Pyrazolinones tautomerism

The most important contribution to pyrazolinone tautomerism is the inclusion of solvent elfects on the theoretical calculations <90JCS(P2)195, 93JA2352). Theoretical calculations of the relative stabilities of the four tautomers of pyrazolinone (3-hydroxypyrazole A (OH form), 3-pyrazolin-5-one B (NH form), 5-hydroxypyrazole C (OH-form), and 2-pyrazolin-5-one D (CH form)) have been considered a challenge to theory <93JA2352> theory predicts the order D < A < C < B in the gas phase, which is different from the experimental results in solution <84CHEC-I(5)167>. 

The spectra of pyrazolinones have been used in tautomeric studies (Section 4.04.1.5.2). The chemical shifts shown in Table 11 are representative of the three classes of fixed derivatives [OR (63) and (64), NR (65) and CR (66)], and many others are discussed in (76AHC(Sl)l). Some tautomeric compounds are also included in Table 11. 

Pyrazolinones and other five-membered compounds (functional tautomerism). These studies have often used the most simple but rather experimentally neglected pyrazolin-5-one 19. Its four tautomers are called CH 19a, NH 19b, 5-OH 19c, and 3-OH 19c (Scheme 10) [76AHCS1, p. 313]. 

Pyrazolin-5-one, 3-methyl-1 -phenyl-in colour photography, 5, 299 Pyrazolinones 13C NMR, 5, 193 H NMR, 5, 185, 187 colour couplers in colour photography in non-silver photography, 1, 384 IR spectra, 5, 200-202 structure, 5, 170 Pyrazolinones, dichloro-reactions with alkali, 5, 78 Pyrazolin-3-ones 1-substituted tautomerism, 5, 214 Pyrazolin-5-ones reactions... 

FVP experiments were carried out between 500 and 800 °G on 3(5)-phenyl- and 3(5)-methylpyrazolinones and on 3(5)-methoxy-5(3)-phenylpyrazole < 1999J(P2)211 >. The origin of the isolated products (mainly indanone, hydroxy-alkynes, and a,/ -unsaturated aldehydes) were explained as arising from the hydroxy tautomers of pyrazolinones. Temperature effects on the tautomeric equilibrium of 1-phenyl-3-methylpyrazolinone in solution show that the percentage of the GFI tautomer increased with increasing temperature. 

Pyrazolinones and pyrazolidinones are oxo derivatives of pyrazo-lines and pyrazolidines, respectively, and are so named in Chemical Abstracts at present. However, the usual method of naming in the earlier literature is the pyrazolone-pyrazolidone system. Although a large number of tautomeric structures are possible for pyrazolinones, the usual assignment of ring structures is as shown in (I), (II) and (III). The Chemical Abstracts names for these are (I) 2-pyrazolin-5-one, (II)... 

The effect of substitution on the structure of pyrazolinones is a very important one, because certain of the tautomeric forms then become impossible. For example, substitution at N-l allows only structures (VII), (VIII) and (X). Substitution at N-2 would make... 

VIII) and (XIII) the only possible tautomers. A combination of N-l and N-2 substitution leaves (VIII) as the only possibility. If there is substitution at N-l and disubstitution at C-4, then only isomer (VII) is possible. There has been little investigation of the possibilities for tautomeric structures of N-2 substituted pyrazolinones. However, Kitamura788 has suggested that such compounds exist as a mixture of forms (VIII) and (XIII). 

All functional groups having nitrogen attached to the pyrazolinone nucleus are considered in this section except nitroso, nitro and some oximes. Only those oximes which cannot be considered as tautomeric with the nitroso group are discussed here. Hydrazones are in this section. These compounds are listed in Tables XXII and XXIII. 

All of these derivatives of 2-pyrazolin-5-ones have a nitrogen atom substituted at the 4-position as the 3-nitrogen-substituted-2-pyrazolin-5-ones are tautomeric with the 5-imino-3-pyrazolidinones and are considered as such in this discussion. The 4-nitrogen-substituted-2-pyrazolin-5-ones are not usually prepared by cyclization directly to the desired pyrazolinone ring but rather by modification of already formed pyrazolinones. The methods used are usually those employed in... 

The stability of the pyrazolinone tautomers was related to their scale of aromaticity.Experimental values for the tautomerism of pyrazolinone derivatives were obtained by flash vacuum pyrolysis (FVP), which allowed the calculation of the effect of the temperature on the equilibrium between tautomers. The variation of Kj with the temperature was determined by H NMR in solution for the model compound l-phenyl-3-methyl-2-pyrazolin-5-one (Scheme 25), showing that at 0 K the only tautomers present are either the NH or the OH ones. Temperature and solvent effects on the prototropic equilibrium of the model compound in solution showed that... 

When I started my PhD Thesis, some 60 years ago, I was involved with pyrazoles tautomerism (annular of pyrazoles and indazoles and functional of pyrazolinones, then called pyrazolones). The knowledge about tautomerism was empirical, qualitative, and unsystematic. Owing to the efforts of Alan Roy Katritzky and those of my thesis supervisor, Robert Jacquier, the situation improved considerably resulting in the publication in 1976 ofthe book The Tautomerism of Heterocycles [1]. ... 

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