Catalyse acide intramoléculaire dans l'hydrolyse des acétals I. Position du proton dans l'état de transition

The hydrolysis of a series of carboxy and methoxycarbonyl acetals, (Ib ‐ IXb and Ia ‐ IXa) derived from substituted salicylic acids and benzaldehydes has been studied. A strong intramolecular acid catalysis has been observed in the hydrolysis of substituted carboxy acetals. The values of the Hammett ρ coefficients measuring the influence of substituents located in the aldehyde and phenol moieties are respectively −2.95 and + 0.93. Comparison with the ρ values obtained with methoxycarbonyl acetals indicates that the C–O bond is largely broken in the transition state. The Bronsted α coefficient for carboxy‐group catalysis is quite small, ∼0.2 ± 0.1, suggesting a low degree of proton transfer in the transition state.