Pyridazinones. tautomerism

The diazinone tautomers are identified by using terminology such as 3(2//)-pyridazinone for the carbonyl tautomer of 3-hydroxypyrazine. The 3(2//) prefix signifies the position of the oxygen (3-pyridazinone) and specifies the NH is at position 2. Note that, in addition to the diazine-diazinone tau-tomerism, when the nitrogens have a 1,3-relationship there is further tautomerism possible, e.g. 4(1//)-pyrimidone 4(3//)-pyrimidone. 

Tautomeric equilibria for pyridazines with potential hydroxyl functions have been measured and the determined Kj, values are smaller compared with those of the corresponding pyridines. Furthermore, it is concluded that tautomerization to zwitterionic forms is repressed when solvents of low dielectric constants are used. On this basis, pyridazinones are expected to possess aromatic... 

Hydrolysis of 3-chloropyridazine 1-oxide with dilute sodium hydroxide solution gives 3-hydroxypyridazine 1-oxide (identical with that synthesized by another method ) and likewise 6-chloro-3-methoxypyridazine 1-oxide (126) with acetic acid and sodium acetate yields 3-methoxy-6-hydroxypyridazine 1-oxide or its tautomeric form, i.e., l-hydroxy-3-methoxy-6(ljH )-pyridazinone (127). ... 

Removal of alkoxy groups from 3-(4-, 5-, or 6-)alkoxypyridazine 1-oxides in alkaline medium produces the corresponding hydroxy-pyridazine l-oxides. - - On the basis of UV spectra and pK values for 3-hydroxypyridazine 1-oxides it is postulated that there is a larger contribution of the phenolic structure (128) than of the tautomeric pyridazinone N-oxide structure (129). ... 

Calculations have been applied to the problem of tautomerism of maleic hydrazide and other pyridazinones. The lactam structure was found to be the most stable, but in the case of 3-amino-4-hydroxypyridazine the aminol form is energetically most favorable. ... 

In a review article (91H(32)127) the concept of heterocyclic aromaticity has been presented, summarized, and applied to discussion of tautomerism for a series of hydroxy-substituted azines. Later, the tautomerism of 2(l/7)-pyridone, 2(177)-pyrimidone, 2(177)-pyrazinone, 4-pyrimidone, and 3(277)-pyridazinone in low-temperature inert gas matrices has been investigated by means of IR spectroscopy. It was found that the relative stability of the oxo and hydroxy tautomers of these compounds depends in a systematic way on the relative position of the lactam group and the second nitrogen atom in the ring (92JPC6250). 

Tautomerism of unsubstituted 3-pyridazinone in low temperature inert gas matrix has been investigated by means of IR spectroscopy (90SA(A)1087, 92JPC6250). In accordance with the theoretical calculations, only oxo tautomer 42a was observed after the deposition of the matrix. The hydroxy tautomer, however, was produced photochem-ically by UV irradiation of the matrix. Aromatic resonance energy difference for tautomers of 3-hydroxypyridazine was calculated to be 4.7 kcal/mol (92T7857). 

There has been a considerable expansion in the use of C NMR in theoretical and structural studies since the publication of CHEC-I in 1985, but in many instances C NMR data is recorded for the purposes of characterization without detailed assignment of the chemical shifts. Selected assigned data from the review period for pyridazines and pyridazinones is shown in Table 1. Compared with these aromatic systems, dihydropyridazinones show normal C shifts as shown by the methylene 25.9 and 23.8) and carbonyl 5 166.6) carbons of 6-benzyl-4,5-dihydro-3(2//)-pyridazinone in DMSO <89JHC1787>. Care should be taken in making comparisons involving potentially tautomeric systems like the pyridazinones note, for instance, the equivalence of 3,6- and... 

A detailed reinvestigation of the 1,3-dipolar cycloaddition of 2-diazopropane to 2-methyl-6-phenyl-3(2//)-pyridazinone has shown that the initial adduct (isolated in 68% yield) is the thermally labile pyrazolopyridazine (79), which undergoes subsequent tautomerization, oxidation, or rearrangement with loss of nitrogen, depending on the conditions (Scheme 36). The iso-propylpyridazine (80) is the major product formed in polar solvents (DMSO) while the diazepine... 

Polarity and lactam-lactim tautomerism in 3(2//)-pyridazinones were studied <97JMS467>. A photophysical analysis of triplet species derived from dihydropyrrolo[3,4-r/]pyridazines 1, diazene precursors of N-arenesulfonyl-3,4-dimethylenepyrroles, was conducted <97JA1416>. 1-Hydroxy-l,2-dihydro-2,3,1-benzodiazaborines 2, boron analogs of phthalazin-l(2//)-one lactims, were extensively characterized by NMR and X-ray analyses <97JA7817>. 

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