Alkylation 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 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... 

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]. 

Gonzalez EJ, Macedo EA (2014) Influence of the number, position and length of the alkyl-substituents on the solubility of water in pyridinium-based ionic liquids. Fluid Phase Equilib... 

Rhodium catalyzed carbonylations of olefins and methanol can be operated in the absence of an alkyl iodide or hydrogen iodide if the carbonylation is operated in the presence of iodide-based ionic liquids. In this chapter, we will describe the historical development of these non-alkyl halide containing processes beginning with the carbonylation of ethylene to propionic acid in which the omission of alkyl hahde led to an improvement in the selectivity. We will further describe extension of the nonalkyl halide based carbonylation to the carbonylation of MeOH (producing acetic acid) in both a batch and continuous mode of operation. In the continuous mode, the best ionic liquids for carbonylation of MeOH were based on pyridinium and polyalkylated pyridinium iodide derivatives. Removing the highly toxic alkyl halide represents safer, potentially lower cost, process with less complex product purification. 

Historically, AlCl j-based ionic liquids were the first to be used for the electrodeposition of metals. As described before, they are easy to synthesize by simple addition of the Lewis acidic AICI3 to a 1,3-dialkyl-imidazolium, alkyl-pyridinium or quaternary ammonium compound under an inert atmosphere. 

Fig. 3 Schematic representation of the transferability premise developed as a built-in characteristic of the CLAP force-field, a cl, c2, c3, and c4 stand for imidazolium- pyridinium-, phosphonium- and guanidinium-based cations, al to a5 stand for the hexafluorophosphate, chloride, nitrate, bis-(trifluoromethanesulfonil)imide, and triflate anions. The c3-a4 combination was selected in this case, b The possibility to grow an alkyl side chain in, for example, imidazolium-based ionic liquids in order to study an entire homologous series of ionic liquids is illustrated...

At 100 °C no product was ever observed. At 150 °C the imidazolium-based ionic liquids could be prepared. For ionic liquids bearing pyridinium cations a reaction temperature of 200 °C was required. The reactions could be scaled up to 50 g and no large excess of alkyl halide was needed. Deetlefs and Seddon studied the synthesis of pyrazolium, thiazolium, imidazolium, and pyridine-based ionic liquids [21]. Selected results are summarized in Table 7.1. The authors reported that the reactions are up to 72 times faster than when using conventional heating. They also found that if the microwave irradiation is prolonged, decomposition of the ionic liquid occurs. Imidazolium halide based ionic liquids could be prepared in 150,... 

The cation in an ionic liquid typically is an organic nitrogen-based ion such as alkyl ammonium, alkyl pyridinium, or dialkylimidazolium, examples of which appear below. 

Other cation combinations with [BF4] and [PFelectrochemical systems. Although N-butylpyridinium tetrafluorobo-rate [bpyr][BF4] is known to be a RTIL [53], the lower electrochemical stability of pyridinium-based cations relative to imidazolium limits their electrochemical applicability. On the other hand, pyrrolidinium-based cations are known to be more electrochemically stable than imidazolium salts, N-alkyl-N-methylpyrrolidinium salts of [BF4] and [PFf,] are made less attractive to researchers by the fact that they are solids at room temperature [54, 55]. Therefore, most of the electrochemical investigations of ionic liquids containing [BF4] and [PF6] have focused on [BMIM][PF6], [BMIM][BF4] and, to a lesser extent, [EMIM][BF4]. 

Functionalised ionic liquids based on cations other than imidazolium have also been developed. For example, pyridinium cations functionalised with pentafluorosulfanyl[89] or alkyl-nitrile groups1901 have been prepared as cheaper alternatives to their imidazolium-based counterparts (see Figure 2.8). The latter have been evaluated in palladium catalysed C-C cross coupling reactions and improved catalyst retention and stability were observed in the nitrile-functionalised ionic liquid compared to the simple alkyl-analogue. Consequently, the nitrile-functionalised ionic liquid solution can be reused repeatedly without significant decrease in activity (see Chapter 6 for further information). 

All the physico-chemical properties strictly dependent upon the precise nature of the cation and anion constituting the IL and they can be changed or modulated by changing the anion or cation or modifying the nature of substituents on cation. Structurally, most of the ILs that have been investigated to date are based on imidazolium, ammonium and pyridinium cations, bearing alkyl chains, associated with polyatomic anions such as chloroaluminates, tetrafluoroborate, hexafluorophosphate and bis-triflimide. In Scheme 1 are reported the above mentioned cations and anions whose combination gives the most commonly employed ionic liquids. 

The earliest appearing ionic liquid is [EtNH3][N03] (m.p. 12°C), which was reported by Walden [1] in 1914, but because of its unstability in the air, its appearance did not attract much attention. However, in the 1960s, the Academy of American Air Force had done the systematic research on the ionic liquids based on alkyl-substituted pyridinium cations, with halides or trihalogenoaluminates... 

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]. 

Recently, Ni and group [66] introduced a new type of chiral ionic liquid based on pyridinium cation having a chiral moiety tethered to a urea unit. The synthesis of salt involves a reaction of 2-aminomethyl pyridine with chiral 2-isocyanate-3-methylbutyrate and then heating in the presence of alkyl halide to form salt (Scheme 17.18). In total, nine chiral pyridinium salts were synthesized with varying amino acids. Currently, the authors are using these salts for asymmetric induction in organic transformation. 

The details of two related patents for the alkylation of aromatic compounds with chloroaluminate(iii) or chlorogallate(iii) ionic liquid catalysts have become available. The first by Seddon and coworkers [35] describes the reaction of ethylene with benzene to give ethylbenzene (Scheme 5.2-8). This is carried out in an acidic ionic liquid based on an imidazolium cation and is claimed for ammonium, phosphonium and pyridinium cations. The anion exemplified in the patent is a chloroaluminate(iii) and the claim includes for chlorogallate(iii) anions and various mixtures of anions. 

The effects of increasing the concentration of initiator (i.e. increased conversion, decreased and broader PDi) and reducing the reaction temperature (i.e. decreased conversion, increased M and narrower PDi) for the polymerizations in ambient-temperature ionic Uquids are the same as observed in conventional solvents. Mays et al. reported similar results and, in addition, used NMR to investigate the stereochemistry of the PMMA produced in (BMIMjlPFej. They found that the stereochemistry is almost identical to that for PMMA produced by free radical polymerization in conventional solvents [28]. The homopolymerization and copolymerization of several other monomers are also reported. Similar to vdiat was found by Noda and Watanabe, in many cases the polymer was not soluble in the ionic liquid and thus phase separated [28,29]. Free radical polymerization of n-butyl methacrylate in ionic liquids based on imidazolium, pyridinium, and alkylammonium salts as solvents was investigated with a systematic variation of the length of the alkyl substituents on the cations, and employing different anions such as tetrafluoroborate, hexafluorophosphate, tosylate, triflate, alkyl sulfates and dimethyl phosphate [31]. 

Imidazolium salts are the most important class of ionic liquids, but preparation of ionic liquids based on different cations has also been reported. Pyridinium salts are prepared in a similar way to imidazolium, by direct alkylation of pyridine. Alkylisoquinolium bis(perfluoroethylsulfonyl)imide salt 13 is similarly prepared by A-alkylation of isoquinoline followed by anion metathesis, and iminium salt 14 is obtained directly associated with the triflate anion (Scheme 6). ... 

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