Journal of Organic Chemistry p. 6274 - 6278 (1994)
Update date:2022-08-11
Topics:
Cevasco, Giorgio
Thea, Sergio
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 (kaKa/Kw) for the hydrolysis of the 2,4-dinitrophenyl ester is some 2500-fold larger than that determined from the Hammett relationship for the BAc2 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 BAc2 mechanism is expected for esters with leaving groups having pKa 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.
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