Reductive Activation Of Molecular Oxygen Research Articles

Reductive Electrochemical Activation of Molecular Oxygen Catalyzed by an Iron-Tungstate Oxide Capsule: Reactivity Studies Consistent with Compound I Type Oxidants

The reductive activation of molecular oxygen catalyzed by iron-based enzymes toward its use as an oxygen donor is paradigmatic for oxygen transfer reactions in nature. Mechanistic studies on these ...

(Keynote) Cathodic Electrocatalysis Mediated By Polyoxometalates: Reduction of Carbon Dioxide and Reductive Activation of Molecular Oxygen for Hydrocarbon Oxidation

The talk will report on recent developments in our laboratory in electrochemical reactions catalyzed by polyoxometalates. First, combining a specifically designed di-Re(I) catalyst with H3PW12O40 in a hybrid compound, we show that upon electrochemical reduction of the polyoxometalate at a relatively low negative potential of 1.3 V versus Ag/AgNO3, photoexcitation of the polyoxometalate with visible light is enabled through excitation of the intravalence charge transfer band. This leads to electron transfer to the Re(I) catalyst for the overall photoelectrochemical reduction of CO2 to CO. Conceptually this means that such a polyoxometalate can be an electron “shuttle” between a water oxidation reaction on the one hand that produces electrons/protons and O2 and a CO2 photoreduction catalyst. Second, tri-transition metal substituted polyoxometalates such as b-[SiW9O37(CuIIH2O)3]10– are shown to be outstanding electrocatalysts for the selective reduction of CO2 to CO with a turnover frequency by foot-of the wave analysis of nearly 3000 sec–1 without addition of an acid. Very high stability in electrolytic reactions was observed with stable currents for a period of 24h. at a potential of 2.5 V versus Fc/Fc+ and a Faradaic efficiency of >90%. Step-wise reduction of the catalyst followed by UV-vis and EPR spectroscopy indicates that the active species is a likely a four-electron reduced polyoxometalate. Additionally, similar compounds such as b-[SiW9O37(FeIII 2NiIIH2O)3]10– are shown to be functional models of CO hydrogenase/CO2 dehydrogenase enzymes that also have Fe-Ni active sites. As in the enzyme, the CO oxidation is one order of magnitude faster than the CO2 reduction. An iron-tungsten oxide inorganic molecular catalyst, a Keplerate polyoxometalate with a capsular structure, with a short-hand notation, {Fe30W72}, was used for the cathodic oxidation of simple hydrocarbons such as ethane to acetic acid, ethylene to formaldehyde and benzene to phenol in water. The {Fe30W72} capsule is stabilized in the core by sulfate/bisulfate anions providing a protic environment where three iron atoms are located at each of the twenty pores of the capsule leading to a unique and potent active site for the aerobic hydrocarbon oxidation. This is the first example of the reductive electrochemical activation of O2 that functionally mimics the reactivity of iron-based monooxygenase enzymes. Turnover frequencies based of 300-600 min1– were obtained based on the surface area of the cathode. With a graphitic carbon fiber cathode and Pt anode Faradaic efficiencies of ~60% at a cell potential of 1.8 V were typically obtained where the uncoupled reduction of O2 to water was minimized. Extensive mechanistic research using EPR spectroscopy, kinetic isotope effect measurements as well as reactions of model compounds such a 1,4-difluorbenzene, indicate the intermediacy of a highly active high spin iron-oxo species as the oxygenating species.

Homogeneous Oxygenase Catalysis.

Oxygenases-catalyzed reductive activation of molecular oxygen and incorporation of O atoms into an organic molecule is undoubtedly one of the most attractive research areas. Typically, these oxygenation reactions proceed with high selectivity and reactivity, which is seldom found in its "biomimetic" chemocatalytic counterparts. Furthermore, enzymatic oxygenation can avoid undesired overoxidation, which is frequently observed in (industrial) chemical transformation. Therefore, it is not surprising that tremendous attention has been paid to enzymatic oxygenation. Their application in organic synthesis has been steadily growing over the years. The goal of the present Review is to provide a handy reference for chemists interested in using homogeneous oxygenase catalysis and those interested in discovering new types of biomimetic oxidations and oxygenations with dioxygen. In this Review, we will review the recent advances in in homogeneous oxygenase catalysis to reveal the great achievements and potentials in this field.

The Oxygen Dilemma: A Severe Challenge for the Application of Monooxygenases?

