DISSECTION OF NUCLEOPHILIC AND GENERAL BASE ROLES FOR THE REACTION OFPHOSPHATE WITH P-NITROPHENYL THIOLACETATE, P-NITROPHENYL THIOLFORMATE, AND PHENYL THIOLACETATE

Phosphate buffers are well-known to catalyze the decomposition of vari ous 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-nitropheny l 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 monoan ion 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 diani on 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 hydrolys is mixtures in H2O and D2O containing phosphate shows the transient fo rmation, and subsequent hydrolysis, of acetyl phosphate. Analysis of t he kinetics of these processes indicates that in H2O at pH = 8.5, phos phate dianion functions as both a nucleophile and general base toward 3b, the nucleophilic role comprising 80-93% of the reaction. In D2O, t he process is entirely nucleophilic. For the reaction of phosphate dia nion with 4b, the H-1 NMR analysis indicates that the nucleophilic rol e comprises 40-50% of the reaction, the general base role being 50-60% . The reaction of phosphate dianion with 5 is entirely nucleophilic, w hile 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 th e nucleophile and leaving groups.