N-alkylpyridinium salt

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]. 

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-... 

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... 

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). 

It is believed that the N-alkylpyridinium salt, generated in situ from condensation of phenacyl bromide and pyridine, is converted into the 1,3-dipole species 43 under basic conditions. Subsequent cydoaddition to ethyl propiolate results in an unstable intermediate, which instantly fadlitates aromatization to afford the fused pyrroles 42a (Scheme 17.32). 

A large number of 3-propenyl derivatives (2.1-11) may be obtained in high yield from the reaction of 3-acetamidopyridine with cyanogen bromide and anilines followed by diazotization (Eq. 11). Similar results have been reported in the diazotization of N-alkylpyridinium salts (Eq. 12). ... 

Acceptor-CH-substituted pyridinium-N-betaines 15 (accessible in situ by deprotonation of the corresponding N-alkylpyridinium salts 14) undergo 1,3-dipolar cycloaddition with activated alkynes and alkenes as dipolarophiles. With alkynes, the cycloadducts (16/19) dehydrogenate spontaneously to indolizines, which are either of the 1,2,3-trisubstituted type 17 or (indicating a regioselective cycloaddition) of the 1,3-disubstituted type 18. With olefinic substrates, the presence of an oxidant for additional dehydrogenation of the primary cycloadduct (20 -> 17) is required [219] ... 

Alkyl halides and tosylates or dialkyl sulfates give rise to quaternary N-alkylpyridinium salts 3, activated haloarenes (e.g., l-chloro-2,4-dinitrobenzene) form N-arylpyridinium salts 4. N-alkylation is likewise brought about by Michael addition of pyridine to acrylic acid derivatives 5 (e.g., R = COOR or CN) in the presence of HX (cf also the King-Ortoleva reaction, p. 380). 

N-Alkylpyridinium salts are readily reduced by NaBH4. Product formation and degree of reduction can be controlled by the pH of the reaction medium. Thus, 1-methylpyridinium chloride (131) is converted to l-methyl-l,2-dihydropyridine (132) in H2O at pH > 7, but to l-methyl-l,2,3,6-tetrahydropyridine (130) at pH 2-5 complete reduction to 1-methylpiperidine is brought about by Sn or Zn in dilute HCl (pH > 1). 

N-ALkylpyridinium salts with a leaving group in 2-position (Mukaiyama reagents, e.g., 234) promote esterification of carboxylic acids with alcohols as well as lactone formation of hydroxy acids in basic medium [138], for example ... 

Acar, E.A., Glarner, E, and Burger, U., Aminocyclopentitols from N-alkylpyridinium salts. A photochemical approach, Helv. Chim. Acta., 81,1095,1998. 

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