An experimental (flash vacuum pyrolysis) and theoretical study of the tautomerism of pyrazolinones at high temperatures
Flash vacuum pyrolysis experiments were carried out between 500 and 800 °C on 3(5)-phenyl- and 3(5)-methyl-pyrazolinones and on 3(5)-methoxy-5(3)-phenylpyrazole. The origin of the isolated products (mainly indanone, hydroxyalkynes and α,β-unsaturated aldehydes) can be explained as arising from the hydroxy tautomers of pyrazolinones. Temperature effects on the tautomeric equilibrium of 1-phenyl-3-methylpyrazolinone in solution show that the percentage of the CH tautomer increases with the temperature. MP2 ab initio calculations on the model compound, pyrazolinone itself, have been used to rationalize these findings. The problem of the aromaticity of the four tautomers of pyrazolinone has been examined through Schleyer’s NICS (nuclear independent chemical shifts) calculations.