Pyridinium-based ionic liquids

In the light of these results, it becomes important to question whether a particular catalytic result obtained in a transition metal-catalyzed reaction in an imidazolium ionic liquid is caused by a metal carbene complex formed in situ. The following simple experiments can help to verify this in more detail a) variation of ligands in the catalytic system, b) application of independently prepared, defined metal carbene complexes, and c) investigation of the reaction in pyridinium-based ionic liquids. If the reaction shows significant sensitivity to the use of different ligands, if the application of the independently prepared, defined metal-carbene complex... 

Also the use of moisture stable ionic liquids as solvents in the Diels-Alder reaction has been carried out, and in all examples an enhanced reaction rate was observed [182,183]. The application of pyridinium-based ionic liquids allowed the utilization of isoprene as diene [184]. The chiral ionic liquid [bmim][L-lactate] was used as a solvent and accelerated the reaction of cyclopentadiene and ethyl acrylate, however, no enantiomeric excess was observed [183]. In addition several amino acid based ionic liquids have been recently tested in the Diels-Alder reaction. Similar exo. endo ratios were found but the product was obtained as racemate. The ionic liquids were prepared by the addition of equimolar amounts of HNO3 to the amino acids [185]. Furthermore, an enantiopure imidazolium salt incorporating a camphor motive was tested in the Diels-Alder reaction. No enantiomeric excess was found [186]. 

Pernak, J. and Feder-Kubis, J., Chiral pyridinium-based ionic liquids, containing the (lR,2S,5R)-(-)-menthyl group. Tetrahedron Asymmetry, 17,1728,2006. 

Zabet-Moghaddam, M. et al., Pyridinium-based ionic liquid matrices can improve the identification of proteins by peptide mass-fingerprint analysis with matrix-assisted laser desorption/ionization mass spectrometry. Anal. Bioanal. Chem., 384, 215, 2006. 

In the light of these results, it is very important to check catalytic results obtained from imidazolium ionic liquid for a possible influence of in-situ formed carbene species. This can be done especially by testing a given reaction as well in ionic liquids which do not form carbene complexes e.g. in pyridinium based ionic liquids. 

Chiral pyridinium-based ionic liquids can be prepared by N-alkylation of pyridines with chloromethyl ()-menthyl ether <2006TA1728>. A room temperature ionic liquid brominating agent is obtained when A- -pcntylpyridinium bromide is reacted with bromine <2004SL1318>. 

The number of alkyl substituents attached to the imidazolium cation was found to be of great importance in the telomerisation of butadiene and methanol with a palladium/phosphine catalyst system, see Scheme 8.8.[27] In the presence of imidazolium ionic liquids with an acidic proton in the 2-position, rapid deactivation of the catalyst took place and it was proposed that formation of stable and inactive palladium-carbene species occurred. In contrast, both a pyridinium-based ionic liquid as well as imidazolium ionic liquids bearing a methyl substituent in the 2-position led to active systems. 

In an attempt to achieve solvent free preparation of ionic liquids, others have reported an improved approach whereby the reaction temperature was moderated by placement of the reaction vessel in a water bath. For example, Chan et al. [108] attempted to moderate the heat of the quatemisation reaction and thereby large scale preparation of imidazolium and pyridinium-based ionic liquids. Water moderation was successfully applied to avoid runaway reactions in the large-scale preparation of a number of ionic liquid precursors under solventless conditions using microwave irradiation ... 

In the Xiao s laboratory [27], the pyridinium-based ionic liquids as the suitable media were used for the Friedel-Crafts alkylation of benzene, which was carried out at a better rate but at relatively lower temperature with high product conversion. They found that the [EtPyKCFjCOO] (i-ethyl-pyridinium trifluoroacetatel-FeClj was an enviromnental friendly, recyclable, and reusable solvent-catalyst system to replace the traditional aluminum catalysis system. During the process, the alkylation... 

Xiao Y, Malhotra SV (2005) Friedel-Crafts alkylation reactions in pyridinium-based ionic liquids. J Mol Catal A Chem 230 129-133... 

Aupoix A, Pdgot B, Vo-Thanh G (2010) Synthesis of imidazolium and pyridinium-based ionic liquids and application of l-alkyl-3-methylimidazolium salts as pie-catalysts for the benzoin condensation using solvent-free and microwave activation. Tetrahedron 66 1352-1356... 

Recently, dynamical rotational motion of various pyridinium-based ionic liquids paired with the frf2N] anion were examined [124], In this work, the principal axes of the cations and anions were assigned as the eigenvectors of an atomic mass weighted covariance matrix, defined as... 

It is finally worth noting that phosphonium tosylates [31], and more recently pyridinium-based ionic liquids [32], have also been used as solvents for the Diels-Alder reactions of isoprene with methyl acrylate, acrylic acids, but-3-en-2-one and acrylonitrile (Scheme 5.1-6). 

