3.5- Dimethylpyrazole, tautomerism

The first observation of the proton transfer in pyrazoles in the solid state was made for the intermolecular tautomerism in 3,5-dimethylpyrazole 10b (85JA5290). The degenerate rearrangement was recorded using the... 

Tautomeric rearrangements of transition-metal complexes with azole ligands are relatively scarce. The fluxional behavior of the rhodium complex 43 with a neutral 3,5-dimethylpyrazole was explained as the result of rapid processes of metallotropy and prototropy occurring simultaneously (Scheme 24) [74JOM(C)51],... 

At low temperature there are two resolved peaks for the methyl substituents, but the observed splitting is reduced on increasing temperature. X-ray diffraction results show that the unit cell consists of a cyclic trimeric arrangement to 3,5-di-methylpyrazole molecules. Within this trimer, the 3,5-dimethylpyrazole molecules have C2v symmetry, the cyclic trimer has threefold symmetry, and the hydrogen involved in the tautomeric process is refined with half occupancy. These results indicate that, at room temperature, a trimer-trimer intermolecular tau-tomerism takes place in 3,5-dimethylpyrazole. In contrast, for pyrazole, the N-H... 

Two other publications examine the effect of alumina and silicagel on the dynamic behavior of pyrazole (7) and 3,5-dimethylpyrazole (19) <94JPC8752> and collect all N chemical shifts of pyrazoles in the solid state and in solution (frozen tautomerism) <94MRC699>. 

We reported in 1994 that prototropy is suppressed in crystalline pyrazole (5) and moderately fast in crystalline 3,5-dimethylpyrazole (6), whereas a fast degenerate proton tautomerism is observed for adsorbed 5 and 6, similar to the liquid solution (94JPC8752).The adsorbates were prepared by grinding pyrazoles with alumina and silica. The method of structure determination was variable-temperature CPMAS NMR. 

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