Phenoxy radicals dimerization

The recognition2 that the a, p-dimer 3 is formed in equilibrium with 1 and not the a,a-dimer 2 was interpreted as a result of the smaller steric strain in 3 than in 23 Also the known strong influence of p-substituents on the equilibrium constants between substituted trityl radicals and their dimers6 found an obvious explanation in this way. The earlier observation that not only those phenoxy radicals 4 carrying three conjugating phenyl substituents 4 (R = C6H5)7a are persistent8 but also their... 

If the five resonance forms of the phenoxy radical (Figure 3.6) can couple to any other phenoxy radical, the theoretical number of dimeric structures possible is 25. The relative frequency of involvement of individual sites in the phenolic coupling reaction depends on their relative electron densities. Quantum mechanical calculations predict that the high electron densities at the phenolic oxygen atom and the carbon atom would give rise to a high proportion of fi-O-4 linkages, which is indeed observed to be the case (Table 3.1). 

The polymerization mechanism of phenols is described as follows. The phenol is adsorbed on the electrode surface and accumulated in the diffusion layer. The adsorbed phenol undergoes one-electron oxidation to the phenoxy radical on the electrode surface. The concentrated phenoxy radical is coupled with each other at p-position to form the dimer, and the dimer repeats the electro-oxidation and coupling. The phenoxy radical is assumed to he adsorbed or oriented upon the electrode surface thus resulted in the selective coupling reaction. 

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). 

The intermolecular coupling of phenols is used extensively in what are believed to be biomimetic alkaloid syntheses. Aqueous solutions of iron(lll) salts are most frequently used as the oxidising agent and the dimerization process must involve phenoxy radicals. Examples are the dimerization of orcinol 21 [114] and the formation of bis-benzyltetrahydroisoquinolines 22 [115],... 

Coupling of phenoxy radicals to give dimeric quinone methides 1. Homolytic or heterolytic cleavage of side-chains (Ca-C/ , alkyl-phenyl) and aromatic rings... 

Autoxidation of mercaptans gives rise to thiyl radicals, but these, like phenoxy radicals, are inert toward oxygen and normally dimerize to disulfides. Their participation in a chain reaction can be achieved in the co-oxidation of olefins and mercaptans, first demonstrated by Khar-asch (12), which takes advantage of the rapid addition of thiyl radicals to double bonds. 

The rate of transfer is accelerated by electron-releasing substituents on the aromatic ring of the antioxidant and retarded by steric protection of the labile hydrogen or its replacement by deuterium. The subsequent fate of the radical A determines the over-all kinetics of the inhibited reaction and the practical usefulness of the antioxidant. If A is a fairly stable phenoxy radical, it will probably add a peroxy radical or dimerize. 

Other methods which have proved of value include the formation of substituted methides by the action of silver oxide on phenols (70JOC3666). It is postulated that upon oxidation of the phenol a phenoxy radical is formed which dimerizes to the quinol ether. Disproportionation to the methide and the original phenol follows. 

In this copper-amine complex, electron transfer from oxygen to copper would give a phenoxy radical which, since the principal reaction of free phenoxy radicals appears to be C-C coupling, in this case must remain bound to the catalyst. Coupling of two of these radicals would then give dimer. 

Apparently, dimerization of the above phenoxy radical through either oxygen or the ring is inhibited by the bulky tert-butyl groups. With fewer or smaller substituents, the phenoxy radicals may form dimerization or disproportionation products. Examples of these reactions follow. 

Thus, a-carbonyl syringols can be oxidized to relatively stable phenoxy radicals in contrast to guaiacol derivatives. The latter probably dimerize... 

Phenols act as acids toward [ (tmpa)Cu 2(02)]2+ (3) and Cu2(XYL—O—)(02)] + (8), hydronating them to give hydrogen peroxide and phenoxo-copper(II) complexes. However, hydrogen-atom abstraction takes place when complex [Cu2(N4)(02)]2+ (15) is reacted with phenols, giving phenoxy radicals that dimerize to produce biphenols or diphenoquinones, depending on the position of the substituents on the phenols. This shows that 15 is a better one-electron oxidant than 3 or 8. 

An Sjuyl-type (S l ) mechanism has been proposed in the synthesis of poly(2,6-dimethyl-l,4-phenylene ether) through the anion-radical polymerization of 4-bromo-2,6-dimethylphenoxide ions (204) under phase-transfer catalysed conditions269. Ions 204 are oxidized to give an oxygen radical 205. The propagation consists of the radical nucleophilic substitution by 205 at the ipso position of the bromine in 204 (equation 144). The anion-radical 206 thus formed eliminates a bromide ion to form a dimer phenoxy radical 207 (equation 145). A polymeric phenoxy radical results by continuation of this radical nucleophilic substitution. 

The final products of phenol oxidation are generated by secondary reactions of the radicals produced by the peroxidase. One very common pathway involves dimerization or oligomerization of the radicals, as illustrated in Fig. 5.14 for the oxidation of a para-substituted phenol such as tyrosine [R = CH2CH(NH2)C02H]. The dimerization can occur between two ring carbon atoms or by addition of the oxygen of one phenoxy radical to a ring carbon of the other. 

Notwithstanding the phenol dimers are builder compounds with respect to the starting substrate, thus making their access to the peroxidase active site more difficult, they have a lower redox potential and compete with monomeric phenols in the reaction with compound I and compound II. Furthermore, the phenoxy radical can oxidize a phenol dimer to its radical form. This results in the formation of oligomeric and then polymeric compounds at longer reaction times [17]. Thus, if the products of interest are the diphenolic compounds, the reaction must be carried out in mildest conditions and only the products formed in the initial phase have to be collected. But in some cases [18-20], the o.o -biphenyl is the principal product of phenol oxidation, such as with p-cresol. tyrosine, etc. All peroxidases can be employed for these reactions, but HRP is usually preferred with respect to other peroxidases due to its higher availability and to its broad specificity. Whole cell... 

The enzymic dehydrogenation reaction is initiated by an electron transfer which results in the formation of resonance-stabilized phenoxy radicals (Fig. 4-4). The combination of these radicals produces a variety of dimers and oligomers, termed lignols (Fig. 4-5). 

Oxidative C-C coupling of phenoxy radicals to give symmetrical biphenyl derivatives has been observed in the anodic dimerization of sodium vanillate to biphenyl (VIII), as in Eq. (3) [18] and in anodic oxidation of 2,6-dihydroxy-acetophenone at a Pt anode in aqueous methanolic NaOH to give biphenyl (IX), as in Eq. (4) [19],... 

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