Oxyl radical mechanism

Similar reactions are catalyzed by Mn and Fe centers of MnSOD and FeSOD. It is obvious that before participation in Reaction (2), superoxide must be protonized to form hydroper-oxyl radical HOO by an outer-sphere or an intra-sphere mechanisms. All stages of dismuting mechanism, including the measurement of elementary rate constants, have been thoroughly studied earlier (see, for example, Ref. [2]). 

To discern the ion-radical nature of reactions, the so-called intramolecular and intermolecular proton/deuterium isotope effects may be of use. Baciocchi et al. (2005) revealed ion-radical mechanism for A-demethylation of A,A-dimethylanilines, (CH3)2NAr, by phthalimide-A-oxyl radical (Scheme 4.14). In this reaction, ( e/ D)intra values were derived for reactivity of (CD3)(CH3)NAr, whereas ( H/ D)inter was referred for the reactivity of (CD3)2NAr. The values of (A e/ D)intra were found to be always different and higher than These results, although are incompat-... 

The A-hydroxyphthalimide is anodically oxidized to the phthaliraide iV-oxyl radical which subsequently performs a hydrogen atom abstraction from the substrate. Regeneration of the active radical takes place at the anode in the presence of a base like pyridine or 2,6-lutidine (mechanism B, Sect. 2.3) (Eq. (113)). The turnover... 

In DNA, the peroxyl radicals at C(3 )-C(5 ) could, in principle, give rise to oxyl radicals that may lead to strand breakage through p-fragmentation. In contrast to the C(4 )-mechanism in the absence of 02, details are, however, not yet established. For developing mechanistic concepts, we still fully rely on model systems such as those described above. 

Hildenbrand K, Schulte-Frohlinde D (1989) E.S.R. studies on the mechanism of hydroxyl radical-induced strand breakage of polyuridylic add. Int J Radiat Biol 55 725-738 Hildenbrand K, Schulte-Frohlinde D (1997) Time-resolved EPR studies on the reaction rates of per-oxyl radicals of poly(acrylic acid) and of calf thymus DNA with glutathione. Re-examination of a rate constant for DNA. Int J Radiat Biol 71 377-385 Hildenbrand K, Mirtsch S, Schulte-Frohlinde D (1993) H-NMR studies of y-irradiated polynucleotides and DNA in N20-saturated aqueous solutions. Release of undamaged and modified bases. Radiat Res 134 283-294... 

In this reaction, a 5 -phosphate end group is formed and the base is released. The products that result from the C(l )-C(3 ) fragment are not yet fully established. Malonaldehyde is a potential one. For this product to be formed, the C(3 ) per-oxyl radical has to be reduced, e.g., via the Russell mechanism (Chap. 8.8). 

Several mechanisms have been suggested to produce the energy required to populate an excited carbonyl, which is at least 290-340 kj mol-1 [8]. Direct homolysis of hydroperoxides [9, 10], disproportion of alkoxy radicals [11] and /2-scission of alkoxy radicals [12] are all exothermic enough. However, the most widely accepted mechanism has been the highly exothermic (460 kj mol-1) bimolecular termination of primary or secondary alkyl per-oxyl radicals, i.e. the Russell mechanism (Scheme 2). It proceeds via an intermediate tetroxide to give an excited carbonyl, an alcohol, and oxygen [13, 14]. 

An oxo/oxyl radical-coupling mechanism between the exchangeable p-oxo bridge (05) and a second oxygen species (oxyl radical) not yet formed/bound in the 2 state [32] see Figure 3.4.5(11). 

Another example of oligomers formed by condensation of nucleus with side chain is the dimer (90) formed when 2,2,6,6-tetramethyl-4-oxopiperidin-l-oxyl (91) is set aside at room temperature for six months 112 the mechanism for the formation of 90 has been assumed112 to be hydrogen abstraction, followed by the coupling reaction of the A7-oxyl radical with the C-radical derived from 91. 

Allylic azides, e.g., 1, were produced by treatment of the triisopropylsilyl enol ethers of cyclic ketones with azidotrimethylsilane and iodosobenzene78, but by lowering the temperature and in the presence of the stable radical 2,2,6,6-tetramethylpiperidine-/V-oxyl (TEMPO), 1-triso-propylsilyloxy-l,2-diazides, e.g., 2, became the predominant product79. The radical mechanism of the reaction was demonstrated. A number of 1,2-diazides (Table 4) were produced in the determined optimum conditions (Method B 16h). The simple diastereoselectivity (trans addition) was complete only with the enol ethers of unsubstituted cycloalkanones or 4-tert-butylcy-clohexanone. This 1,2-bis-azidonation procedure has not been exploited to prepare a-azide ketones, which should be available by simple hydrolysis of the adducts. Instead, the cis-l-triiso-propylsilyloxy-1,2-diazides were applied to the preparation of cw-2-azido tertiary cyclohexanols by selective substitution of the C-l azide group by nucleophiles in the presence of Lewis acids. 

In the bimolecular decay of peroxyl radicals, a short-lived tetroxide is an intermediate. When a hydrogen is present in /3-position to the peroxyl function, a carbonyl compound plus an alcohol and O2 [Russell mechanism, e.g. reaction (42)] or two carbonyl compound plus H2O2 (Bennett mechanism, not shown) may be formed in competition to a decay into two oxyl radicals plus O2 [e.g. reaction (43) for details of peroxyl radical chemistry in aqueous solution, see Refs. 2 and 39]. 

To give a specific example, the advantages of styrene as a substrate for peroxyl radical trapping antioxidants are well known" (i) Its rate constant, kp, for chain propagation is comparatively large (41 M s at 30 °C) so that oxidation occurs at a measurable, suppressed rate during the inhibition period and the inhibition relationship (equation 14) is applicable (ii) styrene contains no easily abstractable H-atom so it forms a polyper-oxyl radical instead of a hydroperoxide, so that the reverse reaction (equation 21), which complicates kinetic studies with many substrates, is avoided and (iii) the chain transfer reaction (pro-oxidant effect, equation 20) is not important with styrene since the mechanism is one involving radical addition of peroxyls to styrene. 

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