Dissection of nucleophilic and general base roles for the reaction of phosphate with p-nitrophenyl thiolacetate, p-nitrophenyl thiolformate, and phenyl thiolacetate

Phosphate buffers are well-known to catalyze the decomposition of various active acyl compounds. This study was undertaken to determine the extent to which it acts as a nucleophile and general base toward some activated esters and thiolesters. Thus, the hydrolyses of p-nitrophenyl acetate (3a), p-nitrophenyl thiolacetate (3b), phenyl acetate (4a), phenyl thiolacetate (4b), and p-nitrophenyl thiolformate (5) have been studied in aqueous phosphate, mu = 1.0 (K2SO4). Both phosphate monoanion and dianion are reactive toward the thiolesters 3b, 4b, and 5. For 3b, reaction of the dianion exhibits a solvent kinetic isotope effect (SKIE) of 1.00 +/- 0.11 while that for the monoanion is 2.13 +/- 1.1. For the reaction of phosphate dianion with 5, the SKIE is 0.8 +/- 0.2 and that for the monoanion at pH 3.05 is roughly 1.5. Phosphate dianion reacts with each thiolacetate and its oxygen analogue at comparable rates: the reactivity ratio of the formyl to acetyl thiolesters, 5:3b, toward phosphate dianion is 685. H-1 NMR analysis of the 3b hydrolysis mixtures in H2O and D2O containing phosphate shows the transient formation, and subsequent hydrolysis, of acetyl phosphate. Analysis of the kinetics of these processes indicates that in H2O at pH = 8.5, phosphate dianion functions as both a nucleophile and general base toward 3b, the nucleophilic role comprising 80-93% of the reaction. In D2O, the process is entirely nucleophilic. For the reaction of phosphate dianion with 4b, the H-1 NMR analysis indicates that the nucleophilic role comprises 40-50% of the reaction, the general base role being 50-60%. The reaction of phosphate dianion with 5 is entirely nucleophilic, while the monoanion reacts as a general base. The data are interpreted in terms of standard carbonyl addition/elimination mechanisms in which the ability of the attacking phosphate di- or monoanion to displace a given leaving group is tied to the pK(a) of the conjugate acids of the nucleophile and leaving groups.