Efficiency of proton transfer catalysis. Intramolecular general acid catalysis of the hydrolysis of dialkyl acetals of benzaldehyde

Abstract

Intramolecular general acid catalysis is demonstrated for the first time for the hydrolysis of dialkyl acetals of benzaldehyde. Effective molarities (EM) ranging from 810 to over 10⁴ mol dm^-3 are observed for the hydrolysis of the carboxylic acid 4 and for the three dimethylammonium systems 5·H⁺–7·H⁺, which show pH–rate profile plateaux extending as far as pH 7–9. Reactivity is shown to depend significantly on the strength of the general acid involved. Efficient catalysis depends on the development of a strong hydrogen bond in the transition state for the reaction, but efficiency is reduced when significant intramolecular hydrogen bonding is present in the ground state. The most reactive acetal, the carboxylic acid 4, is hydrolysed with a halflife of 1.15 s at 20 °C, fast enough to become complicated kinetically by the build up of the hemiacetal intermediate (Scheme 2). It does not, however, show the highest EM.