Abstract
Kinetic ..cap alpha.. secondary deuterium isotope effects, k/sub D//k/sub H/, for formyl group transfer from either the 4-methoxyphenyl or 4-nitrophenyl esters, or both, of formic and deuterioformic acids to a variety of oxygen acceptors have been measured at 25/sup 0/C in aqueous solution: hydroperoxide ion, 1.12; 2-propynol anion, 1.13; hexafluoropropan-2-ol anion, 1.14; and water, 1.22. In addition, corresponding isotope effects have been obtained for general-base-catalyzed formyl group transfer from the same substrates to acetate, 1.21, formate, 1.23, and trimethylamine N-oxide, 1.20. The ..cap alpha.. secondary deuterium isotope effect for acid-catalyzed hydrolysis of both esters has been determined to be 1.24. These data are interpreted to reflect considerable, and perhaps complete, carbon--oxygen bond formation in the transition state for addition of oxygen nucleophiles to phenyl formates. Finally, corresponding isotope effects were determined for reaction of fluoride, 1.19, and azide, 1.14, which also suggest substantial covalent bond formation between ester and nucleophile in the transition state. 4 tables.
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