1 -Aryl pyridinium salts

Carvalho, I., Miller, J. (1995) Synthesis of l-methyl-2-hydroxyiminomethyl-aryl-pyridinium salts with potential as acetyl-cholinesterase reactivators. Heterocycl. Commun. 1 403-10. 

Alkyl or aryl groups may be joined to pyridine compounds (1) through an active methyl group jn the alpha or gamma position, (2) directly at a nuclear carbon atom, or (3) at the nitrogen atom to form quaternary alkyl-or aryl-pyridinium salts. A comprehensive discussion of the alkyl- and aryl-pyridines made by these routes has been presented. ... 

The conversion of A -aUcyl- or -aryl-pyridinium salts into the corresponding pyridine, i.e. the removal of the iV-substitutent, is generally not an easy process however triphenylphosphine or simply heating the iodide salt can work for metho-salts. l-Triphenylmethyl-4-dimethylaminopyridinium chloride and... 

JV-Alkyl- and A-aryl-pyridinium salts are obtained analogously from 2,4,6-trisubstituted pyrylium salts and primary aliphatic or aromatic amines, generally in excellent yields.1028... 

Pyridinium salts, l-aryl-4-methoxy-2,6-dimethyl-synthesis, 3, 762 Pyridinium salts, N-aryloxy-rearrangements, 2, 354 Pyridinium salts, 1-benzyl-covalent amination, 2, 239 Pyridinium salts, N-benzyl-reactions... 

The Zincke reaction is an overall amine exchange process that converts N- 2,A-dinitrophenyl)pyridinium salts (e.g, 1), known as Zincke salts, to iV-aryl or iV-alkyl pyridiniums 2 upon treatment with the appropriate aniline or alkyl amine. The Zincke salts are produced by reaction of pyridine or its derivatives with 2,4-dinitrochlorobenzene. This venerable reaction, first reported in 1904 and independently explored by Konig, proceeds via nucleophilic addition, ring opening, amine exchange, and electrocyclic reclosure, a sequence that also requires a series of proton transfers. By... 

Investigations of reactivity of 3-aryltetrazolo[l,5- ]pyridinium salts 34 with aryl- and aralkylthiolates as nucleophiles recently lead to the preparation of synthetically valuable new mesomeric betaines 38 (Scheme 10) <2002T3613, 2003JOC5652>. 

The Zincke reaction is an overall amine exchange process that converts N- 2,A-dinitrophenyl)pyridinium salts, known as Zincke salts, to A -aryl or A -alkyl pyridiniums upon treatment with the appropriate aniline or alkyl amine. 

The same group expanded the scope of the aza-Diels-Alder reaction of electron-rich dienes to Brassard s diene 97 (Scheme 37) [60]. In contrast to Danishefsky s diene, it is more reactive, but less stable. Akiyama et al. found chiral BINOL phosphate (R)-3m (3 mol%, R = 9-anthryl) with 9-anthryl substituents to promote the [4 + 2] cycloaddition of A-arylated aldimines 94 and Brassard s diene 97. Subsequent treatment with benzoic acid led to the formation of piperidinones 98. Interestingly, the use of its pyridinium salt (3 mol%) resulted in a higher yield (87% instead of 72%) along with a comparable enantioselectivity (94% ee instead of 92% ee). This method furnished cycloadducts 98 derived from aromatic, heteroaromatic, a,P-unsaturated, and aliphatic precursors 94 in satisfactory yields (63-91%) and excellent enantioselectivities (92-99% ee). NMR studies revealed that Brassard s diene 97 is labile in the presence of phosphoric acid 3m (88% decomposition after 1 h), but comparatively stable in the presence of its pyridinium salt (25% decomposition after 1 h). This observation can be explained by the fact that the pyridinium salt is a weak Brpnsted acid compared to BINOL phosphate 3m. 

The authors showed that the above reaction carried out in 7V-hexyl-pyridinium salts gave higher yields than imidazolium salts, that the addition of a phosphine ligand to Pd reduced the yield in pyridinium salts, and that higher reaction temperatures were required to obtain high yields. Howarth and Dallas studied the use of [bmimJpF ] as an IL solvent for the reaction of aryl halides and methyl acrylate. They found that the yields of the products were comparable with that obtained with DMF, and that the solvent and catalyst could be reused several times. 

