Participation of an Extended p-Oxo Ketene Intermediate in the Dissociative Alkaline Hydrolysis of Aryl 4-Hydroxycinnamates
作者:Giorgio Cevasco、Sergio Thea
DOI:10.1021/jo00100a031
日期:1994.10
The alkaline hydrolysis of 4-hydroxycinnamate esters of acidic phenols follows an E1cB mechanism and involves the participation of an ''extended'' p-oxo ketene intermediate. The apparent bimolecular rate constant (k(a)K(a)/K-W) for the hydrolysis of the 2,4-dinitrophenyl ester is some 2500-fold larger than that determined from the Hammett relationship for the B(Ac)2 alkaline hydrolysis of substituted 2,4-dinitrophenyl cinnamates. The positive value of the entropy of activation for the hydrolysis of 2,4-dinitrophenyl 4'-hydroxycinnamate and trapping experiments with nitrogen nucleophiles are consistent with the dissociative pathway. A change from a E1cB to a B(Ac)2 mechanism is expected for esters with leaving groups having pK(a) higher than ca. 6.7. The higher E1cB reactivity of 2,4-dinitrophenyl 4'-hydroxycinnamate compared to that of the corresponding 4'-hydroxybenzoate is due to the vinylene group that further favors the dissociative route, probably increasing the stability of the unsaturated intermediate.