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| - Isotope Effects and the Mechanism of Allylic Hydroxylation ofAlkenes with Selenium Dioxide
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| - The mechanism of the allylic oxidation of 2-methyl-2-butene with selenium dioxide was exploredby a combination of experimental and theoretical studies. A comparison of the experimental 13Cand 2H kinetic isotope effects with predicted values shows that the observed isotope effects areconsistent with an initial concerted ene step mediated by SeO2. However, this comparison alsodoes not rule out the involvement of a selenous ester in the ene reaction or a stepwise reactioninvolving reversible electrophilic addition of HSeO2+ followed by rate-limiting proton abstraction.Becke3LYP calculations strongly favor SeO2 over a selenous ester as the active oxidant, with thepredicted barrier for reaction of 2-methyl-2-butene with SeO2 being 21−24 kcal/mol lower thanthat for reaction with H2SeO3. The possibility of a selenous ester being the active oxidant is alsodisfavored by the observation of oxidations in non-hydroxylic solvents. The involvement of HSeO2+does not appear consistent with a lack of dependence of the reaction on the basicity of the reactionmixture. A concerted ene reaction with SeO2 as the active oxidant appears to be the majormechanistic pathway operative in these reactions.
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