Reactions of carbonyl compounds in basic solutions. Part 25.1 The mechanism of the base-catalysed ring fission of 3,4-diphenylcyclobut-3-ene-1,2-diones

Abstract

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

“…Such a reaction pathway occurs for the related 3,4diphenylcyclobutene-1,2-diones 14 that do not have a fused ring system. 4 The latter is a benzilic acid-type rearrangement. 5 The efficacy of a leaving group has been considered to be a function of the stability of anion formed.…”

“…2,3 This reaction is in stark contrast to that same reaction of simple substituted cyclobutenediones which results in the formation of the anions of substituted 2-oxobut-3-enoic acids. 2, 4 The former reaction involves fission between the carbonyl carbons, while the latter reaction involves fission between the carbonyl carbon and the olefinic carbon. Both these reactions are also in contrast to the benzilic acid rearrangement, the base-catalysed rearrangement of benzils and other α-diketones, in which there is a 1,2-aryl shift.…”

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

“…Comparisons with other systems must be done with care as these may have been studied in different solvent systems and at different temperatures. The same reaction for a series of substituted phenylglyoxals in water at pH 12 and 25 ЊC gave a ρ value of 2.0 36. The base-catalysed ring fission of benzocyclobutenediones in water at 25 ЊC 21 and the cyanide-catalysed cleavage of benzils in alcoholic solutions at 30 ЊC 12 gave ρ values equal to ca.…”

“…The rate-determining step for the ring fission process, with a transition state close to the intermediate 7, appears to be k 1 in Scheme 1. ref 36,. The relative ease of the two types of pathways available for such base-catalysed fission reactions, i.e.…”

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

“…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]. …”