Direct demonstration of tautomeric nature of 4‐bromo‐3(5)‐methylpyrazoles.

Tautomerization is the key feature of pyrazoles, which is connected with the hydrogen transfer in pyrazoles between two nitrogens. It is scholarly accepted that tautomers of 3(5)‐substituted pyrazole derivatives cannot be isolated as individual compounds with the exception of certain extreme conditions. Here using the fact that N‐substitution suppresses tautomerization and allows their separation and isolation, the individual isomers of N‐propanoates of 3(5)‐pyrazole were synthesized, isolated, and further modified several times without change of initial 3‐or 5‐methyl configuration. However, once the group attached to the nitrogen was removed, two individual isomers gained back their tautomeric properties and became the same mixture of tautomers, as expected. Close monitoring by NMR spectroscopy vividly demonstrates the loss and then reverting of tautomeric properties of 3(5)‐pyrazole derivatives along the set of chemical transformations aimed at possible isolation of two tautomers. Failure to isolate them once again proves that the dynamic equilibrium of tautomers in solution is an inherent property of 3(5)‐methylpyrazole. [ABSTRACT FROM AUTHOR]