Kinetic deuterium isotope effects on the reactions of 2-(4- methoxyphenyl)oxirane in water solutions
Victoria C. Ukachukwu, Ram S. Mohan, and Dale L. Whalen*
Department of Chemistry and Biochemistry, University of Maryland, Baltimore County (UMBC), Baltimore, MD 21250 USA
Email:
[email protected]
This manuscript is dedicated to Professor James M. Coxon on his 65th birthday
Abstract
The rates of reaction of 2-(4-methoxyphenyl)oxirane (4-methoxystyrene oxide), trans-3- deutereo-2-(4-methoxyphenyl)oxirane and 3,3-dideutereo-2-(4-methoxyphenyl)oxirane in water solutions were measured as functions of pH. Kinetic deuterium isotope effects for the reactions of the mono- and di-deuterated (4-methoxyphenyl)oxiranes were determined for both the acid- catalyzed hydrolysis to diols and the pH-independent reactions leading mostly to rearranged aldehyde and involving a 1,2-hydrogen migration. The inverse kinetic deuterium isotopes for acid-catalyzed hydrolyses of the deuterated (4-methoxyphenyl)oxiranes to diols are consistent with rate-limiting epoxide ring opening. The magnitudes of the normal kinetic deuterium isotope effects on the pH-independent reactions of deuterated 4-methoxyphenyloxiranes are significantly smaller than the deuterium isotope effect on the aldehyde-forming step, and are rationalized by a reversible epoxide ring opening step that is partially rate-limiting. The magnitude of the partitioning isotope effect on the hydrogen migration step is consistent with isotope effects determined by Professor Coxon’s laboratory on the Lewis acid-catalyzed rearrangements of deuterated phenyloxiranes in organic solvents.
Keywords: Phenyloxiranes, oxirane-carbonyl rearrangement, kinetic isotope effects
Introduction
In the presence of Lewis acids, oxiranes (epoxides) often undergo rearrangement involving a 1,2- hydrogen migration to form aldehydes or ketones. In early work, for example, it was shown that BF3 catalyzes the rearrangement of steroidal endocyclic epoxides to ketones1,2 and steroidal exocyclic epoxides to aldehydes.3 There are many other examples of BF3-catalyzed oxirane- carbonyl reactions published in the literature. Solvents used for this reaction include benzene,
