Reaction Mechanisms Peroxo radical

In fact, as we shall see in more detail later in the chapter, peroxo metal complexes are very versatile oxidants capable of reacting with a variety of substrates through different reaction mechanisms, either polar or radical Thus, it would be of great help to be... 

Snbsequent detailed kinetic stndies revealed that the reaction mechanism for the hydroxy-lation of arenes is mnch more complicated than that indicated above Furthermore, the active intermediate is likely an anion radical species formed upon interaction of two molecules of the vanadium peroxo complex. The sequence of the various steps is indicated in equations 17-24. The steps indicated in equations 17-21 refer to a radical chain which accounts for decomposition of the peroxo complex to form dioxygen, whereas the subsequent steps are those required for the functionalization of the substrate. 

From the evidence described above concerning the uncommon behavior of radical reactions in hydrocarbon oxidation at a titanium center inside a zeolite, the possibility that peroxo radicals are also involved in oxidation reactions of other compounds should be considered. In contrast to the mechanism discussed above for the hydrocarbons, in this case both the peroxo and the reactant molecule would be coordinated at the Tilv center, and the reaction would therefore take place between two coordinated species. 

The data seem to support an alternative mechanism compared with the so-called Lyons mechanism in which O2 is activated forming a metal-oxo (M=0) species.1831 Via reaction of a second metalloporphyrin with a primarily formed superoxo (Me-OO0) or peroxo species, a metal-oxo is formed, reacting eventually with an alkane according to the oxygen rebound mechanism. Alternatively, radicals present in solution, upon reaction with dioxygen, may form alkyl hydroperoxides that are decomposed by metalloporphyrins.[83]... 

The ORR is a multielectron reaction that may include a number of elementary steps involving different reaction intermediates. The detailed mechanism is still not known, since neither ex situ nor in situ techniques are capable of identifying all reaction intermediates formed under genuine reaction conditions exist. Of the various possibilities [13], it has been proposed that on metal surfaces the most plausible reaction pathway for the ORR in both alkaline and acidic electrolytes can be described by the so-called serial reaction pathway (Scheme 3.1) where after the transfer of two electrons and (simultaneous) fast protonation of superoxo/peroxo radicals (not included in the reaction scheme), O is reduced to H O (with rate constant k. ... 

Mechanisms of the photooxygenation reaction in the cases of other than iron metal scans to be different from that postulated for the FeCIs-catalyzed process [60h]. Low-valent species are apparently involved in the oxidation. These species can possibly add an oxygen molecule to produce metal peroxo radicals and peroxo... 

Once an aldehyde radical is formed through the intervention of an OH group, the subsequent step proceeds via a peroxo radical, producing acetic acid. This peroxo radical is consumed and can be regenerated via the direct interaction of aldehyde radicals with oxygen. This chain reaction mechanism explains the findings than one photon can contribute to the photoconversion of several pollutant molecules and can give as a result, quantum yields in excess of 100%. 

Equation (5.172) does not represent an elementary reaction it is a multistep mechanism involving NO3 or the dimer (NO)2. This NO3 (0=N00) is an isomer to the nitrate radical 0=N=0 (O) and the first step in NO oxidation. It is clear that this very instable peroxo radical will almost decompose by quenching to NO + O2, which results in a slow reaction probability (we will meet this reaction later in biological systems) ... 

In [43] mechanism of AP formation on reaction of quadratic chain termination of cumyl peroxo radicals ROO is offered ... 

The half-order of the rate with respect to [02] and the two-term rate law were taken as evidence for a chain mechanism which involves one-electron transfer steps and proceeds via two different reaction paths. The formation of the dimer f(RS)2Cu(p-O2)Cu(RS)2] complex in the initiation phase is the core of the model, as asymmetric dissociation of this species produces two chain carriers. Earlier literature results were contested by rejecting the feasibility of a free-radical mechanism which would imply a redox shuttle between Cu(II) and Cu(I). It was assumed that the substrate remains bonded to the metal center throughout the whole process and the free thiyl radical, RS, does not form during the reaction. It was argued that if free RS radicals formed they would certainly be involved in an almost diffusion-controlled reaction with dioxygen, and the intermediate peroxo species would open alternative reaction paths to generate products other than cystine. This would clearly contradict the noted high selectivity of the autoxidation reaction. 

