Exocyclic Cleavage in the Alkaline Hydrolysis of Methyl Ethylene Phosphate. Evidence Against the Significance of Stereoelectronic Acceleration in Reactions of Cyclic Phosphates

The hydrolysis of methyl ethylene Phosphate serves as a mechanistic Probe of phosphate reactivity and as a model for enzyme catalysis. Early reports, based on proton NMR and GC analysis, indicated that observable amounts of exocyclic cleavage products are obtained in the alkaline hydrolysis reaction [Kluger, R.; Covitz, F.; Dennis, E. A.; Williams, L. D.; Westheimer, F. H. J. Am. Chem. Soc. 1969, 91, 6066]. However, later reports based on phosphorus NMR analysis of reactions in 5 M sodium hydroxide by advocates of a stereoelectronic theory indicated that, consistent with the theory, ring cleavage is the exclusive initial process [Taira, K.; Fanni, T.; Gorenstein, D. G. J. Am. Chem. Soc. 1984, 106, 1521. Taira, K.; Gorenstein, D. G. J. Org. Chem. 1984, 49, 4531]. In the present study, the products of the reaction of methyl ethylene phosphate with hydroxide under a variety of conditions were determined by proton NMR and by phosphorus NMR, with and without rapid quenching. Reactions of the products were also analyzed. The products of exocyclic cleavage, methanol and ethylene phosphate, are produced in the initial reaction to the extent that was reported in 1969. Subsequent reaction of the ring-cleaved products is too slow to account for the initial production of methanol. These data suggest that ring strain and apicophilicities of substituents, rather than orbital interactions, account for the substantial differences in the reactivity of phosphate esters.