Peroxy intermediate

The most stable O2 addition adduct below T = 1250 K is 6-peroxyoxepinone (route A in Fig. 12), which can cyclize to form a 1,4-peroxy intermediate which subsequently releases CO2 to form a 5-oxopentanalyl radical. This species can cyclize and fragment, yielding formyl radical, furan, and carbon dioxide. Above T = 1250 K, the... 

Numerous sulfur and phosphorus peroxy compounds such as monopersulfuric acid (Caro s acid, la), monopersulfate (Oxone, Ib), ammonium monopersulfate Ic, tetra n-butyl-ammonium monopersulfate Id, peroxydisulfates 2a and 2b", tetra n-butylammonium peroxydisulfate 2c, symmetrical bissulfonyl peroxide 3 , acyl sulfonyl peroxide 4 , unsymmetrical sulfonyl peroxide 5 , sulfinyl peroxy intermediates 6a, sulfonyl peroxy intermediate 6b, sulfonimidoyl peroxy intermediate 7, bis(diphenyl phosphinyl) peroxide 8 , unsymmetrical phosphorus peroxide 9 and phosphoranyl peroxy intermediate 10" are known. Recently, many researchers have shown interest in the preparation and... 

Recently, sulfinyl and sulfonyl peroxy radical intermediates 6a and 6b have been prepared by the reactions of aryl sulfinyl or sulfonyl chloride with superoxide anion radical, respectively. These peroxy intermediates show strong oxidizing abilities in various oxidations. This chapter will describe the properties and applications of a variety of sulfur and phosphorus peroxy compounds in oxidation reactions. For a more complete picture, readers should consult the original papers cited in the areas that most interest them. 

Various arylsulfonyl chlorides reacted with to form peroxy intermediates. 

Various arylsulfonyl chlorides reacted with 02 to form peroxy intermediates. 2-Nitrobenzenesulfonylperoxy intermediate 51 had shown a larger oxidizing ability towards olefins compared with the 4-nitrobenzenesulfonylperoxy intermediate. When 2-nitrobenzenesulfonyl chloride reacts with 02 at low temperature (—20 to —35 °C) in CH3CN, the corresponding sulfonylperoxy radical 51 or anion 52 is generated. The radical character of the sulfonylperoxy intermediate 51 was further confirmed by the electrophilic oxidizing nature of a 2-nitrobenzenesulfonyl chloride/K02 mixture. 

When 02 reacts with cytochrome c oxidase, it may be bound initially to either the a3 iron or to CuB, but in the peroxy intermediate P it may bind to both atoms. Oxyferryl compound F (Fig. 8-11) as well as radical species, can also be formed by treatment of the oxidized... 

Preceding work on MEK oxidation has identified a number of possible reaction networks whereby formation of DA and the numerous observed by-products could be rationalised. Yamazoe et al. (ref. 2) and Ai (ref. 5) have proposed the formation of peroxy intermediates formed by reaction with 02"(a(jS) to explain the formation of DA and C2 scission products. 

Changes in aliphatic side chains are mainly initiated by the abstraction of a hydrogen atom from a C-H bond. This produces a free radical which in the absence of O2 can combine with similar free radicals to give dimerization products or with CO2 [probably the CO2 ion. (40)] to give monoamino-dicarboxylic acids. In oxygen these are replaced by oxidative reactions which produce carbonyl, hydroxyl, and carboxyl groups through hydro-peroxy intermediates ... 

The peroxy intermediate derived from 02 attack on the activated substrate is proposed to act as a tridentate ligand to the iron(III) center as shown in Figure 15. Molecular modeling of this adduct using the structure of the PCD ES complex as a starting point shows that such a structure is reasonable [133], This coordination mode is precedented in the structure of the 02 adduct of [Ir(III)(tri-phos)(dbc)]+ [150,151],... 

A mechanism for extradiol cleavage is proposed in Figure 18 [5,158], Substrate binds first to the iron(II) center, followed by 02, to form a ternary complex akin to the ES—NO complex described earlier. Electron transfer from metal to 02 in the Fe(II)—02 adduct results in a superoxide-like moiety and imparts nucleophilic character to the bound 02. This in turn generates semiquinone character on the bound substrate, which is attacked by the nascent superoxide to form a peroxy intermediate that decomposes by a Criegee-type rearrangement to the observed product. 

