Participation of an Extended p-Oxo Ketene Intermediate in the Dissociative Alkaline Hydrolysis of Aryl 4-Hydroxycinnamates

Article abstract of DOI:10.1021/jo00100a031

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_aK_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_Ac2 alkaline hydrolysis of substituted 2,4-dinitrophenyl cinnamates.The positive value of 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_Ac2 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.