Aryloxy radicals oxidative coupling

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

The halogen displacement polymerization proceeds by a combination of the redistribution steps described for oxidative coupling polymerization and a sequence in which a phenoxide ion couples with a phenoxy radical (eq. 11) and then expels a bromide ion. The resultant phenoxy radical can couple with another phenoxide in a manner that is analogous to equation 11 or it can redistribute with other aryloxy radicals in a process analogous to equations 7 and 8. 

When chloroform or methanol is used as the solvent for the oxidation of phenols, other products, originating from coupling of aryloxy radicals, e.g., polyphenylene ethers and/or diphenoquinones, are also formed. ... 

The reactive intermediates in oxidative coupling of phenols are aryloxy radicals. Growth may occur by the successive addition of aryloxy units as shown in Reaction 5. 

Oxidation of phenolates leads to dehydro dimers by coupling of two aryloxy-radicals in o- or p-position. Electrolysis of vanilline in acetonitrile-Et4NOH yields 65% dehydrodivanilline (Eq. (160)) 28°). 

Metal-based reagents are often used to promote one- and two-electron oxidations of arenols. Radical-mediated C-0 coupling reactions of aryloxy radicals can, of course, lead to ortho-quinol derivatives, but the preparative value of such an approach is poor and essentially limited to intramolecular cases. For example, certain bis-phenols such as 36a-c have been spiroannulated in good yields by diradical C-0 coupling under favorable one-electron oxidation regimes (Figure 12) [65-67]. 

The oxidative coupling of 2,6-disubstituted phenols to poly-(arylene oxides) is a polycondensation reaction, in which polymer molecules couple with other polymer molecules as well as with monomer. Unstable quinone ketals formed by coupling of a polymeric aryloxy radical at the para position of the phenolic ring of a second radical are believed to be intermediates or the reaction. The ketals may be converted to polymeric phenols either by a series of intramolecular rearrangements or by disproportionation to aryloxy radicals, leading to a mobile equilibrium between polymer molecules of varying degree of polymerization. Both processes have been shown to occur, with their relative importance determined by the reaction conditions. 

It has generally been accepted that aryloxy radicals are intermediates in the polymerization, largely because the effective reagents are those capable of one-electron transfer. This assumption has been confirmed recently by the identification of both monomeric and polymeric aryloxy radicals in the ESR spectra of polymerizing solutions of 2,6-xylenol (21). The first step in the reaction is the oxidation of the phenol to the aryloxy radical by Cu(II). Carbon-oxygen coupling of two aryloxy radicals yields the cyclohexadienone, which tautomerizers to the dimer (II) (Reaction 3). 

The simplest explanation for the formation of high molecular weight polymer through oxidative coupling of aryloxy radicals involves the successive addition of monomer units to the radicals derived from polymer phenols (Reaction 4). 

The experiments cited above show that redistribution, presumably via a quinone ketal intermediate, occurs during the oxidative polymerization of 2,6-xylenol and must be responsible at least partially for the polycondensation characteristics of the reaction. Although the conditions under which Mijs and White demonstrated rearrangement are different from those usually employed for oxidative polymerization of xylenol, it appears certain that this process also contributes to the coupling of polymer molecules. Redistribution and rearrangement are complementary reactions. Dissociation into aryloxy radicals can occur at any point... 

These condensations, like the oxidative coupling of phenols, presumably are free radical chain reactions with aryloxy radicals as intermediates, but the gross features of the two types of reactions are quite different. At low extents of oxidation the oxidative coupling reaction... 

This is the best known and most quoted chemistry in this area. Oxidation of a phenol as in Scheme 7, with loss of one proton and one electron, provides an aryloxy radical, which may dimerize three modes of C—C union are possible, and two modes of C—O bonding. Spin density is greatest, in general, in the para position, but coupling is reversible (before intermediate dienone phenolization) and product ratios depend on pH, temperature, concentration and oxidant, for a given phenol. Mixtures of isomeric dimers may, and often do, result. If different phenols are present then potential products are numerous. A survey of the literature indicates that reasonable chemospecificity is most likely observed either in cases of phe-... 

Oxidative coupling or condensation reactions involving toxic organics are catalyzed by phenol oxidases such as laccases or peroxidases. In the oxidative coupling of phenol, for example, aryloxy or phenolate radicals are formed by removal of an electron and a proton from the hydroxyl group. The resulting phenolate radicals then couple with phenolic or other compounds to yield dimerized or polymerized products (Brown, 1967). 

Copper(II) acetate in MeOH containing morpholine gives o-benzoquinone as the primary product [13,65]. Aryloxy radicals are formed in a side-reaction only. Copper(I)-amine systems in non-polar solvents afford aryloxy radicals from phenols, and the corresponding coupling products [66-69]. Copper(II) complexes of aliphatic and heterocyclic amines are known to catalyze the oxidation of substituted phenols [70-72]. The basicity and steric properties of the ligand influence the nature of oxidation products. 

The coupling reaction involves oxidation of 2-naphthol by electron transfer to give an aryloxy radical, which then dimerizes to yield the product. The mechanism is shown here ... 

---