Phenolic radical

Reaction that can be carried out by the oxidative coupling of radicals may also be initiated by irradiation with UV light. This procedure is especially useful if the educt contains oleflnic double bonds since they are vulnerable to the oxidants used in the usual phenol coupling reactions. Photochemically excited benzene derivatives may even attack ester carbon atoms which is generally not observed with phenol radicals (I. Ninoraiya, 1973 N.C. Yang, 1966). 

Ni(salen)-DNA adduct formation is closely related to that formed by the Ni(peptide) systems, although there are different mechanisms proposed for both types of complexes. In the case of Ni(salen), the addition of a phenol radical to the guanine heterocycle and formation of a covalent bond to guanine C8 (Equation (9)) is suggested. 

Bisbenzylisoquinoline alkaloids are dimeric benzyltetrahydroisoquinoline alkaloids that are known for their pharmacological activities. A well-described example is the muscle relaxant (+)-tubocurarine, which in crude form serves as an arrow poison for South American Indian tribes. In the biosynthesis of this broad class of dimeric alkaloids, it has been postulated that the mechanism of phenol coupling proceeds by generation of phenolate radicals followed by radical pairing to form either an inter- or intramolecular C - O or C - C bond. Enzyme studies on the formation of bisbenzylisoquinoline alkaloids indicated that a cytochrome P-450-dependent oxidase catalyzes C - O bound formation in the biosynthesis of berbamunine in Berberis cell suspension culture.15 This enzyme, berbamunine synthase (CYP80A1), is one of the few cytochromes P-450 that can be purified to... 

Photo-induced electron-transfer decarboxylation reactions have been reviewed. A variety of methyl- and methoxy-substituted phenol radical cations have been generated by either photo-induced electron transfer or photo-ionization in dry solvents such as acetonitrile. In the presence of small amounts of water the radical cations are not detected and the phenoxyl radical is the only transient species observed. The 2-methoxyphenol radical cation was found to be more reactive than the 4-methoxy radical cation. 

Oxidative coupling involves condensation reactions catalyzed by phenol oxidases. In oxidative coupling of phenol, for example, arloxy or phenolate radicals are formed by the removal of an electron and a proton from an hydroxyl group. The herbicide 2,4-D is degraded (Fig. 15.5) to 2,4 dichlorophenol, which can be oxidatively coupled by phenol oxidases (Bollag and Liu 1990). 

Phenols show a two-electron oxidation wave on cyclic voltammetry in acetonitrile at a less positive potential than for the con-esponding methyl ether (Table 6.5) or a related hydrocarbon. Phenol radical-cation is a strong acid with pKg ca. -5 in water [93], so the two-electron oxidation wave for phenols is due to formation of a phenoxonium ion such as 13, where the complete oxidation process is illustrated for 2,4,6-tri-tt rf-butylphenol. Phenoxide ions are oxidised at considerably less positive potentials than the conesponding phenol. They give rise to a one-electron wave on cyclic voltammetry in aqueous acetonitrile or in aqueous ethanol containing potassium hydroxide. 2,4,6-Tri-/ert-butyiphenoxide ion is reversibly oxidised to the radical in a one-electron proces.s with E° = -0.09 V V5. see. The radical undergoes a further irreversible one-electron oxidation with Ep = 1.05 V vs. see on cyclic voltammetry forming the phenoxonium ion which reacts with water [94J. 

A possible reductive role for veratryl alcohol oxidase is proposed in Figure 5. Laccases from C. versicolor can produce both polymerization and depolymerization of lignin (29). In phenolic lignin model dimers, laccase can perform the same electron abstraction and subsequent bond cleavage as found for lignin peroxidase (30). The phenolic radical is however likely to polymerize unless the quinoid-type intermediates can be removed, for example by reduction back to the phenol. Veratryl alcohol oxidase, in... 

Figure 9 shows plots of Hammett fr+ values versus E j2 for the 8-p-X-Ph-dG adducts. In Fig. 9A, the OH (—0.92 ) fr+ value was used and the regression deviated from linearity. However, Fig. 9B shows that the regression is improved to almost unity when the O (2.30 ) fr+ value is used. These results suggested that the oxidation of 8-p-PhOH-dG may be coupled with phenol deprotonation. As shown in Scheme 12, resonance structures for the radical cation of 8-p-PhOH-dG create a p-substituted phenol radical cation, which possess negative pAa values (pifa for phenol radical cation ). Phenolic radical cations undergo deprotonation rapidly in the presence of water (0.6-6 x to yield neutral phenolic radicals. In the anhydrous DMF solvent used for electrochemical measurements, an N-7 adduct atom or adventitious water in the solvent could serve as base to facilitate phenolic radical production. 

