Tautomerism in 3{5}-(dimethoxyphenyl)pyrazoles

The single-crystal X-ray structures of 3{5}-(2',5'-dimethoxyphenyl)pyrazole (HI) and the hemihydrate of 3 {5}-(3/,4'-dimethoxyphenyl)pyrazole (IV) have been determined. Compound (HI) exists purely as the 5-substituted prototropomer in the crystal; the pyrazole pyrollic N-H proton is involved in a three-way hydrogen bond, involving an intramolecular contact with a methoxy oxygen donor and an intermolecular interaction to the pyridinic N atom of a neighbouring molecule, forming discrete hydrogen-bonded dimers. There is no evidence of degenerate proton transfer within the dimeric units from CPMAS ¹³C NMR spectroscopy, in contrast to other known pyrazoles that associate in this manner. In (IV).1/2H₂O, however, the pyrrolic proton is disordered over both N(1) and N(2) via hydrogen bonding to the solvate water molecule. CPMAS ¹³C NMR spectroscopy shows that the prototropic disorder in (IV).1/2H₂O is static at temperatures up to 370 K. Solution ¹H and ¹³C NMR data in DMSO-d₆ show that for both (HI) and (IV) the 3- and 5-substituted tautomeric forms are similarly populated in this solvent, suggesting both that the intramolecular hydrogen bond in (HI) has been disrupted and that the two tautomers of (HI) and (IV) are close in energy.

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