Hydrazine-hydrazone tautomerism in phenylhydrazinopyrimidones

Article abstract of DOI:10.1002/jhet.5570200439

4-Phenylhydrazino-2-pyrimidone and its 1-methyl analog have been shown by ¹H nmr to exist in interconverting hydrazine and hydrazone tautomers in deuteriodimethylsulfoxide solution. Solvent effects indicate that increasing solvent polarity favors the hydrazine forms. In contrast, the 3-methyl analog occurs exclusively as the hydrazone. The hydrazone forms of the parent and 1-methyl derivatives appear to adopt the syn rotamers as a result of intramolecular hydrogen bonding. Variable temperature studies showed relatively high free energy barriers to tautomerization in these compounds, resulting both from solvation and intramolecular hydrogen bonding. The ¹H nmr spectra of 1-methyl-4-hydrazino-2-pyrimidone suggest that it exists predominately as the hydrazone in deuteriodimethylsulfoxide:deuteriochloroform solution, although the barrier to tautomerization is similar to those for the phenylhydrazino compounds.