Reactivity and Activation of Dioxygen-Derived Species in Aprotic Media (A Model Matrix for Biomembranes) [and Discussion]

In aprotic media the electrochemical reduction of dioxygen yields superoxide ion (O$^-_2$), which is an effective Broslashnsted base, nucleophile, one-electron reductant, and one-electron oxidant of reduced transition metal ions. With electrophilic substrates (organic halides and carbonyl carbons) O$^-_2$ displaces a leaving group to form a peroxy radical (ROO$^\cdot$) in the primary process. Superoxide ion oxidizes the activated hydrogen atoms of ascorbic acid, catechols, hydrophenazines and hydroflavins. Combination of O$^-_2$ with 1,2-diphenylhydrazine yields the anion radical of azobenzene, which reacts with O$_2$ to give azobenzene and O$^-_2$ (an example of O$^-_2$-induced autoxidation). With phenylhydrazine, O$^-_2$ produces phenyl radicals. The in situ formation of HO$^\cdot_2$ (O$^-_2$ plus a proton source) results in H-atom abstraction from allylic and other groups with weak heteroatom-H bonds (binding energy (b.e.) less than 335 kJ). This is a competitive process with the facile second-order disproportionation of HO$^\cdot_2$ to H$_2$O$_2$ and O$_2$ (k$_bi \thickapprox 10^4$ mol$^{-1}$ s$^{-1}$ in Me$_2$SO). Addition of [Fe$^{II}$ (MeCN)$_4$] (ClO$_4$)$_2$ to solutions of hydrogen peroxide in dry acetonitrile catalyses a rapid disproportionation of H$_2$O$_2$ via the initial formation of an adduct $[Fe^{II}(H_2O_2)^{2+} \leftrightarrow$ Fe(O)(H$_2O)^{2+}]$, which oxidizes a second H$_2$O$_2$ to oxygen. In the presence of organic substrates such as 1,4-cyclohexadiene, 1,2-diphenylhydrazine, catechols and thiols the Fe$^{II}$-H$_2$O$_2$/MeCN system yields dehydrogenated products; with alcohols, aldehydes, methylstyrene, thioethers, sulphoxides, and phosphines, the Fe$^{II}$(H$_2$O$_2$)$^{2+}$ adduct promotes their monoxygenation. The product from the FeO$^{2+}$-H$_2O_2$ reaction, [Fe$^{II}$(H$_2$O$_2$)$^{2+}_2$], exhibits chemistry that is closely similar to that for singlet oxygen ($^1$O$_2$), which has been confirmed by the stoichiometric dioxygenation of diphenylisobenzofuran, 9,10-diphenylanthracene, rubrene and electron-rich unsaturated carbon-carbon bonds (Ph$_2$C=CPh$_2$, PhC$\equiv$CPh and cis-PhCH=CHPh). In dry ligand-free acetonitrile (MeCN), anhydrous ferric chloride (Fe$^{III}$Cl$_3$) activates hydrogen peroxide for the efficient epoxidation of alkenes. The Fe$^{III}$Cl$_3$ further catalyses the dimerization of the resulting epoxides to dioxanes. These observations indicate that strong Lewis acids that are coordinatively unsaturated, [