## Abstract

In aprotic media the electrochemical reduction of dioxygen yields superoxide ion (O<latex>$^-_2$</latex>), 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<latex>$^-_2$</latex> displaces a leaving group to form a peroxy radical (ROO<latex>$^\cdot$</latex>) in the primary process. Superoxide ion oxidizes the activated hydrogen atoms of ascorbic acid, catechols, hydrophenazines and hydroflavins. Combination of O<latex>$^-_2$</latex> with 1,2-diphenylhydrazine yields the anion radical of azobenzene, which reacts with O<latex>$_2$</latex> to give azobenzene and O<latex>$^-_2$</latex> (an example of O<latex>$^-_2$</latex>-induced autoxidation). With phenylhydrazine, O<latex>$^-_2$</latex> produces phenyl radicals. The in situ formation of HO<latex>$^\cdot_2$</latex> (O<latex>$^-_2$</latex> 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<latex>$^\cdot_2$</latex> to H<latex>$_2$</latex>O<latex>$_2$</latex> and O<latex>$_2$</latex> (k<latex>$_bi \thickapprox 10^4$</latex> mol<latex>$^{-1}$</latex> s<latex>$^{-1}$</latex> in Me<latex>$_2$</latex>SO). Addition of [Fe<latex>$^{II}$</latex> (MeCN)<latex>$_4$</latex>] (ClO<latex>$_4$</latex>)<latex>$_2$</latex> to solutions of hydrogen peroxide in dry acetonitrile catalyses a rapid disproportionation of H<latex>$_2$</latex>O<latex>$_2$</latex> via the initial formation of an adduct <latex>$[Fe^{II}(H_2O_2)^{2+} \leftrightarrow$</latex> Fe(O)(H<latex>$_2O)^{2+}]$</latex>, which oxidizes a second H<latex>$_2$</latex>O<latex>$_2$</latex> to oxygen. In the presence of organic substrates such as 1,4-cyclohexadiene, 1,2-diphenylhydrazine, catechols and thiols the Fe<latex>$^{II}$</latex>-H<latex>$_2$</latex>O<latex>$_2$</latex>/MeCN system yields dehydrogenated products; with alcohols, aldehydes, methylstyrene, thioethers, sulphoxides, and phosphines, the Fe<latex>$^{II}$</latex>(H<latex>$_2$</latex>O<latex>$_2$</latex>)<latex>$^{2+}$</latex> adduct promotes their monoxygenation. The product from the FeO<latex>$^{2+}$</latex>-H<latex>$_2O_2$</latex> reaction, [Fe<latex>$^{II}$</latex>(H<latex>$_2$</latex>O<latex>$_2$</latex>)<latex>$^{2+}_2$</latex>], exhibits chemistry that is closely similar to that for singlet oxygen (<latex>$^1$</latex>O<latex>$_2$</latex>), which has been confirmed by the stoichiometric dioxygenation of diphenylisobenzofuran, 9,10-diphenylanthracene, rubrene and electron-rich unsaturated carbon-carbon bonds (Ph<latex>$_2$</latex>C=CPh<latex>$_2$</latex>, PhC<latex>$\equiv$</latex>CPh and cis-PhCH=CHPh). In dry ligand-free acetonitrile (MeCN), anhydrous ferric chloride (Fe<latex>$^{III}$</latex>Cl<latex>$_3$</latex>) activates hydrogen peroxide for the efficient epoxidation of alkenes. The Fe<latex>$^{III}$</latex>Cl<latex>$_3$</latex> further catalyses the dimerization of the resulting epoxides to dioxanes. These observations indicate that strong Lewis acids that are coordinatively unsaturated, [Fe<latex>$^{II}$</latex>(MeCN)<latex>$_4$</latex>]<latex>$^{2+}$</latex> and [Fe<latex>$^{III}$</latex>Cl<latex>$_3$</latex>], activate H<latex>$_2$</latex>O<latex>$_2$</latex> to form an effective oxygenation and dehydrogenation agent. When catalytic quantities of superoxide ion are introduced into a dry acetonitrile solution that contains excess substrate (Ph<latex>$_2$</latex>SO or PhCH<latex>$_2$</latex>OH), ambient air, 1,2-diphenylhydrazine and iron<latex>$^{(II)}$</latex>, the substrate is rapidly and efficiently monoxygenated, the combination provides a catalytic system for the autoxidation of organic substrates via reaction cycles that closely mimic cytochrome P<latex>$_{450}$</latex> monoxygenase enzymes.