| Abstract
| - The main purpose of this study is to assess the relative importance of diradical or peroxirane(perepoxide) intermediates in the singlet oxygen cycloaddition reactions with alkenes that lead to dioxetanes.The relevant nonconcerted pathways are explored for ethene, methyl vinyl ether, and s-trans-butadiene byCAS-MCSCF optimizations followed by multireference perturbative CAS-PT2 energy calculations and byDFT(B3LYP) optimizations. The two different theoretical approaches gave similar results (reported below).These results show that methoxy or vinyl substitution does not affect qualitatively the reaction features evidencedby the unsubstituted system. Peroxirane turns out to be attainable only by passing through the diradical, dueto the nature of the critical points involved. The energy barriers for the transformation of the diradical toperoxirane in the case of ethene (ΔE⧧ = 13−15 kcal mol-1) and methyl vinyl ether (ΔE⧧ = 12−13 kcalmol-1) are higher than those for the diradical closure to dioxetane (ΔE⧧ = 8−9 kcal mol-1, for ethene, and 9kcal mol-1, for methyl vinyl ether). In all three systems, the peroxirane pathway to dioxetane is prevented bythe high energy barrier for the second step, leading from peroxirane to dioxetane (ΔE⧧ = 26−27, 27−31 and22 kcal mol-1, for ethene, methyl vinyl ether, and butadiene, respectively). By contrast, peroxirane can veryeasily back-transform to the diradical (with a ΔE⧧ estimate of 3 kcal mol-1, for ethene and methyl vinyl ether,and close to zero, for butadiene). These results indicate that, although a peroxirane intermediate might formin some cases, it corresponds to a dead-end pathway which cannot lead to dioxetane.
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