Reactions of carbonyl compounds in basic solutions. Part 34. The mechanismof 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 a t 30.0 degrees C, as well as for the unsubstituted compound at 40.0, 50.0 a nd 60.0 degrees C. The effects of meta- and para-substituents on the rates have been correlated using the Hammett equation to give a reaction constant , rho, equal to ca 1.2 at 30 degrees 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 ap pear to be mainly polar, rather than steric, in origin. The evidence indica tes a mechanistic pathway which proceeds by addition of hydroxide anion to the ketone, which is rare-determining. The adduct suffers ring fission to g ive the final product via a carbanionic intermediate.