The E1cb mechanism for ester hydrolysis. Evidence for a dianion intermediate in the hydrolysis of aryl malonate anions

Abstract

The hydrolysis of monoesters of malonic acid, but not of dialkylmalonic acids, with good (nitrophenolate) leaving groups, is catalysed by low concentrations of general acids and bases several hundred times more efficiently than expected. This catalysis disappears at high concentrations of catalyst. The same buffers catalyse the exchange of deuterium from solvent D₂O into the methylene group of monoethyl malonate. The results are explained in terms of the E1cb mechanism for ester hydrolysis. Oxyanion bases react only with the undissociated aryl hydrogen malonate, but basic tertiary amines remove a proton from the ester anion, generating an intermediate with a carbanionic centre next to, and significantly stabilised by, a carboxylate group.