HIGHLY EFFICIENT INTRAMOLECULAR GENERAL ACID CATALYSIS OF ENOL ETHER HYDROLYSIS, WITH RAPID PROTON-TRANSFER TO CARBON

The hydrolysis of E and Z enol ether groups in the 8-position of 1-dim ethylaminonaphthalene is catalysed by the neighbouring dimethylammoniu m group with remarkable efficiency. Similar compounds lacking a neighb ouring general acid have been studied in strong acid, but the half-liv es of 4Z and 4E below pH 3 are of the order of 10 s at 39 degrees C an d external acid catalysis cannot be detected. The effective molarity i s estimated as > 60 000 M, the highest known for proton-transfer catal ysis. This is ascribed to effective hydrogen-bond stabilisation of the in-flight proton. So efficient is proton transfer to carbon that the rate-determining step is probably not proton transfer to carbon but op ening of the intramolecular hydrogen bond of the oxocarbocation interm ediate 7. The tight intramolecular H-bonding responsible for the high efficiency of catalysis prevents significant H/D exchange with solvent D2O.