3- Hydroxypyrazole, tautomerism tautomers

The problem of tautomerism is simpler in the case of 1-substituted pyrazolin-3-ones since only two forms, the OH (140a) and the NH (140b), are possible. The OH form is the more stable and is the only one present in the crystal (Section 4.04.1.3.1). In protic solvents, like water or methanol, the equilibrium position is much more evenly balanced between the OH and NH forms. Finally, 4-hydroxypyrazoles (141) exist as such. A CNDO/2 calculation justifies the result that 4-hydroxy tautomers are relatively more stable than... 

Tautomerism of 3-hydroxypyrazoles unsubstituted on nitrogen is more complex. A detailed investigation of 3-hydroxy-5-methylpyrazole disclosed that the major tautomers in aqueous solution (polar medium) are 264 and 265, whereas in cyclohexane solution (nonpolar medium) the major tautomers are 264 and 266 <1976AHC(S1)346>. 

Substituted derivatives. The 4-substituted analogues can in principle exist as two uncharged tautomeric forms 270 and 271 and as the type B mesoionic tautomer 272. All the evidence shows that these compounds exist predominantly as the NH2, OH, or SH tautomers 270. The equilibrium between the pyrazole 273 and the mesoionic tautomer 274 has been shown to favor the hydroxypyrazole 273 strongly (AG 6.15 kcal mol-1) <1972T463>. 

The lactim/lactam tautomerism of hydroxamic acids and their O-alkyl and O-acyl derivatives has also been studied [146], Hydroxamic acids exist in the solid state and in polar solvents as the lactam tautomer only, whereas in nonpolar solvents the hydroximic tautomer is also present. Further analogous solvent-dependent lactim/lactam equilibria have been observed for certain Schiff bases (prepared from anilines and 2(4)-hydroxybenzaldehyde [256] or 2-hydroxynaphthaldehyde [257]), for 3-hydroxypyrazole [258], and for 3-methyl-l-phenylpyrazolin-5-one [259]. 

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>. 

Pyrazol-3-ones are known to be ambient compounds and depending on the solvent in which they are dissolved can exist in several tautomeric forms. Usually is less polar solvents such as chloroform pyrazol-3-one tautomers are dominant, whereas in more polar solvents such as dimethylsulfoxide 3-hydroxypyrazole tautomers are dominant. The most recent review article on this topic has been presented by Katritzky and co-workers (00AHC(76)157). 

Although UV data are available for the 3-hydroxy derivatives of this ring system, no IR details have been published. Accordingly it is unclear which of the possible tautomeric structures (14-16) best represents these compounds. Possibly, as with 5-hydroxypyrazoles, the hydroxy form 14 is less stable than oxo tautomers such as 16. Ultraviolet and infrared data are, however, available for the 6-hydroxy derivatives (17). In the solid state there is surprisingly no evidence for a carbonyl group in the infrare(. The presence of a broad band from 2300 to 3100 cm (v C=NH) is taken as indicating the importance of the dipolar structure 18 at the expense of the neutral forms 17 and 19. 

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