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Relevant academic research and scientific papers

Acid catalysis vs. electron-transfer catalysis via organic cations or cation-radicals as the reactive intermediate. Are these distinctive mechanisms?

Proton transfer to aromatic and olefinic donors (D) leads to the facile interchange of transient carbocations (DH+) and cation-radical (D+.). The same types of cation and cation-radical are reactive intermediates in the acid catalysis and the electron-transfer catalysis of such organic transformations as benzylic coupling, epoxide/pinacol rearrangements and cis-trans isomerization of stilbenes when they are both carried out under otherwise identical reaction conditions. However, the rapid exchange of diamagnetic cations and paramagnetic cation-radicals blurs the traditional view of separate electrophilic and homolytic processes, and rigorous experimental evidence is required to establish whether acid catalysis and electron-transfer catalysis actually represent distinct mechanistic categories. Acta Chemica Scandinavica 1998.

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 3 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.

Electron-transfer reactions of hindered olefins induced by aminium salts

Hindered olefins 1-6, by reaction with aminium salts, can afford, in relations to the reaction conditions and the peculiar features of the reagents, different reaction products such as dioxetanes, epoxides, ketones, or homoallylic, allylic and vinylic hal