Reactions of carbonyl compounds in basic solutions. Part 24.1 The mechanism of the base-catalysed ring fission of substituted benzocyclobutene-1,2-diones

Abstract

The rate coefficients for the base-catalysed ring fission of a series of substituted benzocyclobutenediones to give the corresponding 2-formylbenzoic acids have been determined in water at 25.0 and 60.0 °C. The effects of 4-substituents and 4,5-di-substituents on the rates have been correlated using a modified Hammett equation to give a reaction constant, ρ, equal to ca. 3.6 at 25.0 °C. The activation parameters have been calculated. The effect of solvent composition on the rates has been studied. The kinetic solvent isotope effect, product composition and enrichment in ¹⁸O-enriched water have also been studied. All the evidence indicates a mechanistic pathway which proceeds by rapid reversible addition of hydroxide anion to the dione, followed by intramolecular nucleophilic attack on the second carbonyl group and formation of a carbanionic intermediate.