Theoretical Prediction of a Perepoxide Intermediate for the Reaction of Singlet Oxygen with trans-Cyclooctene Contrasts with the Two-Step No-Intermediate Ene Reaction for Acyclic Alkenes

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B3LYP/6-31G* and CASMP2 calculations have been employed to study the ene reaction of singlet oxygen with trans-cyclooctene. These methods predict that the reaction involves a perepoxide intermediate, whereas alkenes such as tetramethylethylene are predicted by the same methods to occur by a two-step no-intermediate mechanism, with no perepoxide intermediate. The change in mechanism arises because the trans-cyclooctene imposes a substantial strain in the transition state for hydrogen abstraction. The perepoxide is formed through a polarized diradical intermediate that can lead to the observation of alkene isomerization. The polarized diradical also becomes a minimum because of the barrier to abstraction.