Phenoxyl formation

The very rapid oxidation of phenols by solvent radical cations can be expected to yield phenol radical cations as the first products. These species are short-lived, except in highly acidic solutions, and were not observed in the microsecond pnlse radiolysis experiments described above. They were detected, however, in frozen matrices and with nanosecond pulse radiolysis Gamma irradiation of phenols in w-butyl chloride or in l,l,2-trichloro-l,2,2-trifluoroethane (Freon 113) at 77 K produced phenol radical cations, which were detected by their optical absorption and ESR spectra . Annealing to 133 K resulted in deprotonation of the radical cations to yield phenoxyl radicals. Pulse radiolysis of p-methoxyphenol and its 2,6-di-fert-butyl derivative in w-butyl chloride at room temperature produced both the phenol radical cations and the phenoxyl radicals. The phenol radical cations were formed very rapidly k = 1.5 x 10 ° M s ) and decayed in a first-order process k = 2.2 x 10 s ) to yield the phenoxyl radicals. The phenoxyl radicals were partially formed in this slower process and partially in a fast process. The fast process of phenoxyl formation probably involves proton transfer to the solvent along with the electron transfer. When the p-methoxy group was replaced with alkyl or H, the stability of the phenol radical cation was lower and the species observed at short times were more predominantly phenoxyl radicals. 

TTx represents the hydrophobicity of the substituents at position 10. Its positive coefficient (+0.75) suggests that the presence of highly hydrophobic substituents at position 10 increases the activity. The outlier (X = OH) is much more active than expected by 11 times the standard deviation. This may be due to the formation of a phenoxyl radical that interacts with DNA [48]. The other derivative (X = NH2) is also considered as an outher due to being much more active than expected by 14 times the standard deviation. This anomalous behavior may be attributed to its nature as an aniline. This could result in hydrogen abstraction, or involve microsomal N-oxidation [48,49]. 

The central feature of this mechanism is, therefore, that the phenoxyl radical is reversibly reduced and re-oxidised this leads to the continuous consumption of macroalkyl radicals. The phenoxyl radical can, therefore, react with polypropylene radicals and compete with PP-MA adduct formation in the stabilised polymer (Figure 3, curve MA-S). 

Phenoxyl radicals with alkoxyl substituents in the ortho- and pnra-positions decompose with the formation of alkyl radicals [55],... 

C) This group includes phenols with alkoxy substituents. Respective phenoxyl radicals decompose with the formation of chain-propagating alkyl radicals. In addition to inhibitory mechanisms determined by substituents, these phenols can realize mechanism IX. 

The intensive mechanistic studies of phenoxyl self-reactions proved a great variety of mechanisms and rate constants of these reactions [2,3,6], The substituents can dramatically influence the mechanism and kinetics of self-reactions. Due to free valence delocalization the phenoxyl radical possesses an excess of the electron density in the ortho- and para-positions. Mono- and disubstituted phenoxyls recombine with the formation of labile dimers that after enolization form bisphenols [3,6],... 

Trisubstituted phenoxyl radicals disproportionate if the para-substituent bears the a-C—H bond. Two ways were proposed [3] the direct disproportionation and the formation of labile dimer followed by the decay. 

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],... 

Synergism can be observed for mixtures of amines with 2,6-bis(l,l-dimethylethyl)phenol but not with monosubstituted phenols [19], There are two reasons for this. First, 2,6-dialkylphenols are characterized by D0 hn—H therefore, the equilibrium of the above reaction is displaced toward the formation of ArO. Second, phenoxyls like these are sterically hindered and, hence, must be less reactive in abstraction reactions. Thus, the necessary conditions for synergism to occur are the following. 

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). 

The redox potential E]/2 for the [Fenl(L )]+/[FemL] couple is a measure for the relative ease of coordinated phenoxyl radical formation Coordinated (LMe2)3- is most difficult to oxidize, followed by (Llk 2)3-, (LMetBu)3-, (LBuMet)3-, and (LMet2)3 which is most readily oxidized. 

The formation of coordinated phenoxyls in the monocations and dications, [Fe(L )]+ and [Fe(L )]2+, is clearly demonstrated by their electronic spectra (142). Fig. 23 displays the spectra of [Fem(LBuMet)]°, [Fe(LBuMet )]+, and [Fem(LBuMet )]2+. Since the spectrum of the neutral tris(phenolato)iron(III) species shows an absorption minimum at -400 nm it is significant that the monocation and dication both display a new intense asymmetric maximum in this region. This intense maximum is the fingerprint of phenoxyl radicals. It is also remarkable that this maximum doubles in intensity on going from the monocation to the dication. On increasing the oxidation level stepwise, the phenolate-to-iron CT band experiences a batho-chromic shift from 513 nm in the neutral species to 562 nm in the monocation and... 

The search for low-molecular weight (phenoxyl)copper(II) complexes as functional models for GO, which would mimick this reactivity, had a promising start in 1996 when Tolman and co-workers (202) reported that electrochemical one-electron oxidation of Cull(,L,lil 2)(bcnzylalcoholatc) (Fig. 27) resulted in the formation of benzaldehyde (46%) and some other decomposition products of the ligand H L,Bu2 in <5% yield and probably a Cu(I) species of unknown composition. These authors suggest that a (phenoxyl)copper(II) intermediate Cull(,L,l l 2 )(bcn-zylalcoholate)]+ is formed and that the reaction sequence, as in Fig. 8, leads to the observed products. Although this represents a stoichiometric reaction, it demonstrated for the first time that GO chemistry can be successfully modeled. 

Chaudhuri et al. (216) reported that the dinuclear bis(phenoxyl)dicopper(II) species [Cu2(Ls )2]Cl2 (Fig- 30) reacts under anaerobic conditions in dry THF in a stoichiometric fashion with primary and secondary alcohols (ethanol, benzyl alcohol, 2-propanol, diphenylcarbinol, and 2-butanol) with formation of two different products, namely, aldehydes (or ketones) and 1,2-diols (and/or other oxidative C— C coupling products). 

However, the first four products identified 44), viz. dehydro-diconiferyl alcohol (II), DL-pinoresinol (IV), guaiacylglyccrol-p-coniferyl ether (VI) and conifcraldchydc (VII) ipOa) already revealed the nature of most of the secondary reactions taking place after formation of the free phenoxyl radicals by the enzymes, although this was not immediately... 

Nevertheless, there are two highly efficient CL systems which are believed to involve the CIEEL mechanism in the chemiexcitation step, i.e. the peroxyoxalate reaction and the electron transfer initiated decomposition of properly substituted 1,2-dioxetanes (Table 1)17,26 We have recently confirmed the high quantum yields of the peroxyoxalate system and obtained experimental evidence for the validity of the CIEEL hypothesis as the excitation mechanism in this reaction. The catalyzed decomposition of protected phenoxyl-substituted 1,2-dioxetanes is believed to be initiated by an intramolecular electron transfer, analogously to the intermolecular CIEEL mechanism. Therefore, these two highly efficient systems demonstrate the feasibility of efficient excited-state formation by subsequent electron transfer, chemical transformation (cleavage) and back-electron transfer steps, as proposed in the CIEEL hypothesis. 

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