Phenoxy radicals intermediates

Caldwell, E.S. and Steelink, C. (1969). Phenoxy radical intermediates in the enzymatic degradation of lignin model compounds. Biochimica et Biophysica Acta, 184,420-431. 

Thus, almost all the reactions of lignin substructure model dimers by the enzyme are explained on the basis of cation radical intermediates and their subsequent reactions with nucleophiles such as H2O and intramolecular hydroxyl groups, and with radicals such as di oxygen (for non-phenolic substrates), or on the basis of phenoxy radical intermediates (for phenolic substrates). 

Two years later (1989), the same authors reported the oxygenation and oxidation chemistry of the Mn-substituted POMs XMnnWn039"- (X = P, Si, Ge, or B) and a2-P2MnnWi706i8-.316 These POMs undergo reversible oxygenation at low temperature in toluene or benzene solution but irreversible oxidation above 22°C. The 02 adduct can be intercepted by the spin trap 5,5-dimethyl-1-pyrroline A-oxide (DMPO). EPR spectra indicate formation of a polyanion-02-DMP0 intermediate that decomposes to the oxidized POM. The nonpolar organic solutions of these POMs catalyze the oxidation of 2,6- and 2,4,6-substituted phenols to benzoquinones or polyphenyl ethers, and a POM-02-phenoxy radical intermediate can be detected by EPR. 

Capsaicin and capsaicinoids undergo Phase I metabolic conversion involving both oxidative and non-oxidative paths. The liver is the major site of this enzymatic activity. Lee and Kumar (1980) demonstrated the conversion of catechol metabolites via hydroxylation of vanil-lyl ring. In rats, dihydrocapsaicin is metabolized to products that are excreted in the urine as glu-curonides (Kawada and Iwai, 1985). The generation of a quinone derivative occurs via O-demethylation at the aromatic ring with concomitant oxidation of the semiquinone and quinone derivatives or via demethylation of the phenoxy radical intermediate of capsaicin. Additionally, the alkyl side chain of capsaicin is also susceptible to oxidative deamination (Wehmeyer et al., 1990). There is evidence that capsaicinoids can undergo aliphatic oxidation (cu-oxidation) (Surh et al, 1995 Reilly et al, 2003) which is a possible detoxification pathway. Non-oxidative pathways are also involved in the bioconversion of capsaicin, e.g. hydrolysis of the acid-amide bond to yield vanillylamine and fatty acyl moieties (Kawada et al, 1984 Kawada and Iwai, 1985 Oi et al, 1992). 

The mechanism of the oxidation of phenols to benzoquinones with Fremy s salt is fairly well understood [59a], and the kinetics of this reaction have been studied recently [62]. Fremy s radical abstracts the phenolic hydrogen atom to generate a resonance-stabilized phenoxy radical intermediate (Scheme 20). Trapping of the carbon radical with another equivalent of Fremy s salt, followed by elimination of the aminosulfonate group gives the ortho or para benzoquinone products, depending on the ring substitution. 

A phenoxy radical intermediate has four reactive positions the oxygen and para-carbon as well as two ort/io-carbons. Therefore, for o-unsubstituted phenols, it is difficult to regulate the coupling selectivity. Enzyme catalysts and enz5mie model catalysts have been studied for the control of the polymerization of 2- and/or 6-unsubstituted phenols. 

Sheldon and Kochi [71] have discussed the possibility of involvement of phenoxy radical intermediate in the oxygenation of 12 with Co(salpr) [eqs. (14) and (15)]. 

Chemical Properties. Lignin is subject to oxidation, reduction, discoloration, hydrolysis, and other chemical and enzymatic reactions. Many ate briefly described elsewhere (51). Key to these reactions is the ability of the phenolic hydroxyl groups of lignin to participate in the formation of reactive intermediates, eg, phenoxy radical (4), quinonemethide (5), and phenoxy anion (6) ... 

The authors formulate the mechanism in two steps, first an electron transfer from phenoxide ion to diazonium ion forming a radical pair, followed by attack of the diazenyl radical at the 4-position of the phenoxy radical and a concerted proton release, i. e., without involving the o-complex. Admittedly, there is no experimental evidence against such a concerted process, but also none for it It seems that those authors wanted only to demonstrate the occurrence of radical intermediates, but did not consider the question of the mechanism of the proton release. 

The oxidation generates highly delocalized phenoxy radicals (PhO, Scheme 2.21), which may initiate (i) a radical polymerization process, trapping the reactant (CF) to give a benzyl radical intermediate (QMR), or it may (ii) follow a radical coupling to produce the p-QM p-O-QM, which being a reactive electrophile could undergo cationic polymerization. 

The radicals (14) formed may be trapped with, for example, (10) above. Simple alkyl thiyl radicals such as MeS have been detected as reaction intermediates they are highly reactive. Relatively stable oxygen-containing radicals are also known. Thus the phenoxy radical (15),... 

When phenyl acetate (10) is photolyzed in solution with a conventional flash lamp, the transient absorption of the phenoxy radical can be readily observed, with a lifetime of 0.2-0.3 ms. The same intermediate has been detected in different solvents such as ethanol and Freon. Figure 1 presents two transient absorption... 

This supposition was validated by experimental studies that demonstrated the prevalence of different ROS at different temperatures. Using CRDS, Yu and Lin studied the reaction of phenyl radical and oxygen, noting that phenylperoxy radical was the only adduct formed at temperatures ranging up to 473 Venkat et al. completed flow reactor studies of benzene combustion at 1200K and identified phenoxy radical as a key intermediate. 

The overall pathways of benzene oxidation and the decompositions of possible intermediates have been well characterized via theoretical methods. Thus far, we have discussed these species mainly in the context of their oxidation mechanisms, but phenylperoxy and phenoxy radicals have also been investigated as individual experimental targets. 

The structures in brackets are phosphoranyl radicals with nine electrons on phosphorus and are considered to be transient intermediates. The radical, R-, presumably represents some initiator fragment, but since the phosphoranyl radical in Reaction 11 is symmetric, R exchanges can (and do) take place. In this regard, triaryl phosphites autoxidize much more slowly, and it has been suggested (3) that here phenoxy radicals are generated via another exchange (Reaction 13) and then terminate chains. 

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