Phenoxyls peroxides

The spin density of tocopheroxyl radical 2, a classical phenoxyl radical, is mainly concentrated at oxygen 0-6, which is the major position for coupling with other C-centered radicals, leading to chromanyl ethers 5. These products are found in the typical lipid peroxidation scenarios. Also at ortho- and para-positions of the aromatic ring, the spin density is increased. At these carbon atoms, coupling with other radicals, especially O-centered ones, proceeds. Mainly the para-position (C-8a) is involved (Fig. 6.3), leading to differently 8a-substituted chromanones 6. 

With 2,4,6-trialkylphenols used as inhibitors, the formed phenoxyl radicals produce quinolide peroxides by the reactions with peroxyl radicals. At sufficiently high temperatures, quinolide peroxides decompose giving rise to free radicals [18,31,32,53,54] ... 

B) Phenols of this group slowly react with hydroperoxide and dioxygen. Respective phenoxyl radicals are relatively unreactive toward RH and ROOH, but can react with R02 giving rise to peroxides and then to free radicals. For these phenols, appropriate inhibitory mechanisms are I III and VI VIII. 

The 2,4,6-substituted phenoxyl radicals recombine slowly and selectively react with per-oxyl radicals, producing quinolidic peroxides [57]. 

At moderate temperatures (T < 350 K), these peroxides are stable but readily decompose at elevated temperatures to form radicals [58 60]. Peroxyl radicals can add to phenoxyl radicals at both para- and ort/zo-positions with the proportion dependent on the substituent. Thus, the peroxyl radical adds to 2,4,6-tris(l,l-dimethylethyl)phenoxyl in the para-position by eight times more rapidly than in the ortho-position, whereas it adds to 2-methyl-4,6-bis(l,l-dimethylethyl)phenoxyl preferentially in the ortho-position [59], The rate constants of the reactions of the peroxyl radical with 2,6-bis(l,l-dimethylethyl)-4-substituted phenoxyl radicals in benzene were measured in the presence of respective phenol and AIBN as a source of 1-cyano-l-methylethylperoxyl radicals [60,61]. The concentration of the formed phenoxyl radical in these experiments peaked at [ArO ]max (k7/kx)[ArOH]. Using this expression and taking the known kn values, the values of ks for various phenoxyl radicals 4-Y-2,6-(Me3C)2. Q,H20 were estimated at T 353 K (see Database [52]) ... 

The phenoxyl radical has an increased electron density in the ortho- and pura-positions and adds dioxygen similar to alkyl radicals. However, the C—00 bond is weak in this peroxyl radical and back dissociation occurs rapidly. Therefore, the formation of quinolide peroxide occurs in two steps, which was studied for the 2,4,6-tris(l,l-dimethylethyl)phenoxyl radical [100,101],... 

It should be taken into account that the reaction of chain propagation occurs in polymer more slowly than in the liquid phase also. The ratios of rate constants kjlkq, which are so important for inhibition (see Chapter 14), are close for polymers and model hydrocarbon compounds (see Table 19.7). The effectiveness of the inhibiting action of phenols depends not only on their reactivity, but also on the reactivity of the formed phenoxyls (see Chapter 15). Reaction 8 (In + R02 ) leads to chain termination and occurs rapidly in hydrocarbons (see Chapter 15). Since this reaction is limited by the diffusion of reactants it occurs in polymers much more slowly (see earlier). Quinolide peroxides produced in this reaction in the case of sterically hindered phenoxyls are unstable at elevated temperatures. The rate constants of their decay are described in Chapter 15. The reaction of sterically hindered phenoxyls with hydroperoxide groups occurs more slowly in the polymer matrix in comparison with hydrocarbon (see Table 19.8). 

The oxidation of PIB occurs mainly via intramolecular addition of dioxygen to double bonds of polymer. The reaction of peroxyl radical addition to the phenoxyl radical leads to the formation of quinolide peroxide (see Chapter 15). This peroxide is unstable, and its decomposition provokes the degradation of PIB. Another reaction predominates in case of aromatic diamine. 

Various hydroxyl and amino derivatives of aromatic compounds are oxidized by peroxidases in the presence of hydrogen peroxide, yielding neutral or cation free radicals. Thus the phenacetin metabolites p-phenetidine (4-ethoxyaniline) and acetaminophen (TV-acetyl-p-aminophenol) were oxidized by LPO or HRP into the 4-ethoxyaniline cation radical and neutral V-acetyl-4-aminophenoxyl radical, respectively [198,199]. In both cases free radicals were detected by using fast-flow ESR spectroscopy. Catechols, Dopa methyl ester (dihydrox-yphenylalanine methyl ester), and 6-hydroxy-Dopa (trihydroxyphenylalanine) were oxidized by LPO mainly to o-semiquinone free radicals [200]. Another catechol derivative adrenaline (epinephrine) was oxidized into adrenochrome in the reaction catalyzed by HRP [201], This reaction can proceed in the absence of hydrogen peroxide and accompanied by oxygen consumption. It was proposed that the oxidation of adrenaline was mediated by superoxide. HRP and LPO catalyzed the oxidation of Trolox C (an analog of a-tocopherol) into phenoxyl radical [202]. The formation of phenoxyl radicals was monitored by ESR spectroscopy, and the rate constants for the reaction of Compounds II with Trolox C were determined (Table 22.1). 

We have recently described a calibration procedure for the determination of excitation quantum yields on commercial fluorimeters, utilizing the luminol standard , and have thereby determined singlet excitation quantum yields for the peroxyoxalate reaction with bis(2,4,6-trichlorophenyl) oxalate (TCPO), hydrogen peroxide and imidazole, using various activators . The same calibration method has been utilized to determine the singlet quantum yields obtained in the induced decomposition of protected phenoxyl-substituted 1,2-dioxetanes 6 and and compared them to the well-investigated... 

Moreover, under certain conditions these phenolic compounds could also act as pro-oxidants. In the presence of redox-active metal ions such as Cu or Fe, phenolic compounds react with O2 to generate phenoxyl radicals. Under normal growth conditions phenoxyl radicals can be rapidly deactivated by polymerization or enzymatic reduction. However, if the phenoxyl radical concentrations are too high and/or the lifetime is increased, they could initiate DNA damage or lipid peroxidation and exhibit cytotoxicities. Curcumin, demethoxycurcumin, and bisdemethoxycurcumin have been reported to induce... 

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