Reactivity and activation of dioxygen-derived species in aprotic media (a model matrix for biomembranes)

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Abstract
In aprotic media the electrochemical reduction of dioxygen yields superoxide ion (O 2 ), which is an effective Bronsted 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*) 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 situ formation of HO 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 2 to H 2 O 2 and O 2 ( k bi « 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 2 O 2 ) 2+ ↔Fe(O)(H 2 O) 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 - H202/M eCN system yields dehydrogenated products; with alcohols, aldehydes, methylstyrene, thioethers, sulphoxides, and phosphines, the Fen (H2O 2 )2+ adduct promotes their monoxygenation. The product from the Fe02+-H 2O 2 reaction, [FeII(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 = 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, [Fe II (MeCN) 4 ] 2+ and [Fe III Cl 3 ], activate H 2 O 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 2 SO or PhCH 2 OH), 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 P 450 monoxygenase enzymes.

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