Formation of superoxide ion

Kitajima, N., Fukuzumi, S., and Ono, Y., Formation of superoxide ion during the decomposition of hydrogen peroxide on supported metal oxides, /. Phys. Chem., 82, 1505, 1978. 

In the effort to find confirmation on Foote s original mechanistic proposal [84] and discriminate among these two different pathways, a great deal of experimental proofs were achieved. First of all, the DCA and/or 9-cyanoanthracene (CNA)-sensitized reactions on aryl-olefins were studied under inert atmosphere by flash spectroscopic techniques obtaining clear evident for the formation of both olefin radical cations and cyanoaromatic radical anions [95]. In the presence of oxygen, the cyanoaromatic radical anions were rapidly removed, supporting the very rapid formation of superoxide ion and so its direct involvement in these photoinduced oxygenations. 

In the last paper on this topic [113], the same group extended this study to several different unsaturated organic compounds, leading to the conclusion that the DCA-sensitized photooxygenation of electron-rich donors is affected by solvent polarity. The chemical outcome can be connected with the fluorescence data and the formation of superoxide ion, and/or singlet oxygen proceeds from a charge-transfer complex between the donor and the acceptor upon excitation. 

The advantage of the electrochemical method is often to produce intermediates that are not readily accessible by conventional means. The electrogeneration of transient nucleophiles and electrophiles is an example. The formation of superoxide ions by the cathodic reduction of oxygen in aprotic solvents (DMF, DMSO, and MeCN) is another case,... 

The negative rate dependence on acetic acid concentration (-0.5) indicates its strong adsorption on the catalyst, which would inhibit the access of oxygen to the catalyst surface. They assumed that the catalytic action is the formation of superoxide ion (Oj ) followed by a production of hydroperoxy radical as an active species under the reaction condition (pH = 2.9)... 

Saito I, Matuura T, Inoue K. Formation of superoxide ion via one-electron transfer from electron donors to singlet oxygen. J Am Chem Soc 1983 105 3200-6. 

Catalytic oxidation of hydrogen sulfide or methyl mercaptan is also enhanced in the presence of nitrogen-containing functionalities [57], Besides providing the basic pH needed for effective dissociation of HS , they were proposed to activate oxygen via formation of superoxide ion, which participates in the oxidation of thiolate ions to sulfur and sulfuric acid [95],... 

Formation of superoxide ion attached to iron is probable when iron is in the ferrous state or has a ferrous character. Thus, reactivity of superoxide ion has attracted attention in the mechanistic studies of the enzymatic reactions. Reaction of superoxide ion was studied in some details by Moro-oka and Foote in 1976 [13]. Both types of intra- and extra-diol oxygenation products are formed from 1 as shown in Scheme 3. Products 5 and 13-15 are also formed. Since the same products are formed also from the quinone 6, the common intermediate is assumed for both the reactions of catechol and quinone. The product formation is explained by the O2 attack to the activated catecholate anion radical forming a peroxide ion. The initial step forming a radical species is similar to that in... 

The reaction of 1-hydroxy- or 1-aminonaphthoquinone with O2 shows a significant feature of the superoxide ion formation. The superoxide ion forms a van der Waals complex with another product of this reaction, a semiquinone. Hydrogen bonds are formed between Oj" and the OH and NH2 groups of the corresponding semiquinone. As a result, the reaction equilibrium is shifted to the right (Liwo et al. 1997). 

The reaction of superoxide ion with carbon tetrachloride is important for olefin epoxidations. This reaction includes the formation of the trichloromethyl peroxide radical Oj" + CCI4 —> Cl + CI3COO. The trichloromethyl peroxide radicals formed oxidize electron-rich olefins. The latter gives the corresponding epoxides. This peroxide radical is a stronger oxidizing agent than the superoxide ion itself (Yamamoto et al. 1986). 

The major in situ process that results in the formation of H202 is undoubtedly photochemical (e.g., 12, 15, 49, 50). Photochemical formation of H202 in fresh and salt waters probably results from the disproportionation of the superoxide ion radical, 02 (8, 9, 15, 51, 52). The kinetics of superoxide disproportionation are well established (53), and its steady-state concentration can be calculated. Because of the known effects of superoxide ion in cells (47), its presence in surface waters may be important in biologically mediated processes. However, other sources, such as biological formation (e.g., 45, 54), redox chemistry (21, 24, 29, 31, 32), wet (e.g., 55) and dry (50, 56, 57) deposition, and surfaces (e.g., 58) may also be important. 