Scheme 12 Formation of the phenolic radical following oxidation of 8-p-PhOH-dG.

Overall, these results show that attachment of the phenolic ring to dG expands the redox chemistry of the nucleoside to generate the phenolic radical instead of the purine radical cation that reacts with water. Given that the 5-(2 -deoxyuridinyl)methyl radical (T(—H) ) generates the fate of the phe-... 

PCP-phenolic radical that possesses an absorption spectrum in H2O at 440 nm and decays via second-order kineties with k = 9. x 10 /M/s, as evidenced by pulse radiolytie studies. The PCP-phenolic radical can attach covalently to the C8-site of dG to generate the C8-OPCP adductIn the presence of GSH, redox cyeling of the phenoxyl radieal with thiyl radieal generation will yield a GSH disulfide anion radieal that ean reduetively aetivate O2 to generate the superoxide radieal anion (02 ) that ean generate free ferrous iron Free Fe " " may... 

Abbreviations Lf = lumiflavin Rf = riboflavin Me = methyl [Pg.121]

Phenol radical cations exist only in strong acidic solutions (pKa -1) [1, 2]. However, in non-polar media phenol radical cations with lifetimes up to some hundred nanoseconds were obtained by pulse radiolysis [3], The free electron transfer from phenols (ArOH) to primary parent solvent radical cations (RX +) (1) resulted in the parallel and synchroneous generation of phenol radical cations as well as phenoxyl radicals in equal amounts, caused by an extremely rapid electron jump in the time scale of molecule oscillations since the rotation of the hydroxyl groups around the C - OH is strongly connected with pulsations in the electron distribution of the highest molecular orbitals [4-6]. 

From pulse radiolysis lifetimes of phenol radical cations between 300 and 500 ns are known [4, 9]. Laser photolysis (3 ns, 266nm up to 15 mJ) ofN2-purged solutions of up to 10 3 mol dm 3 phenols yields phenoxyl radicals as dominating products (Figure 2). In the spectrum only a little hint for the phenol radical cations exists. The inset shows that the phenoxyl radical formation does not depend linearly from the energy but appears by biphotonic absorption contradictory to the fs-experiments described above. 

Because the p value is positive, negatively charged carbon ions are considered to be the primary transition state complex (TSC). The TSC will dissociate to a substituted phenol radical and a stable anion. It may also be neutralized by the toluene, resulting in the addition of a proton, H+ (Arai and Dorfman, 1964) therefore, eaq is considered to interact with the ir-orbital of the ring as in electrophilic substitution rather than to affect electron distribution and polarizability of a certain substituent. 

Dixon WT, Murphy D (1976) Determination of the acidity constants of some phenol radical cations by means of electron spin resonance. J Chem Soc Faraday Trans 272 1221-1230 Dizdaroglu M, Scherz H, von Sonntag C (1972) y-Radiolyse von meso-Erythrit in wassriger Losung. Z Naturforsch 27b 29-41... 

Recently, a copper-catalyzed synthesis of trimethyl- 1,4-benzoquinone, a key intermediate in the industrial synthesis of vitamin E, has been reported (Eq. 11) [49]. In the proposed mechanism, a tetranuclear cluster [Cu4(a -O)Cl10], isolated from the reaction mixture, deprotonates phenol and oxidizes it to a copper-bound phenolate radical, which reacts with dioxygen. 

We soon realized that the photodecoration of leather was a big challenge. The tanning process introduced a large number of chemicals into hides, which already contained many reactive species, including phenolic substances that could interact with our chemistry. We thus had to learn to put blocking layers onto the hides and, subsequently, embed our chemistry in polyurethane binders, which would provide the required flexibility to these coatings. After imaging, a fix layer, which would contain phenolic radical traps and UV absorbers, was applied. 

Phenolate radical coupling in synthesis and biosynthesis Pummererys ketone... 

The pioneer in phenolate radical coupling was Pummerer. In 1925 he showed1 that one electron oxidation of />-cresol using potassium ferricyanide afforded a nicely crystalline ketonic dimer of the radical in up to 25 % yield. Pummerer s ketone, as it became known, was considered to result from the coupling of two p-cresol radicals to give the dienone 1. This then underwent spontaneous cyclization to furnish 2. As proof of the structure... 

It has not yet been proven that the coupling of phenolate radicals takes place in the biosynthesis of phenolic alkaloids. However, there is no evidence against, and much circumstantial support for, this proposal. The brilliant work of Zenk1517 has shown that the oxidative step in morphine biosynthesis is carried out by a single P-450 type iron porphyrin enzyme, which has been isolated pure and cloned from the appropriate... 

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