Pyrazole —continued tautomerism

Work under this classification (76AHCS1, p. 31) continues to be sparse. Heat-of-solution data provide a useful method for estimating A// for tautomeric processes in nonaqueous solvents, as was illustrated in the case of 2-pyridone 15a/2-hydroxypyridine 15b equilibrium (76TL2685). Heats of dehydration of 4-hydroxypyrazolines into pyrazoles and 5-hydroxyisoxazolines... 

Meanwhile, Nikolay had started his new research on the preparation, reactions, and synthetic applications of p-chloro vinyl ketones and related compounds (p-aminovinyl ketones, p-kctoacetals). Efficient synthetic methods were developed and improved, including C-ketovinylation (introduction of the RCOCH = CH group), and a number of useful heterocyclic compounds (pyrazoles, triazoles, pyridines, and so on) were synthesized. The discovery of enamine-imine tautomerism in p-aminovinyl ketones was another remarkable achievement by Nikolay at that time. For these studies he received in 1953 the degree of Doctor of Science in chemistry. He continued as a lecturer and supervisor of postgraduates at the University, becoming docent (associate professor) in 1951 and full professor in 1955. 

After a paper devoted to GIAO/B3LYP calculations of 13 monosubstituted benzenes and 21 1-substituted pyrazoles [148] that allowed discussion of some structural problems (such as conformation, tautomerism, and structure of salts) we have continued to use this combination of experimental (in solution and in the solid state, CPMAS NMR) and calculated values as a very useful exploratory technique. For instance, we have used II, 13C, and 15NNMR spectroscopy to study compounds 149-154. In para-disubstituted derivatives 149,151,152,153, in the solid state (no free rotation) the signals of the ortho carbons are split (one is close to N2) and, thanks to the calculated values, they can be assigned [149],... 

The excellent review The use of NMR spectroscopy to study tautomerism [3] by Jose Elguero s group was mentioned already in the introduction to this chapter. Jose s working team deals inter alia especially with the annular tautomerism of pyrazoles and pyrazolones. This review gives the perfect overview. This research was (and is) continued, for example, the pyrazole tautomerism of phosphonylpyrazoles was studied by low-temperature H and NMR analysis [89]. Tautomerism of the 3-hydroxyindazoles in solution and in the solid state was studied by the Elguero group [90] as well. 

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