Monooxygenases are promising catalysts because they in principle enable the organic chemist to perform highly selective oxyfunctionalisation reactions that are otherwise difficult to achieve. For this, monooxygenases require reducing equivalents, to allow reductive activation of molecular oxygen at the enzymes' active sites. However, these reducing equivalents are often delivered to O2 either directly or via a reduced intermediate (uncoupling), yielding hazardous reactive oxygen species and wasting valuable reducing equivalents. The oxygen dilemma arises from monooxygenases' dependency on O2 and the undesired uncoupling reaction. With this contribution we hope to generate a general awareness of the oxygen dilemma and to discuss its nature and some promising solutions.

Peroxygenase-Catalyzed Oxyfunctionalization Reactions Promoted by the Complete Oxidation of Methanol.

Peroxygenases catalyze a broad range of (stereo)selective oxyfunctionalization reactions. However, to access their full catalytic potential, peroxygenases need a balanced provision of hydrogen peroxide to achieve high catalytic activity while minimizing oxidative inactivation. Herein, we report an enzymatic cascade process that employs methanol as a sacrificial electron donor for the reductive activation of molecular oxygen. Full oxidation of methanol is achieved, generating three equivalents of hydrogen peroxide that can be used completely for the stereoselective hydroxylation of ethylbenzene as a model reaction. Overall we propose and demonstrate an atom-efficient and easily applicable alternative to established hydrogen peroxide generation methods, which enables the efficient use of peroxygenases for oxyfunctionalization reactions.

Spectroelectrochemistry of the redox activation of anti-cancer drug mitoxantrone

The redox behaviour of the anti-cancer drug mitoxantrone was investigated in aprotic media (dimethylsulfoxide-DMSO) by coupled electrochemical and spectral EPR and UV/VIS absorption techniques. The cyclic voltammetry study with stationary and rotating disc electrode (RDE) of the reductive pathway of mitoxantrone points to two-electron transfers and evidences as intermediate species the anion radical, the dianion and the corresponding protonated species. EPR and optical spectra registered during the electrochemical reduction allow the identification of these species and suggest the possibility of back oxidation of the drug by electron transfer to molecular oxygen. The possibility of reductive activation of molecular oxygen by the intermediate species in the redox processes of mitoxantrone is discussed in connection with the cardiotoxicity of the drug. Gas phase and solvent-dependent AM1 and PM3 semiempirical MO calculations allow a rationalization of the experimental results regarding the reactivity in redox processes.

Reaction Mechanism of H2-Promoted Selective Catalytic Reduction of NO with NH3 over Ag/Al2O3

The rate of the selective catalytic reduction of NO with ammonia (NH 3 -SCR) on Ag/Al 2 O 3 at 473 K increased by a factor of 630, when H 2 (1%) was added to the reaction gas mixture. The mechanistic cause of this dramatic activity improvement was investigated by kinetic and spectroscopic studies. Kinetic studies indicate that H 2 addition increases the rate of NH 3 reaction in NH 3 + O 2 below 673 K and decreases the activation energy for NH 3 + O 2 reaction, indicating that H 2 addition is effective for the oxidative activation of NH 3 . In situ UV-vis results under reaction conditions show that the Ag + ion and Ag n δ+ cluster (n ≤ 8) coexist during H 2 -assisted NH 3 -SCR reaction (H 2 -NH 3 -SCR). ESR spectra after exposing H 2 -NH 3 -SCR reaction gas mixture at 423 K show that the superoxide ion is formed on the catalyst, and its amount increased with H 2 concentration. The steady-state NO reduction rate and relative amount of the cluster during the reaction increased with H 2 concentration, and the rate correlates fairly well with the relative amount of the cluster, indicating that the cluster acts as active species in H 2 -NH 3 -SCR. It is concluded that Ag n δ+ cluster and protons formed by H 2 reduction of Ag + ions are involved in the reductive activation of molecular oxygen into superoxide ion, which should act as effective oxidant for N-H activation of NH 3 to NH X (x ≤ 2) intermediate.

