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.