The obvious candidates for the two competing reactions are those shown in Eqs. (24) and (25). The mechanisms of the two reactions are probably similar, although it was not possible to show whether reaction 25 produced isobutene. The expected chemistry is shown in more detail in Eq. (32). In a less appealing possibility, the tentative peroxo intermediate would cleave homolytically to yield alkyl and alkoxyl radicals, a route that is thermodynamically much less favorable than reaction 32. 

Based on the analysis of the reactions in Scheme 3 and on previous studies (46, 47), a mechanism for the reaction was proposed in which the /x-peroxo complex, 16, may simultaneously abstract two hydrogen atoms from iso-propyl groups on the pyrazolyl ligands. Alternatively, because of the weak 0—0 bond, 16 may homolytically dissociate to form two Tp"Co(0-) oxo-radical moieties, and these species would then abstract hydrogen from the iso-propyl groups. In either case, the resulting carbon-centered radical can either react with solvent, as was observed for the Tp complex (46), or with another carbon-centered radical so as to regenerate the Tp"Co(OH) complex and produce a derivative of the Tp" complex with an iso-propenyl substituent, 18. Ultimately, either route would produce the (/x-OH)2 complex, 17. 

As was mentioned in Section V.C.3.b, when competitive oxidation of 1-octene and -hexane is carried out, the alkene is preferentially oxidized. Correspondingly, alkenes react at lower temperatures than alkanes. It is therefore surprising that under noncompetitive reaction conditions, the rate of oxidation of n-hexane is higher than that of 1-octene (Huybrechts et al., 1992). One possible explanation for this observation is that the reaction conditions were different (Clerici et al., 1993b). At 373 K titanium peroxo compounds decompose, thereby giving rise to radical chain reactions that are negligible at lower temperatures. Thus there could be a different mechanism for low- and high-temperature oxidations made more complex by secondary uncatalyzed oxidation of initial products (Spinace et al., 1995). 

Hydroxylation of [Cu2(R—XYL—H)]2+ (10) by 02. As described in Section II.C.l, the complete kinetic analysis reveals an initial reversible binding of 02 by 10 to give [Cu2(H—XYL—H)(02)]2+ (11), followed by an irreversible hydroxylation reaction described by k2. The kinetics preclude that a Fenton-type mechanism (production of hydroxyl radical) is involved in the reaction (i.e., that an intermediate peroxo species is further attacked by LCu(I)). We note that [Cu2(H—XYL—H)]4+ (34) cleanly reacts with H202 to give product [Cu2(H—XYL—O—)(OH)]2+ (12), whereas reaction of [Cu2(H—XYL—H)]2+ (10) with hydrogen peroxide does not (unpublished observation). Addition of radical traps to solutions of 10 and 02 also does not affect the hydroxylation (unpublished observation), and all the evidence points to intramolecular hydroxylation by the peroxo-dicopper species 11. 

In 1983, Mimoun and co-workers reported that benzene can be oxidized to phenol stoichiometrically with hydrogen peroxide in 56% yield, using peroxo-vana-dium complex 1 (Eq. 2) [20]. Oxidation of toluene gave a mixture of ortho-, meta-and para-cresols with only traces of benzaldehyde. The catalytic version of the reaction was described by Shul pin[21] and Conte [22]. In both cases, conversion of benzene was low (0.3-2%) and catalyst turned over 200 and 25 times, respectively. The reaction is thought to proceed through a radical chain mechanism with an electrophilic oxygen-centered and vanadium-bound radical species [23]. 

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