Associative and Dissociative Pathways in the Alkaline Hydrolysis of Aryl 2-Hydroxycinnamates

Article abstract of DOI:10.1021/jo00106a016

Aryl 2-hydroxycinnamate esters hydrolyze in alkaline solutions (20percent dioxane-water v/v) obeying the rate law k_obs = k_a + k_b/(1 + a_H/K_a), where K_a is the ionization constant of the hydroxy group of the ester and k_b is the second-order rate constant for the attack of hydroxide ion on the ionized ester.Kinetic data and activation parameters for the hydrolysis of the 2,4-dinitrophenyl ester show that the mechanism giving rise to the k_a term cannot be a simple B_Ac2 type process and suggest the occurrence of a E1cB mechanism involving an "extended" o-oxoketene intermediate.The Broensted plot of the apparent second-order rate constante (k_aK_a/k_w) versus the pK of the leaving group indicates that the reaction mechanism changes from E1cB to B_Ac2 for esters with leaving groups having pK higher than about 6.