Stereochemical dependence of isotope effects in the singlet oxygen--olefin reaction

Cis and trans tetramethylethylene-d6 react smoothly with singlet oxygen generated by sun lamp irradiation of Rose Bengal in acetone-d6 at -100C under an atmosphere of oxygen. The products of hydrogen or deuterium extraction are distinguished by 1HNMR. The NMR analysis was performed directly on hydroperoxides from the reaction. From the cis-1 a very small isotope effect is observed (k/sub H//k/sub D/ = 1.04 to 1.09). From trans-1 a significant effect is observed (k/sub H//k/sub D/ = 1.38 to 1.44). Both ranges are the result of duplicate runs and multiple integrations. Because of the symmetrical nature of the deuterium substitution, the isotope ratio must be derived from competition in the hydrogen abstraction step and not from differences due to the rehybridization of the CO bond site, or secondary effects. These primary isotope effects require a mechanism in which groups cis to each other are competitive in the singlet oxygen reaction, while geminal or trans dispositions are only weakly so. The results appear to be incompatible with any single-step reaction and thus require an intermediate. The pereperoxide appears to be the intermediate which accomodates the present data