1- alkylpyridinium ionic liquid

The most common cations used for ionic liquids to date, e.g., tetraalkylammo-nium, tetralkylphosphonium, 1,3-dialkylimidazolium, 1,2,3-trialklyirnidazolium and 1-alkylpyridinium ionic liquids all have negligible concentrations of neutral molecules in the pure ionic liquid, certainly under ambient conditions. However, even ionic liquids with these archetypical cations may contain significant amounts of neutral molecules if the chosen anion is characterized by a significant nucleophilicity and they are being used at elevated temperatures. 

Stepnowski, R, Mrozik, W., and Nichthauser, J., Adsorption of alkylimidazo-lium and alkylpyridinium ionic liquids onto natural soils. Environ. Sci. TechnoL, 41, 511, 2007. 

First, ionic liquid surface tensions are unremarkable and lie in the range of conventional molecular liquids [79]. The surface tension of a series of [C,jCjim]X n = 2, 4, or 6 X = [OTf or [BF4] ) ionic liquids has been shown to decrease as the alkyl chain length increases [82]. This would be expected from standard relations that show that surface tension is inversely proportional to molecular volume [79]. Similar results were found for the N-alkylpyridinium ionic liquids [C pyr][NTf2] (n = 2, 4, or 5) [83]. Rebelo et al. [84] found that for the ionic liquids [C,jCjim][NTf2] (n=l, 2, 4, 6, 8, 10, 12, or 14) this trend held, whereas Maier et al. [85] found that for many of the same ionic liquids [C,jCjim][NTf2] (n= 1, 2, 4, 6, 8, 10, or 12) this trend held up to n = 8, after which the surface tensions leveled. Although... 

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

Room-temperature ionic liquids, salts with V-alkylpyridinium cations in synthesis and catalysis 99CRV2071. 

Beckmann rearrangements of several ketoximes were performed in room-temperature ionic liquids based on l,3-dialkylimida2olium or alkylpyridinium salts containing phosphorus 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. 

In recent years ionic liquids have also been employed as media for reactions catalyzed both by isolated enzymes and by whole cells, and excellent reviews on this topic are already available [47]. Biocatalysis has been mainly conducted in those room-temperature ionic liquids that are composed of a 1,3-dialkylimidazolium or N-alkylpyridinium cation and a noncoordinating anion [47aj. 

D.1.2. N-alkylpyridinium-Containing Ionic Liquids. The melting point of N-alkylpyridinium chloride increases from 70 to 80°C when the alkyl chain is lengthened from Ci2 to Cig. The melting point of iV-alkylpyridinium [NiCl4]J changes only from 80 to 86°C as the substituent changes from C12 to Cig 62). 

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. 

The alkylpyridinium cations suffer from being relatively easy to reduce, both chemically and electrochemically. Charles Hussey (Figure 1.3) and I set out a program to predict cations more resistant to reduction, to synthesize ionic liquids on the basis of those predictions, and to characterize them electrochemically for use as battery electrolytes. 

Biocatalysis in ionic liquids was first reported in 2000 [7, 8, 9]. The early work involved ionic liquids composed of a 1,3-dialkylimidazolium or N-alkylpyridinium cation and a weakly-coordinating anion (Figure 10.1). More recently, attention is shifting toward new structural types. A number of reviews of this rapidly expanding subject have appeared [10, 11, 12, 13, 14]. 

Lipases, which are noted for their tolerance of organic solvents, were obvious candidates for biocatalysis in ionic liquids. Indeed, stable microbial lipases, such as CaLB [8, 54, 55, 56] and Pseudomonas cepacia lipase (PcL) [28, 55, 57] were cat-alytically active in the ionic liquids of the l-alkyl-3-methylimidazolium and 1-alkylpyridinium families, in combination with anions such as [BF4], [PF6], [TfO] and [ Tf2N]. Early results were not always consistent, which may be caused by impurities that result from the preparation of the ionic liquid. Lipase-mediated transesterification reactions (Figure 10.3) in these ionic liquids proceeded with an efficiency comparable to that in tert-butyl alcohol [8], dioxane [57], or toluene... 

Recently, Deetlefs and Seddon83 reported the solvent-free synthesis and scale-up of ionic liquids under microwave irradiation. Using a commercial microwave reactor they prepared ionic liquids based upon the 1-alkylpyridinium, l-alkyl-3-methylimida-zolium, l-alkyl-2-methylpyrazolium and 3-alkyl-4-methylthiazolium cations, on scales from 50 mmol to 2 mol (Scheme 9.2). Under microwave irradiation, because of efficient... 

Pyridinium hydrobromide perbromide salt was introduced by Djerassi and Scholz as an alternative brominating agent to bromine in 1948. Salazar and Dorta rationalized that since alkylpyridinium salts are well documented and commercially available room temperature ionic liquids, a combination of an alkylpyridinium cation with tribromide anion 1 should therefore lead to a room temperature ionic liquid bromine analogue (Equation 1). 

