Amines pyridinium salts

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... 

The configuration of the amine was retained, except in the case of amino acid derivatives, which racemized at the stage of the pyridinium salt product. Control experiments showed that, while the starting amino acid was configurationally stable under the reaction conditions, the pyridinium salt readily underwent deuterium exchange at the rz-position in D2O. In another early example, optically active amino alcohol 73 and amino acetate 74 provided chiral 1,4-dihydronicotinamide precursors 75 and 76, respectively, upon reaction with Zincke salt 8 (Scheme 8.4.24). The 1,4-dihydro forms of 75 and 76 were used in studies on the asymmetric reduction of rz,>S-unsaturated iminium salts. 

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... 

Methoxybut-3-en-2-one with primary amines and acids gives the pyridinium salts, e.g., 222. In this case two molecules of methoxybutenone per one molecule of the amine enter the reaction (62M586). 

Substituted pyridinium salts of this nature were difficult to obtain by other methods. This sequence is initiated with condensation of the anions of imine 50 with malonaldehyde monoacetals 51 to access glutaconaldehyde monoacetals 52. The monoacetal-aldehyde was then reacted with primary amines ( -BuNH2) to give 3-substituted pyridinium salts 49. When the silylimino derivative (R1=Si(CH3)3) was used, substantially higher yields of intermediate glutaconaldehydes were obtained. 

Most of the synthetic approaches toward this ring system utilize N-amino pyridinium salts functionalized at the a-position with a carbonyl group. Thus, the amination of 2-(l,3-dioxolan-2-yl)pyridine with tosyl-hydroxylamine gave 78, whose reaction with urea in the presence of boron trifluoride-acetic acid gave 79, which gave the thermally unstable... 

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. 

N-NeoDentyl)-4-DihexylaminoDvrldinium Bromide (3h) The neopentyl salt was prepared in a similar manner from neopentyl mesylate, but reaction was carried out neat at 130 for 72 hr. Higher temperatures cannot be used, due to decomposition of neopentyl mesylate. The crude product was dissolved in water, basifled to neutralize any pyridinium salt, and was washed with petroleum ether to remove amine and unreacted neopentyl mesylate. The aqueous phase was acidified with HBr, and extracted with methylene chloride, to afford crude salt. Recrystallization from 20 1 ethyl acetate/acetonitrile affords the product (mp = 169-170 ). 

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. 

N-Amination of 2-[o-(l,3-dioxolan-l-yl)phenyl]pyridine (121) with O-tosylhydroxylamine gave an A-amino derivative, which underwent ring closure on the action of 70% perchloric acid to yield a pyrido[2,l-a]phthal-azinium salt (122, R = H) (92JHC1049). The 7-hydroxy derivative of a pyrido[2,l-fl]phthalalzinium salt (122, R = OH) was prepared similarly by cyclization of l-amino-2-(o-methoxycarbonylphenyl)pyridinium salt, ob-... 

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). 

Similar 3-substituted pyridinium salts give stable solid isolable adducts on reaction with aliphatic amines such as piperidine (equation 109) (Table 16) (80T785). These adducts are stable in apolar solvents, but in water they rapidly dissociate into pyridinium ions and amines, and reactions occur by several pathways (Scheme 94). All may be considered to arise from the covalent hydrates (159) and (160) which are formed under the aqueous basic conditions. 

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