Electron-rich alkenes, reaction with singlet oxygen

Since [4 + 2]cycloaddition and ene reactions are generally assumed to proceed in a concerted manner via isopolar activated complexes, they should exhibit virtually the same small, often negligible, response to changes in solvent polarity. This is what, in fact, has been found cf. for example [138, 682, 683]. However, two-step [2 + 2]-cycloaddition reactions of singlet oxygen to suitably substituted electron-rich alkenes proceed via dipolar activated complexes to zwitterionic intermediates (1,4-dipoles or perepoxides). In this case, the relative amounts of 1,2-dioxetane and allylic hydroperoxides or e do-peroxides should vary markedly with solvent polarity if two or even all three of the reaction pathways shown in Eq. (5-145) are operative [681, 683, 684]. 

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