A mechanism was proposed in which the perferryl iron-oxeme, resulting from heterolytic cleavage of the 0-0 bond of the iron-peroxy intermediate, abstracts an electron from the 0=0 double bond of the carbonyl group of the aldehyde. The reduced perferryl attacks the 1-carbon of the aldehyde to form a thiyl-iron-hemiacetal diradical. The latter intermediate can fragment to form an alkyl radical and thiyl-iron-formyl radical. The alkyl radical then abstracts the formyl hydrogen to produce the hydrocarbon and C02 (Reed et al 1995). 

Peroxy intermediates are also probably involved in the catalytic cycle, since Mn(III) species are not readily regenerated from Mn(II) with oxygen. 

RlSR2 —> R S(0)R2. This sulfinyl chloride is oxidized by potassium superoxide to a sulfinyl peroxy intermediate that oxidizes sulfides to sulfoxides at —25° in >90% yield with only traces of the corresponding sulfones.1... 

Mechanisms for those enzymes acting as monooxygenases, e.g. mammalian tyrosine hydroxylase [87], Pseudomonas oleovorans monooxygenase [88] or 4-methoxybenzoate monooxygenase [84] can be devised that are essentially identical to those of cytochrome P-450 (Fig. 4) in this case the transient compound I-type intermediate is formalised as Fev=0, as there is no porphyrin ring to accept a cation radical. However, alternative mechanisms have been suggested that involve a direct attack on the substrate by the peroxy intermediate, Fe3+-C>2 [78,84],... 

Figure 22. Comparison of oxygen intermediates. A Electronic absorption spectra of the peroxy-intermediate in laccase versus oxyhemocyanin and oxytyrosinase. B Proposed structural differences between peroxide binding in oxyhemocyanin and oxytyrosinase relative to the end-on bound hydroperoxide intermediate at the trinuclear copper cluster in laccase.

The rate constants and the associated free-energy snrfaces available to the peroxide and native intermediates deserve comment since they differ overall by nearly 10 (or ca. Vkcalmol in absolnte valne). Given the relatively electronentral nature of electron transfers between the copper sites (the E° values for the three sites differ overall by only 60 mV), the differences in rate in the first instance reflect the difference in the E° value for le versus 2e reduction of dioxygen (leading to the peroxy intermediate) and peroxide (leading to the native intermediate). Second, the differences reflect the work available from the favorable 4e reduction that drives the turnover from native intermediate to fully reduced enzyme primed, now, to react with O2. This latter process, k 100 s (compare to k = 0.34s for decay of the native intermediate to fully oxidized enzyme), is functionally equivalent to the reductive release of Fe + from Fe +-transferrin catalyzed by the membrane metalloreductase, Dcytb in both cases, the lower valent metal species is more loosely coordinating. Whereas Fe + dissociates in the latter case, in MCO turnover the bound water(s) dissociate. 

Recent studies pointed to the formation of a peroxy intermediate on the Pt surface, suggesting that the more complex associative mechanism is at work on a Pt electrode. However, the DFT calculations by Nprskov et al. suggested that the associative mechanism was only the dominant pathway at ORR overpotentials greater than 0.8 V. At realistic ORR overpotentials (<0.8 V), the two pathways run... 

In the mechanism of Scheme 18.4, hydroxylation is carried out homolytically by the same species proposed for alkane hydroxylation [25]. The oxidation of the cyclohexadienyl intermediate by hydrogen peroxide produces the diphenol and regenerates the active species, closing the catalytic cycle. Scheme 18.5 illustrates the heterolytic routes proposed by Wilkenhoner and others for hydroquinone and catechol production, based on cationic peroxy intermediates [47]. Both types of mechanism, however, are little more than working hypotheses, needing validation by experimental evidence. 

The compehtion of one-electron pathways is sometimes detectable in the epoxidations catalyzed by transition metal catalysts [67]. However, in the epoxidahon of unhindered olefins on TS-1, the typical radical products are below the detection limits. Their presence could no longer be neglected when the rate of epoxidation is so slow as to become comparable to that of homolytic side reactions, for example with bulky olefins (see also Section 18.11). It is possible that, within these limits only, the epoxide is produced in part through the addition of a radical peroxy intermediate to the double bond [68, 69]. Even so, a homolytic pathway has again been proposed as a generally vahd epoxidation mechanism [7]. 

M. Wikstrom and J.E. Morgan. 1992. The dioxygen cycle Spectral, kinetic, and thermodynamic characteristics of ferryl and peroxy intermediates observed by reversal of the cytochrome oxidase reaction J. Biol. Chem. 267 10266-10273. (PubMed)... 

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