Heterodienesyntheses—XXV: Kinetics of the reaction of ethoxyethenes with 1-Phenyl-4-benzyliden-5-pyrazolone. Activation energies vs. energy gained in the cycloaddition

The kinetics of the reaction between 1-phenyl-4-benzyliden-5-pyrazolone and ethoxyethenes have been studied by quantitative spectroscopic analysis in xylene at various temperatures. These 1,4-cycloadditions are HOMO-ethoxyethene controlled but Sustmann's approximation of the general perturbation equation cannot be used to rationalize the results since meaningless relationships are obtained by plotting the log of the rate constants vs the inverse of ethoxyethene HOMO energies. If the energy gained in the cycloaddition, calculated by MO energies and coefficients, is plotted vs the log of the rate constants a good relationship is obtained which results better if the entropy factor is excluded.