Peroxide anion

Under illumination, some of the luciferin molecules (LnH) that absorbed photons are changed into free radicals (Ln ), probably at carbon-2 of the imidazopyrazinone ring. The free radical instantly binds with an oxygen molecule to form a peroxide radical (LnOO ), an extremely fast reaction (k = 109M 1 s"1 Pryor, 1976). The peroxide radical formed reacts with a luciferin molecule, generating a new free radical of luciferin and a luciferin peroxide anion (LnOO"),... 

The radical 85 yields the peroxide anion 86 the decomposition of which gives rise to excited N-methylacridone and cyanate. In addition to previous suggestions put forward in respect of this exergonic decomposition 3.139) (see also p. 97), a recombination of the radical 85 and the peroxy radical 87 (from 86 and O2) according to... 

In the case of the experiments performed by Hohman and co-workers [149], the fluoride anion would readily displace the silicon-leaving group. The peroxide anion could then further react via an intramolecular nucleophilic attack, resulting in cyclization to form the reactive intermediate responsible for the chemiluminescence that was observed. A recent kinetic study by Stevani and Baader [150] of the reaction of 4-chlorophenyl-O,O-hydrogen monoperoxyoxalate with various oxygen and nitrogen bases suggested that the intermediate formed must be 1,2-dioxetandione. 

The electropositive metal center polarizes the peroxo group so that is much more electrophilic than free peroxide anion. Evidence has been accumulated, and it will be summarized later, that such polarization happens to the extent that nucleophilic reagents can attack a peroxo oxygen when it is coordinated to a d° metal. In other words, this situation... 

Cyclic enones can be oxidatively cleaved by a range of reagents to yield keto acids. As ozonolysis can be quite hazardous for large-scale preparations with the build up of ozonides, the procedure has been modified using quaternary ammonium salts to catalyse the transfer of peroxide anion for a rapid oxidative work-up [32]. Two methods are available but, in the safer procedure (10.7.15.A), there is no effective build-up of the ozonide. 

In the early 1980 s Julia and Colonna published a series of papers which, to some extent, filled the gap left by the natural biocatalysts. The Spanish and Italian collaborators showed that a, -unsaturated ketones of type 1 underwent asymmetric oxidation to give the epoxide 2 using a three-phase system, namely aqueous hydrogen peroxide containing sodium hydroxide, an organic solvent such as tetrachloromethane and insoluble poly-(l)-alanine, (Scheme 1) [12]. The reaction takes place via a Michael-type addition of peroxide anion (the Weitz-Scheffer reaction). 

Julid and Colonna showed that, as expected, poly-(L)-leucine and poly-(i)-isoleucine could be employed in place of poly-(L)-alanine, while poly-(L)-valine gave poorer results in terms of yields and stereoselectivity [13]. Some other eno-nes were tried without success and nucleophiles other than peroxide anion were... 

At this point it is impossible to guess the architecture of the active catalyst. The folding of 10-mers of leucine and alanine in organic solvents is clearly of critical importance, and studies are in progress to understand the preferred shapes. Obviously, in its active form the catalyst binds and activates peroxide anion and/or the electron-poor alkene near its chiral surface, perhaps in a chiral cavity, but the precise orientation of catalyst and reactants in the initial bondforming Michael reaction remains unsolved. 

In a similar fashion to those results obtained for the oxidation process, on switching from poly-(L)-leucine to poly-(D)-leucine the opposite configuration of the polyether was observed (absolute configuration of products unknown). Unexpected, however, was the observation that poly-(L)-phenylalanine furnished the opposite enantiomer to that observed employing poly-(L)-leucine. Thus, it has been shown that addition of nucleophiles other than peroxide anion can be catalysed by polyamino acids with significant stereocontrol [22]. 

Several mechanisms for the peroxide oxidation of organosilanes to alcohols are compared. Without doubt, the reaction proceeds via anionic, pentacoordinate silicate species, but a profound difference is found between in vacuo and solvated reaction profiles, as expected. In the solvents investigated (CH2CI2 and MeOH), the most favorable mechanism is addition of peroxide anion to a fluorosilane used as starting material or formed in situ, followed by a concerted migration and dissociation of hydroxide anion. In the gas phase, and possibly in very nonpolar solvents, concerted addition-migration of H2O2 to a pentacoordinate fluorosilicate is also plausible. ... 

The mechanism of 1,2-dioxetane formation in the reaction of lucigenin with hydrogen peroxide suggests a nucleophilic attack of peroxide anion on position 9 of the acri-dinium ring, followed by deprotonation and subsequent formation of 1,2-dioxetane ring... 

Radical cations can be generated by many chemical oxidizing reagents, including Brpnsted and Lewis acids, the halogens, peroxide anions or radical anions, metal ions or oxides, nitrosonium and dioxygenyl ions, stable aminium radical cations, semiconductor surfaces, and suitable zeolites. In principle, it is possible to choose a reagent with a one-electron redox potential sufficient for oxidation-reduction, and a two-electron potential insufficient for oxidation-reduction of the radical ion. 

Notice—and I want to make a point of this—the hydrogen peroxide anion is a better nucleophilic reagent than the hydroxide ion by a factor of about 50, even though hydrogen peroxide is a stronger acid than water by a factor of about 104. We convert water to the same units as hydrogen peroxide. 

This is an example of what John Edwards and I call the alpha effect. I think it is valuable because it does enable us to generate quite easily a very powerful nucleophilic reagent in water, this hydrogen peroxide anion. 

The role of oxygen in metabolism involves a paradox. Combustion of food to release and store its energy content requires a stepwise four-electron reduction of oxygen to produce harmless water, carbon dioxide, and ammonia (Figure 10.1). The first electron produces superoxide anion radical, the second produces peroxide anion, the third produces hydroxyl radical, and the fourth produces water. When this process is compartmentalized... 

In this mechanism, the peroxide anion of the substrate makes a nucleophilic attack on the carbonyl carbon of a-ketoglutarate so that a peroxide bridge is formed between the two compounds. In this way, one atom each of molecular oxygen is incorporated into the substrate and a-ketoglutarate to form the product and succinate, The incorporation of molecular oxygen into succinate has been shown in most of the reactions that require a-ketoglutarate. 

Oxygen dissolves in the melt by reaction with carbonate, forming hyperoxide and peroxide anions ... 

Appleby and Nicholson 173, 174) report the rate laws obtained at flag electrodes that are consistent with the reduction of hyperoxide and peroxide anions, respectively. 

While there is no doubt that the iron atom moves upon oxygenation, it is not obvious that this will lead to the observed cooperativity. Oxygenated heme has some of the characteristics of an Fe(III)-peroxide anion complex.152 The iron atom acquires an increased positive charge upon oxygenation by donating an electron for bond formation. 

Formation of H202 by flavin oxidases can occur via elimination of a peroxide anion HOO from the adduct of Eq. 15-31 with regeneration of the oxidized flavin. 

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