The Anomalous Hydrolytic Behavior of 1-Phenylvinyl Phosphate

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Abstract
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°C and µ = 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 deuterium isotope effect (kH/kD) of 3.20. Also, the 1H-NMR spectrum of acetophenone formed upon complete hydrolysis of 1 in D2O (pD 1.2) revealed that only one deuterium atom was incorporated into the methyl group. Hence, the evidence was consistent with a rate-limiting and nonreversible proton transfer from the solvent to 1. In addition, using an H218O labeling study in conjunction with 31P-NMR analysis, the hydrolytic mechanism appeared to involve nucleophilic attack by water at both the α-carbon and the phosphorus atom with concurrent C–O and P–O bond fission. Second, in the pH region where the monoanionic species of 1 predominated, buffers had a pronounced catalytic effect on the hydrolysis rate; there appeared to be a normal solvent deuterium isotope effect; and the rate constant, k′o, showed a positive deviation from the established Brønsted relationship. The dissimilarities between 1 and alkyl and aryl phosphomonoesters supported the involvement of an alternate dephosphorylation pathway. One potential mechanism for the hydrolysis of 1, consistent with the experimental findings, might be rate-limiting and nonreversible protonation of the β-carbon of the olefmic bond, resulting in the formation of a rapidly hydrated carbonium ion intermediate, a mechanism similar to that proposed for the more acidic pH region. Alternatively, a concerted mechanism involving proton transfer with expulsion of a monomeric metaphos-phate anion might be operating.