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

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_2O_2 and O_2 (k_bi thickapprox 10^4 mol^-1 s^-1 in Me_2SO). Addition of [Fe^II (MeCN)_4] (ClO_4)_2 to solutions of hydrogen peroxide in dry acetonitrile catalyses a rapid disproportionation of H_2O_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_2O_2 to oxygen. In the presence of organic substrates such as 1,4-cyclohexadiene, 1,2-diphenylhydrazine, catechols and thiols the Fe^II-H_2O_2/MeCN system yields dehydrogenated products; with alcohols, aldehydes, methylstyrene, thioethers, sulphoxides, and phosphines, the Fe^II(H_2O_2)²⁺ adduct promotes their monoxygenation. The product from the FeO²⁺-H_2O_2 reaction, [Fe^II(H_2O_2)²⁺_2], exhibits chemistry that is closely similar to that for singlet oxygen (^1O_2), which has been confirmed by the stoichiometric dioxygenation of diphenylisobenzofuran, 9,10-diphenylanthracene, rubrene and electron-rich unsaturated carbon-carbon bonds (Ph_2C=CPh_2, PhCequivCPh and cis-PhCH=CHPh). In dry ligand-free acetonitrile (MeCN), anhydrous ferric chloride (Fe^IIICl_3) activates hydrogen peroxide for the efficient epoxidation of alkenes. The Fe^IIICl_3 further catalyses the dimerization of the resulting epoxides to dioxanes. These observations indicate that strong Lewis acids that are coordinatively unsaturated, [Fe^II(MeCN)_4]²⁺ and [Fe^IIICl_3], activate H_2O_2 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_2SO or PhCH_2OH), ambient air, 1,2-diphenylhydrazine and iron^(II), 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 P₄₅₀ monoxygenase enzymes.