N-alkylpyridinium cation

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. 

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]. 

Bronich, T.K., Kabanov, A.V., Kabanov, V.A, Yu, K. and Eisenberg, A. (1997) Soluble complexes from poly(ethylene oxide)-block-polymethacrylate anions and N-alkylpyridinium cations. Macromolecules, 30, 3519-3525. 

Other polymerization processes. Polyisobutene, traditionally prepared by the Cosden process, is a valuable lubricant, and also a route to higher value-added materials. In general it was observed that the catalytic activity of the ionic liquids increases towards higher degrees of polymerization from short-chain ohgomers as the alkylchain length of the l-alkyl-3-methylimidazohum or N-alkylpyridinium cation is increased. ... 

The research groups of Mariano and West developed a photoinduced electrocydi-zation/nucleophilic addition sequence. Thus, irradiation of N-alkylpyridinium perchlorates as 5-19 in an aqueous solution led to the aziridine cations 5-20, which react in a nucleophilic addition with OH to give the isolable azabicyclo[3.1.0]hex-2-enols 5-21. These can be further transformed by a nucleophilic ring-opening of the aziridine moiety under acidic conditions to lead to useful unsymmetrically trans,trans-trisubstituted cyclopentenes 5-22 (Scheme 5.5) [10]. 

We note in this context that k /kw is greater than unity both for nucleophilic attack upon neutral aromatic substrates, e.g. 2,4-dinitrochloro-benzene and cationic N-alkylpyridinium ions (Tables 3 and 4). 

Calixarenes modified with a reporter site are equally competent chemosen-sors for the detection of a variety of analytes according to Scheme 6, especially when the analyte is cationic. The tt-cavity of a tetraanionic resorcin[4]arene readily binds cationic guests, a feature that Inouye et al. have exploited in the development of an acetylcholine chemosensor [358], The fluorescence from a pyrene-modified N-alkylpyridinium is strongly quenched upon its association with the tetraphenolate form of resorcin[4]arene (24). This quenching is consis-... 

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). 

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. 

Fig. 3.5-4 Basic skeleton of the nitrogen-containing cations that can generate ILs N-alkylpyridinium 1,3-dialkylimidazolium 1,1 -...

The most common salts in use are those with alkylammonium, alkylphosphonium, N-alkylpyridinium and N,N -dialkyl imidazolium cation (Scheme 1). 

Ionic liquids have been known for nearly a century the first to be discovered was ethylammonium nitrate, CH3CH2NH3 " N03, with a melting point of 12 °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 energy and disfavor formation of the solid. Typical cations are quaternary ammonium ions from heterocyclic amines, either 1,3-dialkylimidazolium ions, N-alkylpyridinium ions, or ring-substituted N-alkylpyridinium ions. 

Most ILs are based on quaternary ammonium cations such as 1,3-diaIkyHmidazoHum, N,N,N,N-tetraalkylammonium, N,N-dialkylpyrrolidinium, N-alkylpyridinium, or N-alkylpiperidinium. Other options are, for example, P,P,P,P-tetraaIkylphosphonium and S,S,S-trialkylsulfonium cations. Preferred anions can be either organic anions such as dicyanamide (DCA ), triflate... 

The N-alkylpyridinium-based cations do react with solvated electrons, producing a pyridinyl radical that can transfer an electron to many solutes (Equations 5.5 and 5.6). 

Treatment of 2-amino-1-alkylpyridinium salts with butyllithium affords -aminopentadienenitriles ring opening involves initial double deprotonation of the pyridinium cations This possibility is illustrated by the conversion of the N-methylpyridinium salt (50) to the aminonitrile (51).75... 

Many ILs are prepared from a cation containing a heteroelement (N, P) such as tetralkylammonium, alkylpyridinium, dialkylimidazolium, or tetralkylphospho-nium. A number of these cations have been extensively studied for Friedel-Crafts reactions and have shown a rather good stability [10]. Therefore, they represent good potential candidates. 

---