Occurrence of an elongated p-oxo ketene intermediate in the dissociative alkaline hydrolysis of aryl (2E,4E)-5-(4 '-hydroxyphenyl)pentadienoates

The alkaline hydrolysis of title esters possessing acidic leaving groups fo llows an E1cB mechanism involving the participation of an "extra extended" p-oxo ketene intermediate. For the hydrolysis of the 2,4-dinitrophenyl este r kinetic data, activation parameters and trapping of the intermediate clea rly indicate that the dissociative pathway carries the reaction flux. Break in the Bronsted plot of the apparent second-order rate constants versus th e pK(a) of the leaving group suggests that the reaction mechanism changes f rom E1cB to B(Ac)2 for esters having pK(a) higher than about 6.