Structure of chiral pyrazoles in the solid state and in solution

The X-ray molecular structures of three chiral pyrazoles bearing (4S)-4-benzyloxazolidin-2-ones or (2R)-bornane-10,2-sultams at position 3(5) have been studied by single crystal X-ray diffraction and by NMR. In the solid state, the same pyrazole tautomer a has been found for the three compounds, that with the chiral group in the 3-position. Their crystal structures consist of infinite chains (catemers) formed by N-H···O=X (X: C, S) hydrogen bonds. The use of solid-state 13C NMR spectroscopy demonstrated that the pyrazole is also an a tautomer, 13C NMR spectra in methanol solution yield average signals even at -80°C; nevertheless, interpolation allows one to estimate that in solution tautomer a also predominates.