Early experiments by a number of workers demonstrated that dye excited states could function as electron donors (69,92-94). Lindquist (69) showed that the triplet state of fluorescein was quenched by oxygen in basic media with the formation of superoxide anion, by ferric ion in acid media to form ferrous ion, and by peroxydisulfate ions in alkaline solutions. The photo-... 

In the discussion of the biochemistry of copper in Section 62.1.8 it was noted that three types of copper exist in copper enzymes. These are type 1 ( blue copper centres) type 2 ( normal copper centres) and type 3 (which occur as coupled pairs). All three classes are present in the blue copper oxidases laccase, ascorbate oxidase and ceruloplasmin. Laccase contains four copper ions per molecule, and the other two contain eight copper ions per molecule. In all cases oxidation of substrate is linked to the four-electron reduction of dioxygen to water. Unlike cytochrome oxidase, these are water-soluble enzymes, and so are convenient systems for studying the problems of multielectron redox reactions. The type 3 pair of copper centres constitutes the 02-reducing sites in these enzymes, and provides a two-electron pathway to peroxide, bypassing the formation of superoxide. Laccase also contains one type 1 and one type 2 centre. While ascorbate oxidase contains eight copper ions per molecule, so far ESR and analysis data have led to the identification of type 1 (two), type 2 (two) and type 3 (four) copper centres. 

On ceria, unreduced gold seems to play a major role in the reaction (Section 6.3.3.6). Ceria may provide sites for the formation of superoxide and peroxide-type species,100 or act as an oxygen supplier for a reaction of Mars and van Krevelen-type.97 It is occasionally suggested that the reaction might go entirely on the support itself modified by gold ions, and forming a solid solution of type Cei a Au3 02 5-98... 

However, Schaap s results are not consistent with a mechanism involving attack of superoxide ion on the opened radical cations. In fact, the striking stereochemical course could be accommodated only by admitting the formation of the most stable E-E conformation of epoxide radical cations, and by showing that the ring closure of the zwitterion intermediates could occur faster than the bond rotation. 

Another route to the formation of H00./02. is UV irradiation of HOOH in aqueous solutions. Aerobic organisms produce minor fluxes of superoxide ion (02, and thereby HOO.) during respiration and oxidative metabolism for example, possibly up to 10-15% of the O2 reduced by cytochrome c oxidase and by xanthine oxidase passes through the H00./02. state. Thus, the chemistry of HOO. (and of 02 ) may be important to an understanding of oxygen toxicity in biological systems. In aprotic media, the rate of protonation of 02. is proportional to the acidity of the associated Bronsted acids (equation 96). Electrolytic oxidation of... 

Combination of superoxide ion with the cation radical (MV+0 of methyl viologen (l,T-dimethyl-4,4 -bipyridinium ion, Paraqnat) is one of the best-documented examples of a stoichiometric 02 - radical coupling reaction. The initial addition to give an unstable diamagnetic product is assumed to be at a-carbon atoms (which possess the maximum unpaired spin density in the cation radical), followed by formation of a dioxetane-like intermediate that decomposes to a complex mixture of products (Scheme 13). This process may provide a means to explain the mechanism of Paraquat toxicity. Several other reports have proposed direct coupling of 02 - to cation radicals. ... 

Scheme 5. Electrochemically detectaUe reactions of superoxide ion in the first steps of silanone formation. The bimolecular reaction with k is rate determining.

Significant enhancement effects of electron acceptors (additives) such as hydrogen peroxide, ammonium persulfate, potassium bromate, and potassium peroxymono-sulfate (oxone) on the Ti02 photocatalytic degradation of various organic pollutants were observed already in early investigations [376]. The results showed that these additives markedly improved the degradation rate of 2,4-dichlorophenol. The enhanced photocatalytic oxidation of sulfide ions on phthalocyanine modified titania was ascribed [377] to the additional formation of superoxide radicals. 

A paper by Gierer et al. [63] probably resolves the mechanism. These workers found that a number of hydroxy- or methoxy-substituted stilbenes would react with hydroxyl radicals, or with a combination of superoxide and hydroxyl radicals. The formation of superoxide and hydroxyl radicals by hydrogen peroxide decomposition is hard to prevent and incomplete chelation of metal ions might explain the inconsistent results reported by the earlier workers. 

---