Isotope effects and the mechanism of triazolinedione ene reactions. Aziridinium imides are innocent bystanders
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Kinetic isotope effects were determined for the ene reactions of 4- phenyl-1,2,4-triazoline-2,5-dione with 2-methyl-2-butene and 2-methyl-1- pentene. In each case highly inverse (k(H)/k(D) = 0.87-0.88) vinylic deuterium isotope effects were observed. Large 13C isotope effects were observed for the less-substituted olefinic carbons for each alkene and much smaller effects were found in the more-substituted olefinic carbon. Becke3LYP calculations of the mechanistic pathway for the ene reaction of 1,2,4- triazoline-2,5-dione with isobutene, 2-methyl-2-butene, and tetramethylethylene predict a new mechanism for these reactions involving an open biradical as the key intermediate. This biradical may either form the ene product or reversibly form an intermediate aziridinium imide. The validity of these calculations is supported by a comparison of experimental and theoretically predicted isotope effects. A curved Arrhenius plot of product ratios was found for the ene reaction of 2-methyl-1-pentene in the presence of methanol. This is inconsistent with a simple two-step ene mechanism involving only an aziridinium imide intermediate, but the curvature of this plot could be modeled well by the theoretical mechanism using enthalpy and entropy parameters close to theoretically predicted values. A variety of observations with these reactions are examined and found to support the new mechanism.