Reactions of Carbonyl Compounds in Basic Solutions. Part 34. The Mechanism of the Base-Catalysed Ring Fission of 2,3-Diphenylcycloprop-2-en-1-one

The rate coefficients for the base-catalysed ring fission of a series of 2-phenyl-3-(2-, 3- or 4-substituted phenyl)cycloprop-2-en-1-ones to give the corresponding (E)-2,3-diphenylacrylic acids have been determined in water at 30.0 °C, as well as for the unsubstituted compound at 40.0, 50.0 and 60.0 °C. The effects of meta- and para-substituents on the rates have been correlated using the Hammett equation to give a reaction constant, ρ, equal to ca 1.2 at 30 °C. For the unsubstituted compound, the activation parameters have been calculated and the kinetic solvent isotope effect has been studied. The effects of ortho-substituents on the rates appear to be mainly polar, rather than steric, in origin. The evidence indicates a mechanistic pathway which proceeds by addition of hydroxide anion to the ketone, which is rate-determining. The adduct suffers ring fission to give the final product via a carbanionic intermediate.