Highly efficient intramolecular general acid catalysis of enol ether hydrolysis, with rapid proton transfer to carbon

Abstract

The hydrolysis of E and Z enol ether groups in the 8-position of 1-dimethylaminonaphthalene is catalysed by the neighbouring dimethylammonium group with remarkable efficiency. Similar compounds lacking a neighbouring general acid have been studied in strong acid, but the half-lives of 4Z and 4E below pH 3 are of the order of 10 s at 39 °C and external acid catalysis cannot be detected. The effective molarity is estimated as >60 000 M, the highest known for proton-transfer catalysis. 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 opening of the intramolecular hydrogen bond of the oxocarbocation intermediate 7. The tight intramolecular H-bonding responsible for the high efficiency of catalysis prevents significant H/D exchange with solvent D₂O.