Dioxygen radical mechanism

The possibility of a radical mechanism is supported by the observation of the accelerating effect of molecular oxygen on the cyclopropanation. Miyano et al. discovered that the addition of dioxygen accelerated the formation of the zinc carbenoid in the Furukawa procedure [24a, b]. The rate of this process was monitored by changes in the concentration of ethyl iodide, the by-product of reagent formation. Comparison of the reaction rate in the presence of oxygen with that in the... 

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 kinetics of the catalytic oxidation of cyclopentene to glutaraldehyde by aqueous hydrogen peroxide and tungstic acid have been studied and a compatible mechanism was proposed, which proceeds via cyclopentene oxide and /3-hydroxycyclopentenyl hydroperoxide. " Monosubstituted heteropolytungstate-catalysed oxidation of alkenes by t-butyl hydroperoxide, iodosobenzene, and dioxygen have been studied a radical mechanism was proved for the reaction of alkenes with t-BuOOH and O2, but alkene epoxidation by iodosobenzene proceeds via oxidant coordination to the catalyst and has a heterolytic mechanism. ... 

Nitric oxide is only formed at very high temperatures (equilibrium concentration about 10 vol% NO (500 K), 0.04% (1000 K), 0.1% (1500 K), 0.5% (1800 K), and 1% at 2100 K). Around 2300 K, the NO concentration reaches a maximum at higher temperatures the NO decomposition is faster than the NO formation. The nitric oxide formation has a radical mechanism, starting with the homolytic dissociation of dioxygen to form oxygen radicals. 

Figure 19. Proposed dioxygen activation mechanism for the diiron sites in MMO and RRB2. In RRB2, the diiron site is responsible for the generation of the tyrosine radical. The diiron site of MMO oxidizes methane to methanol no evidence for the production of a tyrosine radical is seen in this system.

Oleic acid has been reported to undergo epoxidation with dioxygen around 30 °C in the presence of cobalt acetate catalyst and added benzaldehyde [114]. A free radical mechanism involving the C H CO ... 

They further propose that the chemisorption of the O2 occurs inside the bulk of the FePc or (FePc) to produce a dioxygen radical which then diffuses to the catalyst-electrolyte interface. It is difficult, however, to accept their mechanism, particularly step 4, which leads to the formation of two OH radicals with the simultaneous desorption of the peroxide. Such a complex single step seems very unlikely on the basis of the high free energy needed to form OH free radicals in alkaline electrolytes (see Figure 1). 

ABSTOACT. The electrochranistfy of a series of cobalt chelates derived from cobaloxime (bis dimethylglyoximato cobalt comf xes) and the bdiaviour towanls molecular oxygen are reviewed. Correlations between the structure of the equatorial ligand, the relative stability of Go(lll), Co(II). Co(l) oxidation states, the extent of interaction with O2, the foimation of dioxygen adducts, mechanisms and pioducts of the redox catalysis for the reduction of Oj in the presence of protons, initiation of a radical mechanism of autoxidation reactions are pointed out and discussed. 

The scorpionate vanadium complexes [VCl3 HC(pz)3 ] (10) and [VCl3 S03C(pz)3 ] (15), which catalyze cyclohexane oxidation with H2O2 (Section 22.2.1,), also operate with dioxygen under solvent-free conditions. Cyclohexane is oxidized to cyclohexanol (the main product) and cyclohexanone (13% conversion), with a high selectivity, typically at the O2 pressure of 15 atm, at 140 °C, 18 h reaction time [6]. The reaction is further promoted (to 15% conversion) by pyrazinecarboxylic acid. The reactions proceed via radical mechanisms with possible involvement of both C-centered and 0-centered radicals. 

Radical initiated polymerization of vinylpyridinium ions has been recognized since 1964 when Shyluk reported 1,2-addition polymerization of 1,2-dimethyl-5-vinylpyridinium methylsulfate initiated by potassium peroxydisulf-ate [39]. Later, Ringsdorf proposed a radical mechanism for the spontaneous polymerization of 1 (R = H) at concentrations greater than 1.0 M in water [37, 42]. The postulated mechanism was supported by experiments in which inhibition of polymerization was demonstrated by radical inhibitors such as dioxygen, copper (II) ions, and 5-butylcatechol. Kinetic analysis of these reactions was also consistent with the proposed radical pathway. 

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