Diradical and Peroxirane Pathways in the [π2 + π2] Cycloaddition Reactions of1ΔgDioxygen with Ethene, Methyl Vinyl Ether, and Butadiene: A Density Functional and Multireference Perturbation Theory Study

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 by CAS-MCSCF optimizations followed by multireference perturbative CAS-PT2 energy calculations and by DFT(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 evidenced by the unsubstituted system. Peroxirane turns out to be attainable only by passing through the diradical, due to the nature of the critical points involved. The energy barriers for the transformation of the diradical to peroxirane in the case of ethene (ΔE⧧ = 13−15 kcal mol-1) and methyl vinyl ether (ΔE⧧ = 12−13 kcal mol-1) are higher than those for the diradical closure to dioxetane (...