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 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.

Reactions of carbonyl compounds in basic solutions. Part 23.1 The mechanism of the base-catalysed ring fission of 2,2-dihydroxyindane-1,3-diones

“…Similar behaviour was so found for the base-catalysed ring fission of benzocyclobutenediones. 21 A possible cause for this behaviour has been discussed previously. 31 1,2-Dicarbonyl substitution in a benzene ring may result in powerful cross-conjugation effects with substituents so modifying the expected substituent effects.…”

“…However, the dramatic increase in rate with increasing dioxane content would be that expected from the formation of dianionic transition state from two monoanions as an initial state with the greatly decreased relative permittivity of the medium as dioxane content increases. 25 This is quite unlike and opposite to the changes observed for the reaction of dipolar molecules such as methyl benzoate 26 and benzocyclobutenedione 21 with hydroxide anions in aqueous dioxane. The significant increases in rate observed for increased ionic strength of the medium clearly indicate a positive primary salt effect.…”

“…Thus, no significant, detectable primary isotope effect occurs. Values of k 2 O have been found for the basecatalysed ring fission of benzocyclobutenedione in water at 25 ЊC, 21 for the benzilic acid rearrangement in 2 : 1 dioxanewater at 50 ЊC 22 and for alkaline hydrolysis of methyl benzoate in 70% aqueous dioxane at 25 ЊC 23 to be 0.58, 0.54 and 0.87, respectively. Such results have been ascribed to the greater basicity and nucleophilicity of deuteroxide anions in D 2 O, than of hydroxide anions in H 2 O.…”

A computational study of base-catalyzed reactions of cyclic 1,2-diones: cyclobutane-1,2-dione

“…This change in the position of the proton is accompanied by a shortening and lengthening, respectively, of the C1–O3 and C2–O2 distances: in TS5 r (C1–O3) = 1.383 Å and r (C2–O2) = 1.233 Å, while in P3a r (C1–O3) = 1.210 Å and r (C2–O2) = 1.367 Å. Finally, it should be noted that intermediates of the type Int4 have been proposed [9] to be involved in the base-catalyzed reactions of benzocyclobutenediones (reaction 5 → 6 in Scheme 2). …”

“…In contrast, cyclobut-3-ene-1,2-diones 3 react to 2-oxobut-3-enoates 4 (at least formally according to path B) [8], whereas benzocyclobutene-1,2-diones 5 lead to 2-formylbenzoates 6 (path C, Scheme 2) [9]. …”