Evans et al. also showed that the 1 1 mixture of BAN and (3, y-distearoyl-phos-photidylcholine (DSPC) gives a smectic A texture in the temperature range of 57.3 to 100°C [21]. This is the first notice of lyotropic lamellar liquid crystals formed in the ionic medium. Additionally, Seddon et al. [28] and Neve et al. [29] have described the long-chained A-alkylpyridinium or l-methyl-3-alkylimidazolium ions to display smectic liquid-crystalline phases above their melting points, when Cl or tetrachloro-metal anions like CoCl " and CuCl " are used as the counter ions. Lin et al. have also noted the liquid crystal behavior of 1-alkylimidazolium salts and the effect on the stereoselectivity of Diels-Alder reactions [30]. However, liquid crystals are classified as ionic liquid crystals (ILCs), and they are distinguished from liquid crystals that are dispersed in ionic liquids. Although the formation of micelles and liquid crystal phases in ionic liquids have been thus reported, there has been no mention of the self-assembly of developed nano-assemblies that are stably dispersed in ionic liquids. In the next section the formation of bilayer membranes and vesicles in ionic liquids is discussed. 

N-Alkylpyridinium salts with linear alkyl chains have recently found interesting applications as ionic liquids, along with imidazolium cations. Another application of pyridinium salts with two hydrophobic tails is as non-viral gene transfer agents (cationic lipids) (06JMC3872). 

C. More generally, however, the ionic liquids in use today are salts in which the cation is unsymmetrical and in which one or both of the ions are bulky so that the charges are dispersed over a large volume. Both factors minimize the crystal lattice energ) and disfavor formation of the solid. Typical cations are quaternary ammonium ions from heterocyclic amines, either 1,3-dialkyliraidazolium ions, iV-alkylpyridinium ions, or ring-substituted A -alkylpyridinium ions. 

Seddon [110] described a process using a commercial multimode microwave reactor and apart from the classical [Cnmim]-derivatives his report also describes the preparation of iV-alkylpyridinium-, lV-alkyl-3-methylthiazolium- and TV-alkyl-2-methyl-pyrazolium-based ionic liquids (Scheme 19). 

From conventional homogeneous to green homogeneous and heterogeneous catalysis with Lewis acids (ring opening and electrocyclic formation of heterocycles, reactions in ionic liquids, 7V,7V -dialkylimidazolium and Y-alkylpyridinium salts) 03CRV4307. 

Dialkylimidazolium, 1,1-dialkylpyrrolidinium, and 1-alkylpyridinium salts as ionic liquids in catalysis 04CCR(248)2459. 

The most common ionic liquids include alkylammonium, alkylphosphonium, A/-alkylpyridinium, and A/A/ -dialkylimidazolium cations. Two general methods for their preparation are acid-base neutralization reactions and metathesis of halide salts with a metal or ammonium salts. Alkylammonium, pyridinium, and imidazo-lium halides can be prepared by the reaction of the appropriate alkyl halide and amine. Preparation of l-ethyl-3-methylimidazolium chloride [emimjCl requires a sealed tube since it has a low boiling point. On the other hand, synthesis of [bmim] Cl can be achieved under conventional reflux conditions [33, 34]. 

Reactions in ionic liquids (mainly in N-alkylpyridinium and 1,3-dialkylimida-zolium salts) 00PAC1391, 00PAC2275, 01CC2399. 

Table 8.10 Examples of ionic liquids prepared from reactions of [XjCl and a Lewis acid where [X] is an alkylpyridinium or dialkylimidazolium ion.

Ionic liquids differ from classical molten salts by being liquids at room temperature, or their melting points are below 100 °C. They have an ionic structure and usually consist of an organic cation and an inorganic or organic anion. The most common cations found in ionic liquids are the tetraalkylammonium, tetraalkyl-phosphonium, N-alkylpyridinium, 1,3-dialkylimidazolium, and trialkylsulfonium moieties [11]. Some common ionic liquid cations and anions are presented in Fig. 7.1. 

Cations The types of cations used in synthesizing ionic liquids are tetraalkylam-monium, triaUcylsulfonium, tetra-alkylphosphonium, 1-3-dialkylimidazolium, A -alkylpyridinium, A -A -dialkylpyrrolidinium, iV-aUcylthiazolium, A -A -dial-kyltriazolium, iV-A -dialkyloxazolium and A -A -dialkylpyrazolium. The most commonly used cations are 1-3-dialkylimidazolium and A -alkylpyridinium, due to their important physicochemical properties (60). 

This chapter will concentrate on the preparation of ionic liquids based on 1,3-dialkylimidazolium cations, as these have dominated the area over the last twenty years. The techniques discussed in this chapter are, however, generally applicable to the other classes of cations indicated in Fig. 2.1-1. The original decision by Wilkes et al. to prepare l-alkyl-3-methylimidazolium ([RMIM]+) salts was prompted by the requirement for a cation that had more negative reduction potential than Al(iii) [6]. The discovery that the imidazolium-based salts also generally displayed lower melting points than the 1-alkylpyridinium salts used prior to this cemented their position as the cations of choice since this time. Indeed, the method reported by... 

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