Pyridyl pyridinium salts

Fluoropyridine. This isomer can be prepared in 54—81% yield by dia2oti2ation of 4-aminopyridine in anhydrous hydrogen fluoride (370,371,400). Eree 4-fluoropyridine readily undergoes self-quaterni2ation to give pyridyl pyridinium salts (401) stabili2ation can be effected as the hydrochloride salt (371,400). Numerous 4-fluoropyridinium salts, eg, 4-fluoro-l-methylpyridinium iodide, have been converted to novel penicillins (387,402). 

Phosphorous acid attack on l-(4-pyridyl)pyridinium salts, again proceeding with relatively low yield (<30%) (Figure... 

Figure 6.4 Reaction of a pyridyl (pyridinium) salt with phosphorus acid.

Pyridine itself will substitute a- and y-halogenopyridines, giving A-(2- or 4-pyridyl)pyridinium salts. Self-quaternization of halogenopyridines can also occur and is most common with 4-fluoropyridines (58BSF424). 

Synthesis of N-(4-pyridyl)pyridinium salts from trichloromethylarenes and pyridine 93KGS980. 

A hydrogen atom is replaced by an ammonium group when l-4 -pyridyl-pyridinium salts are formed from pyridine and thionyl chloride. Since these salts are often applied to the preparation of 4-substituted pyridines that are hard to obtain by other methods,241 the preparation of l-4 -pyridylpyridinium chloride hydrochloride241 is described below ... 

Quaternary pyridinium salts503 having electron-attracting substituents on nitrogen — e.g., l-(2,4-dinitrophenyl)- or l-(4 -pyridyl)-pyridinium salts — can react in two ways with nucleophilic reagents such as amines. Either the quaternary pyridinium nucleus is replaced by the new amine group (reaction... 

This reaction (a) has considerable preparative interest in the case of l-(4-pyridyl)pyridinium salts since it permits introduction of an amino group at position 4 of pyridine. For example, 4-aminopyridine is formed when l-(4-pyridyl)pyridinium chloride hydrochloride is heated with aqueous ammonia for 8 hours at 150°1136 or when ammonia is led into a solution of this salt in phenol for 3 hours at 180-190° (80% yield).1137 Analogously, passing dimethyl-amine into a phenol melt of the salt gives 4-(dimethylamino)pyridine.1138 Under these conditions the quaternary pyridine ring is easily cleaved by aromatic amines, so that 4-(arylamino)pyridines are better synthesized from the pyridylpyridinium salts by means of the hydrochloride of aromatic amines.1139... 

Poly(3-alkylthiophene)s are chemically robust, withstanding strong reductants including boranes [67] and LiAlH4 [72]. The electron-rich backbone is, however, readily functionalized by oxidative methods. Li and co-workers exploited this to replace the 4-proton with Cl, Br or NO2 functionality [73-75]. Reaction at the a-methylene was noted in some instances. Subsequent Pd-catalyzed cross-coupling of the perbrominated polymer could effect >99% derivatization. Oxidation renders the backbone susceptible to nucleophilic attack. Li et al. found that pyridine derivatives efficiently reacted at the 4-position of the radical cation, functionalizing up to 60 % of the putative polaron pentads. Use of l-methyl-4-(4 -pyridyl)pyridinium salts yielded viologen substituents [76]. 

Aminopyridines from ethylenepyridinium salts via (3-pyridyl)pyridinium salts... 

H-Pyrido[l,2-alpyrimidin-4-ones, 2-hydroxy- 17, 811 G-Pyridylethylation 17, 763 (3-Pyridyl)pyridinium salts... 

Bromine converts pyridine-3-sulphonic acfd, in boiling aqueous solution, into 4-amino-3,5-dibromopyridine, presumably through 3,4,5-tribromo-pyridine and a derived 3,5-dibromo-4-pyridyl-pyridinium salt (see p. 266)159, 160. 

A special type of ammonio group is represented by 4-( 1 -pyridinium)-pyridine and other azinium analogs. Such products often result from self-quaternization of highly reactive derivatives. A -(4-Pyridyl)-and A -(3-nitro-4-pyridyl)-pyridinium chloride hydrochlorides (121) react with aniline, chloride ion, and water to give 4-substituted pyridines plus pyridine. l-(2-Quinolyl)- and l-(4-quinolyl)-pyridinium salts undergo 2- and 4-substitution, respectively, with amines, anilines, hydroxylamine, phenols, alkoxides, mercaptans, and chloride... 