Kinetic and in situ infrared studies on SCR of NO with propane by silver–alumina catalyst: Role of H 2 on O 2 activation and retardation of nitrate poisoning

The mechanistic cause of enhanced C 3H 8-SCR activity of Ag/Al 2O 3 by H 2 addition was investigated by kinetic and in situ infrared (IR) studies. Kinetic studies found that hydrogen addition results in decreased activation energy for NO reduction (from 224 to 61 kJ mol −1), decreased reaction orders with respect to O 2 (from 1.38 to 0.01) and C 3H 8 (from 1.91 to 0.76), and increased order with respect to NO (from −2.53 to 0.49). The essential role of H 2 is in the reductive activation of molecular oxygen into reactive oxygen species involved in the oxidative activation of C 3H 8. Addition of hydrogen can also retard nitrate poisoning by reducing nitrates and thus decreasing nitrate coverage. N 2 formation via the unsteady-state reaction of nitrates with hydrogen and NH 3 formation via the NO + H 2 reaction also occur, demonstrating an additional effect of hydrogen on the promotion of NO reduction under the unsteady-state and unselective conditions.

Integrated photocatalysts for hydrocarbon oxidation: polyoxotungstates/iron porphyrins systems in the reductive activation of molecular oxygen

The photochemical and photocatalytic properties of (nBu 4N) 3PW 12O 40(PW 12O 40 3).(nBu 4N) 4W 10O 32 and (Et 4N) 4W 10O 32(W 10O 32 4) were investigated in the presence of several Fe(III) meso-tetraarylporphyrins (Fe(III)Por). Laser flash photolysis experiments show that in the (nBu 4N) 3PW 12O 40/Fe(III)Por integrated systems the electron transfer from the photoreduced polyoxotungstate to the Fe(III) Por is the first step for the subsequent reductive activation of O 2 by the ferrous complex. In contrast, the direct reduction of O 2 by the photoreduced polyoxotungstate is the kinetically favored process in the (nBu 4N) 4W 10O 32/Fe(III) Por systems. A comparison between the results obtained using (nBu 4N) 4W 10O 12 and (Et 4N) 4W 10O 32 shows that the decrease in the steric hindrance of the counterion, surrounding the photochemically active polyoxoanion, increases the rate of electron transfer between W 10O 32 and Fe(III)Por, thus enabling the decatungstate to contribute to the activation of dioxygen through the reduction of the Fe(III) Por. Continuous irradiation of the POT 11/Fe(III) Por integrated systems under controlled aerobic conditions leads to the oxidation of cyclohexane to cyclohexanol and cyclohexanone. In comparisn with POT 11 a slone a higher cyclohexanol/cyclohexanone concentration ratio is obtained, while in comparison with Fe(III)Por alone, higher yields of oxidation products and a higher stability of the photocatalysts are observed. The effect of the iron porphyrin on the products distribution can be ascribed to the ability of the complex in both the ferrous and the ferric forms to generate monooxygenating intermediates as a consequence of its reactions with O 2 alkylhydroperoxide and radical species.

A manganese(III) porphyrin/rhodium(III) bipyridine/formate catalyst system for the reductive activation of molecular oxygen

A two-phase system utilizing a rhodium(III) bipyridine complex and a manganese(III) porphyrin in the presence of sodium formate for the reductive activation of molecular oxygen and the subsequent epoxidation of alkenes is described. In this system, the reducing equivalents for the conversion of Mn III to Mn II are obtained from the rhodium-catalyzed oxidation of formate to carbon dioxide.

Development of supramolecular metalloprotein mimics

Abstract

Oxygen activation and olefin oxygenation by iron(III)porphyrin as a model of cytochrome P-450.

Tetraphenylporphinatoiron (III) catalyzed the reductive activation of molecular oxygen in the presence of NaBH4 and Me4NOH. Several olefins were oxidized with this system and oxidized products were isolated in high yields.

チトクロムＰ‐４５０をモデルとした反応

Our recent developments related to the cytochrome P-450 model reactions by using iron porphyrin complex are summarized. This article is divided into three sections according to the type of reaction.The first is the oxidant supported oxidation. Porphinatoiron-oxene complex generated by chloro α, β, γ, δ-tetraphenylporphinato iron (III) (FeIII TPPCl) and 2-iodoso-m-xylene as oxidant smoothly dealkylated several tertiary amines to the corresponding secondary amines.The second is the oxidation accompanied by the reductive activation of molecular oxygen. Reductive dioxygen activation was achieved by reductant (Na2S2O4, NaBH4 or Zn) -FeIII TPPCl system in protic media and the active intermediate formed by this system oxidized tertiary amines, sulfides and hydrocarbons.The last is the reductive deoxygenation from several tertiary amine oxides and arene oxides by reduced porphinatoiron (FeII TPP).Detailed studies on the reaction mechanism are also described.