Leaving group effects in thiolester hydrolysis. Part 2. On the possibility of an elimination–addition (keten) mediated pathway in S-acetylcoenzyme a basic hydrolysis and acetyl transfer

Abstract

Alkaline hydrolysis rates (k_HO–) at 25 °C in aqueous solution for a series of S-alkyl and S-aryl thiolacetates, including S-acetylcoenzyme A, were correlated (as their logarithms) with the pK_a of the conjugate acid of the thiolate leaving group to give a slope (β_l.g.) of –0.33. In comparison with the corresponding oxygen esters, thiolesters are, for the basicity of a given leaving species, one to two orders of magnitude less reactive towards hydroxide ion and show little dispersion into aryl and alkyl leaving groups, ascribed to the lower steric sensitivity of thiolacetate esters compared with the oxygen analogues. The small value of β_l.g. and the lower reactivity of S- than O-esters are offered as evidence of a bimolecular associative (B_Ac2) mechanism for basic hydrolysis. The E2 route is excluded by the lack of deuterium incorporation into the (acetate) product of hydrolysis. In spite of the accepted acidity of thiolacetates, a kinetically insignificant amount of ester conjugate base is formed in aqueous solution even at high, non-physiological pH and thus S-acetylcoenzyme A does not hydrolyse by an E1cB pathway.