Heterodiene syntheses—XIX: Correlation of the kinetic data with lumo energies in the reaction between 1-aryl-4-benzylidene-5-pyrazolones and isopropyl vinyl ether

The kinetics of the reaction between 1-p-substituted phenyl-4-benzylidene-5-pyrazolones and isopropyl vinyl ether have been studied by quantitative spectroscopic analysis and liquid chromatography. The rate increases with the electron attracting character of the substituents and a correlation is obtained with σ- constants. The polarographic one-electron half-wave reduction potentials E^red_{$\text{1}\text{2}$}, which represent an experimental measure for the relative energy of the lowest unoccupied molecular orbital (LUMO), were measured. Good correlations were obtained between these and the kinetic data, thus demonstrating the frontier-controlled character of the cycloaddition and the dominant interaction between the LUMO of the pyrazolone and the highest occupied molecular orbital (HOMO) of the vinyl ether.