Gong and Zhao et al. have independently presented the application of pyridinium-based ionic liquid as recyclable solvent for the DABCO-catalyzed MBH reaction. Compared with the commonly used imidazolium-based ionic liquids [bmimlPFg] and [bdmim][PF6], which evidently react with aldehydes under basic conditions, the pyridinium-based ionic liquid [EPyJpFJ and [BuPy][N03] are inert and the MBH reactions of aldehydes with methyl acrylate, MVK or acrylonitrile proceed quickly in good yields. Zhao et al. also examined hexamethyl-enetetramine (HMTA)-catalyzed MBH reactions by using the ionic liquid [EPy][BF4] as a reaction media short reaction times and good to excellent yields was obtained (Scheme 1.120). ° ... 

Table 3. For the pyridinium-based ionic liquids, a much stronger interaction is expected between the pyridinium ring of the cation and the aromatic solute [24], However, it can be observed that the incorrect choice of anion and the alkyl substituents on the pyridinium ring can adversely affect this. Pyridinium ionic liquids based on the [mebupy] cation, such as [mebupy] [BF4], have proven to be quite effective as entrainers, as observed by Meindersma et al. [17] in liquid-liquid equilibria studies.

Based on the evidence of the research findings presented above, pyridinium-based ionic liquids, in particular, those with alkyl substituents, and aromatic-based anions seem to show the greatest promise in being the most effective for the separation of saturated-unsaturated/aromatic hydrocarbons. Consequently, an ideal candidate for the above separations would be obtained through the coupling of the [mebupy] cation and the [Tos] anion, i.e. [mebupy] [Tos]. 

Cadena, C., Zhao, Q., Snurr, R.Q. and Maginn, E.J., Molecular modelling and experimental studies of the thermodynamic and transport properties of pyridinium-based ionic liquids, J. Phys. Chem. B 110, 2821-2832 (2006). 

Zhang, C., Wang, H., Malhotra S.V, Dodge C.J. and Francis, A.J., Biodegradation of pyridinium based ionic liquids by an axenic culture of soil Corynebacteria, Green Chem. 12, 851-858 (2012). 

Example 6.8 Using pyridinium-based ionic liquid (bmpy)Cl and AICI3 as the catalyst, an interesting insertion of acetylene into the C-Cl bond of acyl chlorides was achieved (Scheme 6.12) [36]. 

Isothermal Compressibilities of Imidazolium and Pyridinium-Based Ionic Liquids. 

Molecular Modeling and Experimental Studies of the Thermodynamic and Transport Properties of Pyridinium-Based Ionic Liquids. 

Xiao, Y., Malhotra, S.V. (2005). Friedel-crafts acylation reactions in pyridinium based ionic liquids. Journal of Organometallic Chemistry, 690., 15., 3609-3613., 0022-328X Xu, ]., Yang, ]., NuLi, Y., Wang, ]., Zhang, Z. (2006). Additive-containing ionic liquid electrolytes for secondary lithium battery. Journal of Power Sources 160., 1., 621-626., 0378-7753... 

Xiao, Y. Malhotra, S.V. (2005). Fiiedel-Crafts Acylation Reactions in Pyiidinium-Based Ionic Liquids, J. Orgcmometallic Chemistry, 690, pp. 3609-3613 Xiao, Y. (2006). Study of Organic Reactions in Pyridinium-Based Ionic Liquids, PhD Dissertation, Department of Chemistry Environmental Science, New Jersey Institute of Technology, N.J., USA. 

Another example of how the selection of the anion will affect solubility is the study by Papaiconomou et al. A portion of their results is shown in Table 16.2.1. For each ionic liquid, the organic cation was l-octyl-4-methylpyridinium. When combined wifli one of nine inereasingly hydrophobic anions, solubility decreased by foiu orders of magnitude. The most soluble ionic liquid was with dicyanamide as the anion, while tetraphe-nylborate rendered the pyridinium-based ionic liquid as the most hydrophobic. 

The adsorption of ionic liquids appears to be an endothermic reaction. Reinert et al. measured the adsorption of five imidazolium- and pyridinium-based ionic liquids by... 

Guerrero H, Martin S, Perez-Gregorio V, Lafuente C, Bandres I (2012) Volumetric characterization of pyridinium-based ionic liquids. Fluid Phase Equilib 317 102-109... 

Pereiro AB, Rodriguez A, Blesic M, Shimizu K, Lopes INC, Rebelo LPN (2011) Mixtures of pyridine and nicotine with pyridinium-based ionic liquids. J Chem Eng Data 56 4356-4363... 

Garcia-Mardones M, Cea P, Gascon I, Lafuente C (2014) Thermodynamic study of the surface of liquid mixtures containing pyridinium-based ionic liquids and alkanols. J Chem Thermodyn 78 234-240... 

---