N-alkylpyridinium

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

Figure 4.1-5 Schematic showing the changes in structure of N-alkylpyridinium tetrachloro-...

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. 

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]. 

The application of the concept of a reactive-ion micelle is illustrated by addition of cyanide ion to N-alkylpyridinium ions (Fig. 3) and rate constants are compared in Table 4. Second-order rate constants are essentially independent of substrate hydrophobicity and are only slightly affected by added inert salts. They are also very similar to second-order rate constants in water. 

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). 

Just as pyridine is a weaker base than piperidine, it is also a poorer nucleophile. Nevertheless, it reacts with electrophiles to form stable pyridinium salts. In the examples shown, primary alkyl halides form N-alkylpyridinium salts, whereas acyl halides and anhydrides react to give N-acylpyridinium salts. 

N-Alkylpyridinium salts give mainly N,N -dialkyltetrahydro-4,4 -dipyridyl derivatives on reduction in neutral and slightly alkaline aqueous solution [76]. These products can be oxidised to the N,N -dialkyl-4,4 -dipyridyl. The radical-zwitterion derived from 4-cyano-l-methylpyridine couples and then loses cyanide ion to form N,N -dimethyl-4,4 -dipyridyl in 39 % yield [77]. 

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). 

Alkyl halides and related alkylating agents react with pyridines to form N-alkylpyridinium salts (Scheme 2.7). These compounds are much more stable than their 7V-acylpyridinium equivalents and can often be isolated as crystalline solids, particularly if the halide ion is exchanged for perchlorate, tetrafluoroborate or another less polarizable counter anion. 

Many of the systems studied are based on [MCl4] anion. Neve et al. have extensively studied the formation of liquid-crystalline phases of N-alkylpyridinium salts with alkyl chain lengths of n = 12-18 with tetrahalometalate anions based upon Pd(II) [22] and Cu(II) [23]. In general, the liquid-crystalline phases exhibit lamellar-... 

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]. 

Oxidation of N-alkylpyridinium salts to the 2-pyridones (157) is an old, high-yield process that utilizes a mediator, usually potassium ferricyanide (Scheme 51).216-218 Selectivity for the 2-position was quite high. Iron anodes, an economic and readily machined electrode material, in basic electrolyte are satisfactory. Large variations in current density did not affect the yield. The... 

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. 

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-... 

Radius can be generated by reduction of carbonium ions or onium salts. Reduction of the N-alkylpyridinium salt 203 yields in either buffered aqueous KC1 or CH3CN/Bu4N+C104 the radical 204 in 100%efficiency 597). 

N-Alkylpyridinium salts are easily prepared and reduced by borohydride in protic solvents to give tetrahydropyridines having the double bond at the 3,4-position, i.e. to cyclic allylamines283. If the latter could be isomerized to enamines, a very useful simple... 

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). 

Hydrophilic anions will stay in the outer, water-rich regions of micelles and the more hydrophobic organic substrates may be located more deeply in the micelle. These effects, and those due to the preferred orientation of substrates in the micelle, do not seem to be of major importance in determining k /k, because values are not very different for reactions of substrates which have the same reactive groups but different hydrophobici-ties. This conclusion is illustrated by comparing values of k lk, for reactions of OH with 4-nitrophenyl acetate and octanoate or of CN with a series of N-alkylpyridinium ions (Tables 3 and 4). Despite large differences in hydro-phobicities, as shown by variations in K there is little change in (Il is... 

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

---