THE ANOMALOUS HYDROLYTIC BEHAVIOR OF 1-PHENYLVINYL PHOSPHATE

The kinetics of hydrolysis of 1-phenylvinyl phosphate, 1, were studied over a pH range of 1 to 8.3 and over a pD range of 1 to 5.6 at 25-deg rees-C and mu = 0.5 M with sodium chloride. The hydrolytic behavior of 1 was found to differ, in many respects, from that of alkyl and aryl phosphomonoesters. First, the rates of hydrolysis of 1 were extremely rapid and, in the hydronium ion-catalyzed region, gave a solvent deute rium isotope effect (k(H)/k(D)) of 3.20. Also, the H-1-NMR spectrum of acetophenone formed upon complete hydrolysis of 1 in D2O (pD 1.2) rev ealed that only one deuterium atom was incorporated into the methyl gr oup. Hence, the evidence was consistent with a rate-limiting and nonre versible proton transfer from the solvent to 1. In addition, using an (H2O)-O-18 labeling study in conjunction with P-31-NMR analysis, the h ydrolytic mechanism appeared to involve nucleophilic attack by water a t both the alpha-carbon and the phosphorus atom with concurrent C-O an d P-O bond fission. Second, in the pH region where the monoanionic spe cies of 1 predominated, buffers had a pronounced catalytic effect on t he hydrolysis rate; there appeared to be a normal solvent deuterium is otope effect; and the rate constant, k'o, showed a positive deviation from the established Bronsted relationship. The dissimilarities betwee n 1 and alkyl and aryl phosphomonoesters supported the involvement of an alternate dephosphorylation pathway. One potential mechanism for th e hydrolysis of 1, consistent with the experimental findings, might be rate-limiting and nonreversible protonation of the beta-carbon of the olefinic bond, resulting in the formation of a rapidly hydrated carbo nium ion intermediate, a mechanism similar to that proposed for the mo re acidic pH region. Alternatively, a concerted mechanism involving pr oton transfer with expulsion of a monomeric metaphosphate anion might be operating.