Packing modes in eight 3-ethoxycarbonylpyrazole derivatives. Influence of the substituents on the crystal structure and annular tautomerism

The crystal and molecular structures of eight N-unsubstituted ethoxycarbony lpyrazole derivatives have been determined by X-Ray analysis. The molecules are linked by bifurcated N-H ... N/O bonds giving rise to two hydrogen-bon ding motifs: catemers and tetramers that are further joined by weak interac tions. Different packing modes are observed depending on the substituent at C5. The 3-ethoxycarbonyl-4-bromo-5-phenylpyrazol presents temperature depe ndent solid-solid phase transition. Having determined that all these compou nds are 3-ethoxycarbonyl tautomers in the solid state, their annular tautom erism was studied by C-13 NMR spectroscopy.