Reactions of carbonyl compounds in basic solutions. Part 22.1 The mechanisms of the alkaline hydrolysis of methyl cis-3-(substituted benzoyl)-2,3-diphenylacrylates and the 5-methoxy-5-(substituted phenyl)-3,4-diphenylfuran-2(5H)-ones, as well as the ionisation of the corresponding acids

Abstract

Rate coefficients have been measured for the alkaline hydrolysis of a series of chain (normal) esters, the methyl cis-3-(substituted benzoyl)-2,3-diphenylacrylates, and the corresponding ring (pseudo) esters, the 5-methoxy-5-(substituted phenyl)-3,4-diphenylfuran-2(5H)-ones, in 70% (v/v) dioxane–water at 30 °C. Those for the two parent esters were also measured at 45.0 and 60.0 °C and the enthalpies and entropies of activation have been evaluated. A consideration of relative rates, activation parameters and substituent effects indicates hydrolysis of the chain esters with neighbouring group participation by the keto-carbonyl group. The ring esters demonstrate the high reactivity of the five-membered lactone-like system. The steric effects of the phenyl groups does not switch mechanisms, but only modifies reactivity. The equilibrium constants for ring-chain tautomerism in a series of cis-3-(substituted benzoyl)-2,3-diphenylacrylic acids have been determined by using the observed pK_a values in 80% (m/m) 2-methoxyethanol–water together with estimated true pK_a (pK^T_a) values. The equilibrium constants for these acids have been correlated using the Hammett equation to give ρ as ca. 1.3. The results are discussed in terms of steric ‘bulk’ and resonance effects.