On the Mechanism of Photooxygenation Reactions. Computational Evidence against the Diradical Mechanism of Singlet Oxygen Ene Reactions

Ab initio calculations with the STO-3G basis set and semiempirical MINDO/3 calculations at the unrestricted Hartree-Fock level show that both ab initio and semiempirical techniques predict that 1,4-diradicals formed from singlet oxygen and simple alkenes are more stable than perepoxides. Thus, both techniques now agree that perepoxides are too high in energy to qualify as viable intermediates in singlet oxygen ene reactions, as are zwitterions, for weakly polar alkenes. However, calculations on putative substituted diradical intermediates show that reactions involving such intermediates should be nonstereospecific and show significant regioselectivities. The high stereospecificity and low regioselectivity found experimentally argue strongly against the intermediacy of diradicals in this and related reactions.