Making Radical Cations Live Longer
Abstract: The acid-catalyzed rearrangement of 1,4,5,8-tetramethylnaphthalene was used to determine the acidity of media used for EPR spectral studies of radical cations. The radical cations of methyl-substituted naphthalenes were shown to undergo dimerization reactions. The apparent persistency of the radical cation of hexamethylbenzene was due to the slow oxidation when thallium(III) trifluoroacetate was used as the electron acceptor. Phenyliodine(III) bis(trifluoroacetate) was shown to be a versatile reagent for the generation of radical cations. The combination of thallium(III)-based oxidants and 1,1,1,3,3,3-hexafluoropropan-2-ol (HFP) was shown to increase the persistency of radical cations with a factor of 10-100 compared to the previously best methods available. HFP was also shown to strongly attenuate the reactivity of nucleophiles. The radical cations of dibenzothiophene, dibenzofuran and some of their methyl-substituted derivatives were studied by EPR spectroscopy, and the results were compared with the calculated energy states and spin densities. The photolysis of a solution of alpha-phenyl-N-tert-butylnitrone (PBN) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone in HFP produced the EPR spectrum from the radical cation of 2-tert-butyl-3-phenyl-5,5-dimethylisoxazolidine. The treatment of the spin traps PBN and 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) with the strong oxidant xenon difluoride gave fluoro spin adducts via oxidation of the spin trap followed by nucleophilic addition of fluoride ion. A similar reaction was obtained by treating PBN and DMPO with strong oxidants in the presence of various carboxylic acids. This procedure resulted in the formation of acyloxyl spin adducts. The weak oxidants N-fluorodibenzenesulfonamide and trichloroacetonitrile gave spin adducts via an acid promoted version of the Forrester-Hepburn mechanism. HFP was shown to have advantageous properties in distinguishing between true spin trapping and spin trapping via nonconventional mechanisms.
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