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

Abstract

In aprotic media the electrochemical reduction of dioxygen yields superoxide ion (O2-), which is an effective Brønsted base, nucleophile, one-electron reductant, and one-electron oxidant of reduced transition metal ions. With electrophilic substrates (organic halides and carbonyl carbons) O2- 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 O2- with 1,2-diphenylhydrazine yields the anion radical of azobenzene, which reacts with O2 to give azobenzene and O2- (an example of O2--induced autoxidation). With phenylhydrazine, O2- produces phenyl radicals. The in situ formation of HO2. (O2- 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 HO2. to H2O2 and O2 (kbi approximately equal to 10(4) mol-1 s-1 in Me2SO). Addition of [FeII(MeCN)4] (ClO4)2 to solutions of hydrogen peroxide in dry acetonitrile catalyses a rapid disproportionation of H2O2 via the initial formation of an adduct [FeII(H2O2)2+----Fe(O)(H2O)2+], which oxidizes a second H2O2 to oxygen. In the presence of organic substrates such as 1,4-cyclohexadiene, 1,2-diphenylhydrazine, catechols and thiols the FeII-H2O2/MeCN system yields dehydrogenated products; with alcohols, aldehydes, methylstyrene, thioethers, sulphoxides, and phosphines, the FeII(H2O2)2+ adduct promotes their monoxygenation. The product from the FeO2+-H2O2 reaction, [FeII(H2O2)22+], exhibits chemistry that is closely similar to that for singlet oxygen (1O2), which has been confirmed by the stoichiometric dioxygenation of diphenylisobenzofuran, 9,10-diphenylanthracene, rubrene and electron-rich unsaturated carbon-carbon bonds (Ph2C = CPh2, PhC = CPh and cis-PhCH = CHPh). In dry ligand-free acetonitrile (MeCN), anhydrous ferric chloride (FeIIICl3) activates hydrogen peroxide for the efficient epoxidation of alkenes. The FeIIICl3 further catalyses the dimerization of the resulting epoxides to dioxanes. These observations indicate that strong Lewis acids that are coordinatively unsaturated, [FeII(MeCN)4]2+ and [FeIIICl3], activate H2O2 to form an effective oxygenation and dehydrogenation agent.(ABSTRACT TRUNCATED AT 400 WORDS)

Strategies for the Direct Catalytic Valorization of Methane Using Heterogeneous Catalysis: Challenges and Opportunities

“…This reaction can result in different intermediate species that can be further classified depending on the total number of electrons transferred from the metal center to the oxygen molecule (Figure ). The complete reduction requires four electrons (eq ) and is highly influenced by the proton availability, and thus, the redox potential of these intermediate species depends heavily upon pH or effective acidity . One electron reduction of oxygen is often difficult because it possesses the highest barrier.…”

Synthesis of Phenol–Pyridinium Salts Enabled by Tandem Electron Donor–Acceptor Complexation and Iridium Photocatalysis

“…Phenoxyl radical cation intermediates have previously been proposed en route to unsymmetrical phenol dimers using stoichiometric persulfate. Phenol radical cations [p K a (C 6 H 5 OH •+ ) ∼ −2.0] are thermodynamically prone to deprotonation by the superoxide anion (O 2 •– ) [p K a (HOO • ) = 4.9] . Unproductive deprotonation would result in the formation of neutral phenoxyl radicals, which would likely undergo dimerization (a main reaction byproduct) .…”

Oxidative Photocatalytic Homo- and Cross-Coupling of Phenols: Nonenzymatic, Catalytic Method for Coupling Tyrosine

“…The reduced photocatalyst can then be reoxidized by dioxygen ( III to I ) to afford a superoxide anion and ground state MesAcr + BF 4 – ( I ). Thereafter, the oxidized phenol radical cation V ( pK a ∼ −2.0) is deprotonated by a superoxide anion ( p K a of HO 2 • → O 2 – • = 4.9) ( V to VI ) and attacked by a neutral phenol ( VI to VII ). A peroxyl radical or the excited state MesAcr + BF 4 – subsequently oxidizes intermediate VII to provide VIII .…”