By analogy with the conversion of 1,2-dialkylpyridinium salts into 2-alkylaminopyridines known as the Kost-Sagitullin rearrangement, the pyridinium salt 252 was treated with two different hydrazides (Equation 33). The [l,2,4]triazolo[4,3-tf]-pyridines 253 with Y = 4-pyridyl and NH2 were obtained with 35% and 38% yields, respectively <2003CHE275>. 

The oxidation of8-f-hutyl-l-(2-pyridyl)-2-naphthol illustrates the reaction between a produced cationic center and a tertiary amine (Scheme 29) [40]. The produced pyridinium salt reacts in a basic medium with loss of isobutylene. 

A special rearrangement in the 1,2,4-thiadiazole series concerns the 2-phenylamino-l,2,4-thiadiazolo [2,3,-a]pyridinium salts (275 R = H, Me), which are obtained by oxidation of N-(2-pyridyl)thioureas (Scheme 45). In the case of the unsubstituted derivative (275 R = H X = Br), neutralization with sodium acetate in ethanol produces directly benzothiazole 277... 

A number of syntheses of substituted 2,3 -bipyridines are worthy of note. Tetracyclone heated at 215°C with nicotinonitrile affords 3,4,5,6-tetraphenyl-2,3 -bipyridine, whereas 3,4-di(2-pyridyl)pyridine is obtained by an oxidative degradation of the corresponding 6,7-disubstituted thiazolo[3,2-a]-pyridinium salt. Nicotinic acid on UV irradiation in aqueous solution at pH 4-6 gives 2,3 -bipyridine-5-carboxylic acid, whereas irradiation of picolinic acid in the same pH range in the absence of metal ions gives some 2,3 -bipyridine. 6,6 -Diphenyl-2,3 -bipyridine is thought to be formed from... 

Further evidence for the presence of l-(4-pyridyl) pyridinium ion in the reaction mixtures was provided by a separation technique based on the precipitation of the l-(4-pyridyl)pyridinium ion in the presence of pyridine with Ph4B in a solution with a pH of 8. This procedure was necessary, since the ultraviolet spectrum of l-(4-pyridyl)pyridinium ion in a solution prepared by acid hydrolysis of the crude reaction products was completely obscured by the intense spectrum of pyridinium ion, which also was formed by the hydrolysis of the reaction products. However, as shown in Figure 3, the l-(4-pyridyl) pyridinium ion was easily identified after its separation as the tetraphenylborate(III) salt... 

It was necessary to assume that l-(4-pyridyl) pyridinium dichloride was formed without the formation of pyridinium chloride in the chloride reaction mixture, in order to account for the stoichiometries observed. The absence of the characteristic spectra of pyridinium chloride from samples of chloroform-soluble residues supported this assumption. However, for similar reasons it was necessary to conclude that pyridinium bromide was a product of the bromide reaction. The difference between the two reactions in this respect may be explained by the relative solubilities of the two halide salts of the protonated l-(4-pyridyl) pyridinium ion in pyridine. This point, however, was not pursued further in this investigation. 

Figure 7.11 2-Pyridyl analogs (pyridinium salts) closely mimic the carboxyl of proline.

Chloro-l-methylpyridinium iodide (683) functions as an important mediator in a variety of different reactions. Esters are formed in high yield from acids and alcohols when reacted with one equivalent of (683) in the presence of two equivalents of triethylamine (Scheme 158) (75CL1045). cv-Hydroxy acids are lactonized under similar conditions (76CL49). The pyridinium salt (683) will also convert thioureas into carbodiimides (77CL575) and pyridine-2-thiones to pyridyl sulfides (75CL1159). 

Base-catalyzed rearrangement of Af-(aryloxy)pyridinium salts (11) leads to 2-arylpyridines (12) via intramolecular attack on the ortho position of the aryloxy ring by the 2-pyridyl carbanion. When X was 3-CChMe, pyrido[3,2-J]coumarins (13 R = NO2, CN) were directly obtained in useful yields from this reaction. When in (11) X was 3-COMe and R was 4-NO2, 10-hydroxy- 10-methyl-6-nitropyrido[2,3-J]benzopyran (14) was obtained in 10.8% yield.89... 

Only one report of synthetic value of anodic transformations of pyridyl aldehydes and ketones is known.210 In this work, a series of hydrazones was converted to the s-triazolo[3,4-a]pyridinium salts (Scheme 50). The yields were generally high (80-90%). 

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