Ring–chain tautomerism. Part 6. Base- and acid-catalysed rearrangement of pseudo to normal methyl 8-(3- or 4-substituted benzoyl)-1-naphthoates and 2-(3- or 4-substituted benzoyl)benzoates

Abstract

Rate coefficients have been measured for both the base- and acid-catalysed rearrangements of a series of methyl pseudo- 8-benzoyl-l-naphthoates and pseudo- 2-benzoylbenzoates in methanol at 60.0 °C to form the corresponding normal esters. The entropies and enthalpies of activation for the parent pseudo-esters have been evaluated, as have the solvent isotope and solvent (methanolic dioxan and dimethyl sulphoxide) effects for these esters. The reaction constants ρ for the methoxide anion-catalysed rearrangement are ca. 2.1 (naphthoates) and 1.1 (benzoates), respectively. The evidence is consistent with a stepwise mechanism in which the formation or decomposition of tetrahedral intermediates is rate-determining: for the pseudo-benzoates, the formation by addition of methoxide to the pseudo-ester carbonyl group; for the pseudo-naphthoates. the decomposition by dissociation of methoxide from the normal ester carbonyl group. This mechanistic switch appears to arise from greater strain in the five-membered ring affecting its initial state and primary addition product stabilities. The reaction constants ρ for the acid-catalysed rearrangement are ca.–0.1 (naphthoates) and –0.2 (benzoates), respectively. All the evidence for the latter reaction indicates an A_AC2 path and likely transition states are discussed.