If a ketone rather than an ester group were present at position 2 of the pyridinium salt (266, Scheme 131), ammonium acetate produced 2-azaquinolizinium salts with an alkyl or aryl group at position 1 (Table 15). 

A novel way of preparing s-triazolo[3,4-a]pyridinium salts (105) by anodic oxidation of aryl hydrazones (104) of 2-acetylpyridine, 2-benzoylpyri-dine, and 2-formylpyridine in CH3CN-Et4NX (X=C104) p-TsO, or BF4) has been reported165 [Eq. (80)]. 

Both 4-arylimino-2,6-dimethylpyran salts (167) and l-aryl-4-methoxy-2,6-dimethyl-pyridinium salts (168) result from the reaction of 4-methoxy-2,6-dimethylpyrylium salts with primary aromatic amines (78JCS(P1)1373). The ratio of the products varies with the basicity of the amine, the less basic amines giving mainly the pyran salt. The free iminopyrans (169), which have limited stability at room temperature, are readily obtained from the salts. 

In the reaction of activated aryl halides with phenoxide and thiophenoxide in chloro- or o-dichloro-benzene at reflux, pyridinium salts (48 R, R = —-CH2CH2CHMeCH2CH2— R = Me, Bu", n-C6Hi3 R = CH2CHEtBu, CH2BU1) proved to be superior catalysts to simpler onium salts, like TBAB, due mainly to their greater thermal stability.162... 

The Mukaryama reagent,18 N-methyl-2-chloropyridinium iodide (6). transforms carboxylic acid 5 into the amide 7 The acid Is first activated in situ in the form of pyridinium salt 17 by an SN reaction with the Mukoixama reagent (6). This activation is a result of preventing resonance stabilization of the C-O double bond in the positively charged aryl ester 17. 

The 13C-NMR spectra of l-arylbenzo[c]pyrylium salts have been investigated in comparison with those of 1-arylnaphthalenes, N-methyliso-quinolinium, and 1-arylisoquinoline derivatives 75ZN(B)943]. The correlation obtained is analogous to relationships found for a series of pyrylium and pyridinium salts, and pyridine derivatives [82AHC(Suppl)]. The, 3C shifts of 1 -aryl-benzo[c]pyrylium salts are also a valuable source of information for ptfa values, aromatic character, and conjugative effects of these compounds (760MR324). 

All these various pyrylium salts are converted by ammonia, or alkyl, arylkyl or aryl primary amines into the corresponding pyridinium salts.25 The probable reaction sequence is given in the illustrative example 1-benzyl-2,4,6-triphenyl-pyridinium tetrafluoroborate26 (Expt 8.31). 

This method is not general and only a limited number of 5-alkyl-l,2,3,4-thiatriazoles were prepared by this reaction. Ikeda and co-workers have described a convenient method for the synthesis of both 5-alkyl- and 5-aryl-l,2,3,4-thiatriazoles 9 by reaction of l-methyl-2-thioacylpyridinium salts 143 with sodium azide (Scheme 33). Compound 143 can be prepared from pyridium salt 142 by reaction with dithiocarboxylic acid derivatives. The synthesis can conveniently be carried out as a one-pot reaction from 2-chloro-l-pyridinium salt 142 and carbodithioate leading to final compounds 9 in high yields. The 5-alkyl-l,2,3,4-thiatriazoles 9 were isolated as oils <1990S415, 1990ZC67>. 

Interest in the use of pyrylium salts in synthesis continues. 2-Methylpyridinium salts are converted into the 2-arylpyridinium compound on reaction with 2,4,6-triarylpyrylium salts in the presence of base. The methylpyridinium salt is deprotonated to the anhydrobase which behaves as an enamine, attacking the pyrylium salt at C-2, promoting a ring opening - ring closure sequence which culminates in the effective arylation of the pyridinium salt. 

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