Pyridones weak acids

When carried out in dilute acid, diazotization of 2-aminothia2ole may provide unstable diazohydroxides (164, 335, 336), differing in that respect from 2-aminopyridines which give 2-pyridones when the reaction is carried out in weak acids (337). 

Hydroxypyridines (776) are both weak acids and bases and can therefore exist as zwitterions (777) (see Section 2.2.5.1). The zwitterions of 2- and 4-hydroxypyridines are known as 2- and 4-pyridones because of their uncharged canonical forms, e.g. (778) and (779). a- and y-Hydroxypyridines exist in aqueous solution very predominantly as the oxo or pyridone form. For a- and y-hydroxy-benzopyridines and -benzazines, the equilibrium favors the benzopyridone form still more, with the exception of 3-hydroxyisoquinoline. The reactivity of the pyridones and azinones is considered in Sections 3.2.3.7.2-4. 

Hydroxypyridines 806 are both weak acids and bases and can therefore exist as zwitterions 807 (see Section 2.2.5.1). The zwitterions of 2- and 4-hydroxypyridines are known as 2- and 4-pyridones because of their uncharged canonical... 

Again coordination of the boron with the carboxylic acid nucleophile activates the R group for cleavage, and labilizes the proton that is involved in rate-determining attack. In agreement with this concept very weak acids, with properly placed nucleophiles, react with triethylboron, e.g., 1,3-diketones, acetamide, 2-pyridone, pyrollidone, and 8-hydroxyquinoline. 

These dyes are invariably monoazo compounds with the reactive system attached to the diazo component, owing to the ready availability of monosulphonated phenylenediamine intermediates. Pyrazolone couplers are most commonly used, as in structure 7.82 (where Z is the reactive grouping), and this is particularly the case for greenish yellow vinylsulphone dyes. Catalytic wet fading by phthalocyanine or triphenodioxazine blues is a characteristic weakness of azopyrazolone yellows (section 3.3.4). Pyridones (7.83), barbituric acid (7.84) and acetoacetarylide (7.85 Ar = aryl) coupling components are also represented in this sector, with the same type of diazo component to carry the reactive function. 

Like amides, 2- and 4-pyridones are also very weak bases, mnch weaker than amines. Like amides, they actnally protonate on oxygen rather than nitrogen (see Section 4.5.4). This fnrther emphasizes that the nitrogen lone pair is already in nse and not available for protonation. On the other hand, the N-H can readily be deprotonated pyridones are appreciably acidic abont 11). The conjngate base benefits from considerable resonance stabilization, both via... 

Considerable interest has been shown in the formation of hydrogen bond-mediated (discrete) cyclic assemblies incorporating between three and ten component molecules. A few hydrogen bonding motifs have dominated these studies. These include cyanuric acid-melamine contacts, 2-aminopyridine-carboxylic acid contacts and carboxylic acid or pyridone dimer contacts. While individual hydrogen bonding interactions of this type are often weak (with association constants of... 

The amino function in 4-amino-3-halopyridines belraves unexceptionally. Thus 4-amino-3-diloropyridine gives the 3-chloro-4-pyridinediazonium salt dien treated with nitrous acid the diazonium salt decomposes in the presence of potassium iodide to yield 3-chloro-4-iodopyridine. 4-Amino-2,3,5,6-tetrafluoropyridine is a very weak base but it can be diazotized in 80% hydrofluoric acid. The diazonium salt is converted to 4-bromo-2,3,5,6-tetrabromopyridine with cuprous bromide, but its reaction with water or with A, lV-dimethylaniline are complex. 4-Amino-2-chloro-3-nitropyridine is not converted to 2-chloro-3-nitro-4-pyridone (K-97) on diazotization with mineral acids and sodium nitrite or with isoamyl nitrite in glacial acetic acid with the latter reagent 2-chloro-3-nitropyri-dine (K-98) is formed." Nitrous acid reacts with 4-amlno-2-pyridone to give 4-amino-3-nitroso-2-pyridone instead of the diazonium salt. ... 

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