Alkylpyridinium salts

Quaternary Salts. Herbicides paraquat (20) and diquat (59) are the quaternary salts of 4,4 -bipyridine (19) and 2,2 -bipyridine with methyl chloride and 1,2-dibromoethane, respectively. Higher alkylpyridinium salts are used in the textile industry as dye ancillaries and spin bath additives. The higher alkylpyridinium salt, hexadecylpytidinium chloride [123-03-5] (67) (cetylpyridinium chloride) is a topical antiseptic. Amprolium (62), a quaternary salt of a-picohne (2), is a coccidiostat. Bisaryl salts of butylpyridinium bromide (or its lower 1-alkyl homologues) with aluminum chloride have been used as battery electrolytes (84), in aluminum electroplating baths (85), as Friedel-Crafts catalysts (86), and for the formylation of toluene by carbon monoxide (87) (see QuaternaryAA ONiUM compounds). 

The methodology based on the addition of nucleophiles at the a- and y -positions of A -alkylpyridinium salts to give substituted 1,2- and 1,4-dihydropyridines (often not isolated) as intermediates, respectively, which can be further elaborated into complex polycyclic alkaloids, was reviewed by Joan Bosch and M.-Lluiesa Bennasar in 1995 (95SL587). 

Partial hydrogenation of A -alkylpyridinium salts in hydrolytic media provides a convenient route to oj-alkylaminovaleraldehydes (87). 

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. 

On 100% acrylic materials the quaternary ammonium retarders are used almost exclusively. Other types have been evaluated, however. For example, saturated alkylamines (RNH2 R = C10, C12, C14 and C16 hydrophobes) were found to be just as effective as the quaternary types although other factors, such as aqueous solubility at the optimal dyebath pH and resistance to subsequent discoloration, favour the quaternary compounds [34]- On the other hand, bis (hydroxyethyl) coco amine (12.10) had relatively little effect and the amphoteric carboxymethyldimethylcocoamine (12.11) none at all, although dimethyl-cocoamine oxide (12.12) was quite an effective retarder [34]- Other cationic compounds used [43,44] have included alkylpyridinium salts, imidazoles and imidazolinium salts, alkyldiamines, alkylpolyamines, as well as sulphonium and phosphonium derivatives. 

Another classic reaction of pyridinium salts is reduction of the pyridine ring. Donohoe and co-workers reported the partial reduction of A-alkylpyridinium salts <060BC1071>, which is accompanied by subsequent alkylation and hydrolysis to furnish a range of 2,3-dihydropyrid-4-ones. This sequence has the potential to introduce a variety of functional groups at the C-2 position of 2,3-dihydropyrid-4-ones. Reduction of pyridinium ylides with sodium borohydride has also been reported in fair to good yields <06JHC709>. 

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. 

Electrochemical reduction of 1-alkylpyridinium salts 1 leads to Are addition of one electron with the formation of a ii-delocalised radical-zwitterion. This is a formally neutral species. Botlr this species and the N-protonated pyridine radical-anion are essentially n-delocalised radicals. The radical-zwitterion from 1-methylpyridinium shows a long wavelength absorption band in water with 7, ax 750 nm [19]. The nitrogen ring radical-zwitterions take up further electrons at more... 

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. 

A number of other routes are available for the syntheses of diquaternary salts of 4,4 -bipyridines. One method that has been extensively studied involves reaction of a 1-alkylpyridinium salt with sodium amalgam (or sodium in liquid ammonia) to form the 1,1 -dialkyl-1,1, 4,4 -tetrahydro-bipyridine, which is readily oxidized to the corresponding l,l -dialkyl diquaternary salt. This reaction is analogous to the synthesis of 4,4 -bipyridine by the action of sodium on pyridine, followed by oxidation of the intermediate tetrahydrobipyridine. " The reduction may be achieved electrolytically or by reaction with zinc or magnesium. Various oxidizing agents have been used to assist the conversion to the di-quaternary Another synthesis of diquaternary salts of... 

Alkaline solutions of alkylpyridinium salts contain increasing amounts of pseudobases (106 equation 83) in equilibrium with the charged form as the series 1-methylquinolinium, 2-methylisoquinolinium, 10-methylphenanthridinium and 10-methylacridinium is traversed. Such species were first postulated as a result of the observation that alkaline solutions of quaternary salts do not obey the Beer-Lambert law. Pseudobase formation at... 

Amination of AT-alkylpyridinium salts with amide ions, which in principle should be easier than the reaction with the parent pyridine, has been little studied. The main reason for this is that solvent selection is difficult. Metal amides are only soluble in liquid ammonia (with which pyridinium salts react easily, vide infra), and pyridinium salts are soluble in solvents that are not suitable for use with metal amides. The A/ -methylacridinium cation undergoes direct imination to give (153) in 35% yield by treatment with potassium amide and iron (III) nitrate in liquid ammonia. Two other products (154) and (155) are also formed, probably by hydrolysis and subsequent disproportionation (Scheme 90). One might question whether sodamide is necessary to the above transformation in light of the fact that quin-olinium, isoquinolinium and certain pyridinium ions give cr-complexes (156), (157) and (158) in liquid ammonia alone at 0 °C (73JOC1949). 

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-... 

Illustrative examples of the reductive alkylation of support-bound amines are listed in Table 10.7. Further examples have been reported [134-138]. Closely related to the reduction of iminium salts is the reduction of A-alkylpyridinium salts, which has been successfully accomplished on Wang resin using NaBH4 as the reducing agent (DMF/ MeOH (1 1), 2 x 4 h Entry 11, Table 15.23 [139]). 

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... 

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). 

Kanomata et al. carried out the reduction of iV-alkylpyridinium salt 168 via hydride transfer from diolate 169 to afford mainly the 1,4-dihydropyridine 170 (Equation 89) <1998AGE1410>. 

Lavilla and co-workers developed a dithionite reduction of a-substituted iV-alkylpyridinium salts to afford the corresponding 1,4-dihydropyridines or piperidines. In the absence of NaHCC>3, full reduction occurred to give the piperidine derivatives in high yield (Scheme 32) <2005TL3513>. 

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). 

Photolysis of 1-alkylpyridinium salts, e.g., 53, is considered to involve bicyclic aziridine allylic cations 54 as intermediates (Scheme 8) <2005JOC5618>, and similar behavior has been found in certain pyrylium cations (Schemes 9 and 10). Diazabenzvalenes are implicated in the rearrangement at 300 C of certain perfluoropyridazines to pyrimidines and pyrazines (Scheme 11). 

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