Catalytic Epoxidation of Hindered Olefins with Dioxygen. Fast Oxygen Atom Transfer to Olefin Cation Radicals from Nitrogen Oxides

Hindered olefins are efficiently converted to epoxides by dioxygen at 25 °C in dichloromethane containing catalytic amounts of nitrogen oxides (NO₂, NO⁺, NO, etc.). Nitrogen dioxide also effects the direct (stoichiometric) epoxidation of the same hindered olefins. Olefin cation radicals are spectrally identified as the first observable intermediate, and separate experiments confirm the facile transfer of an oxygen atom from nitrogen dioxide to olefin cation radicals to produce epoxides. At low temperature (−78 °C), the epoxidation is rapidly initiated by added 1-electron oxidants such as tris(2,4-dibromophenyl)amine cation radical and nitrosonium (NO⁺). Scheme presents the complete sequence of redox changes that are mediated by the nitrogen oxides in the catalytic conversion of hindered olefins to epoxides via the cation radical. The deliberate irradiation of the charge-transfer absorption band of the corresponding olefin electron donor−acceptor complexes with tetranitromethane also establishes the formation of epoxides to occur via the same reactive pair, i.e., the olefin cation radical and NO₂. The mechanistic implication of rapid oxygen atom transfer to olefin cation radicals is underscored in the general consideration of catalytic epoxidations with dioxygen.