arene with 10 phosphonium cations (3.56) is amphiphilic and acts as a substrate-selective phase-transfer catalyst. Oxidation of linear alkane 1-hexene to 1-pentanal by KMn04 vwis >99%, whereas that of the branched alkene, 4-methyl-l-hexene, was only 31%, even under ideal conditions. An etherification reaction between an allqrl-halide and phenolic compounds is a good method to synthesize functionalized pillar[5]arene. Etherification of pillar[5]arene containing 10 bromide with a coumarin derivative with a phenolie moiety gave a pillar[5]arene eanying 10 coumarin moieties (3.57) 3.57 aeted as a fluorescence sensor for methyl parathion. Pillar[5]- and pillar[6]arenes with 10 and 12 bromide moieties should be good key compounds for the synthesis of various functionalized pillar[5]- and pillar[6]arenes. 

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