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 ЊC. 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, , equal to ca. 3.6 at 25.0 ЊC. 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 18 O-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.

The rate coefficients for the base-catalysed ring fission of a series of substituted 3,4-diphenylcyclobut-3ene-1,2-diones to give the corresponding (Z)-2-oxo-3,4-diphenylbut-3-enoic acids have been determined in 50% (v/v) aqueous dimethyl sulfoxide (DMSO) at 25.0 and 45.0 ؇C, as well as for a limited series in 70% (v/v) aqueous DMSO. The activation parameters have been calculated. The effect of both mono-and di-substitution of the phenyl groups has been studied and gives a Hammett value of ca. 1.3 in 50% aqueous DMSO at 25.0 ؇C. The kinetic solvent isotope and solvent effect and enrichment in 18 O-enriched water have been studied. The product composition of the reaction of the monosubstituted compounds gave a value of ca. 0.9. All the evidence indicates a mechanistic pathway which proceeds by a rapid, reversible addition of hydroxide anion to the dione, followed by a benzilic acid-type rearrangement to form a 1-hydroxycyclopropene-1-carboxylic acid. The latter then suffers ring fission to give the final product.

Reactions of Carbonyl Compounds in BasicSolutions. Part 25. The Mechanism of the Base-Catalyzed Ring Fission of 3,4-Diphenylcyclobut-3-ene-1,2-diones. --(AL-NAJJAR, A.; BOWDEN, K.; HORRI, M. V.; J. Chem. Soc., Perkin Trans. 2 (1997) 5, 993-996; Dep. Biol. Chem. Sci., Univ. Essex, Colchester, Essex CO4 3SQ, UK; EN) 1

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