Radical aryloxy

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. 

This process, which predominates at low temperatures, causes migration of internal ketal stmctures along a chain but does not involve the dissociation to separate aryloxy radicals that occurs during the redistribution process. 

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 homolyses of 104 to 105 would be expected on the basis of findings that diaryl peroxides are stable only at very low temperatures, and decompose upon warming to aryloxy radicals (Walling, 1957). The conversion of 105 to 106 finds analogy in the known conversion of triphenylmethoxy radical to the more stable diphenoxymethyl radical (Wieland, 1911). The four-membered peroxide 107 would be expected by precedent to cleave to the ketoaldehyde 108 (Wei et al., 1967 White et al., 1969 Fenical et al., 1969). 

In addition to the absorptions attributable to aryloxy radicals. Fig. 1 displays a broad shoulder around 315 nm, much longer-lived, which is assigned to a 2,4-cyclohexadienone. This intermediate decays with a rate of 1.25 0.1 sec in hexane solution, to give 2-hydroxyacetophenone (11) via 1,3-hydrogen shift. The rate of appearance of 11 is coincident with the decay rate of the dienone. 

So far, the work dealing with the PFR mechanism has paid more attention to the detection and fate of aryloxy radicals than to acyl radicals. It is generally accepted that aryloxy radicals afford phenols (or naphthols) through H abstraction from the solvent, but little has been said about the other partners of the radical pair, namely the acyl radicals. Their detection has been accomplished both directly and indirectly. 

Polyethylene films influence the product distribution in a different way, depending on the nature of the aryloxy radical. Thus, the ortho/para ratio in the PFR of phenyl esters is 2, both in hexane and films. However, this ratio is very different for 1-naphthyl esters 2 in hexane and > 6 in the films. This has been ascribed to the more available rotational movement for phenoxyl than for naphthoxyl radicals. As a matter of fact, van der Waals volumes are quite different, being 81 A for the former and 124 A for the latter [292]. 

The most useful class of inhibitors are molecules that react with chain radicals to yield radicals of low reactivity. Quinones such as benzoquinone and chloranil (2,3,5,6-tetrachloro-benzoquinone) are an important class of inhibitor. The behavior of quinones is quite complex [Eastmond, 1976a,b,c George, 1967 Small, 1975 Yamamoto and Sugimoto, 1979 Yassin and Risk, 1978a, 1978b]. Two major types of products are obtained—quinone and ether—formed by reaction at the C and O atoms of a quinone, respectively. Attack of a propagating radical at oxygen yields the aryloxy radical XXVIII, which can terminate by... 

Recently, syntheses of the model compounds 156 and 157 were reported [107,108], which are closely related to earlier approaches [103,109] (Fig. 25). In agreement with theoretical calculations the CO complexes of the Fe(II)por-phyrins 156 and 157 display a split Soret band at 370/446 nm and 383/456 nm, respectively, but no experiments with molecular oxygen were reported. But it was demonstrated that 157 catalyzed the formation of stable aryloxy radicals from the corresponding phenols in the presence of e.g. feri-butylhydroperoxide (TBHP). These results indicate a thiolate mediated 0-0 bond cleavage of TBHP accelerated 240 fold in comparison to iron(III)tetraphenylporphyrin [108],... 

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. 

In one of the mechanisms advanced to explain the coupling of polymeric aryloxy radicals to a polymeric phenol polymer, polymer redistri-... 

Aryloxy radicals react with phenolic species, either by direct hydrogen transfer or by oxidation-reduction reactions with the catalyst as carrier, to form new aryloxy radicals, which continue the redistribution. 

There are, however, other possible routes to block copolymers successive addition of units of the reactive monomer to the polymer already present, Reaction 5 termination reactions between polymer molecules —side reactions of unknown nature lead to loss of reactive hydroxyl groups (18) possible reactions are ortho carbon-carbon coupling followed by dimerization, addition of amine or water to the ketal intermediate, etc. Block copolymers might even be formed by polymer-polymer redistribution assuming that such redistribution in polymers of greatly different reactivities (such as DMP and DPP), takes place almost exclusively in one type of polymer sequence—that is, that bond scission in a "mixed ketal such as IV occurs always in the same direction—to produce the aryloxy radical corresponding to the more reactive monomer. None of these possible sources of block copolymer can be ruled out on the basis of available evidence. All could produce homopolymer in addition to block copolymer. All of the polymers produced in this work, except for those characterized as completely random copolymers, probably contained at least small amount of one or both homopolymers. 

Reaction 3 provides a means for transferring an aryloxy unit from one radical to another. Hydrogen transfer reactions between phenols and aryloxy radicals occur so rapidly that the over-all reaction is a chain process which requires only initiation to proceed to the equilibrium mix-... 

Termination occurs when the reaction of two aryloxy radicals produces a single polymeric phenol, as when a monomer unit is added (Reaction 8). 

Thus, for example, Stone and Waters (1962) have prepared a range of aryloxy radicals ArO by a method involving oxidation by aqueous ceric ion in a flow system which enabled continuous spectral sampling across the important region of magnetic field for radicals whose half-life was of the order of only 10-2 sec. 

Selected Proton Hyperfine Coupling Constants for Aryloxy Radicals (G)... 

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°). 

The paramagnetic 1 1 adduct is probably the active catalyst in these reactions. The initial step may involve hydrogen transfer or electron transfer to give aryloxy radicals that react further immediately or diffuse out of the solvent cage and react with another molecule of catalyst or with themselves (Scheme 3). More work is necessary to resolve the mechanism of this interesting and synthetically useful reaction. 

Contrary to our results, other workers (4, 9, 20, 36) state that in the stabilization of carotene, paraffin wax, and lard the activity of pyrocatechol is favorably affected by substitution at position 4, not only by normal but by tertiary alkyl groups as well. Disparate influences of substitution are not surprising when comparing the activity in different substrates owing to the possibility of directive influences in the process of inhibited oxidation. The participation of phenolic antioxidants in the inhibition of autoxidation can be demonstrated (1, 2, 3) simply as a reaction between the molecule of antioxidant AH and the alkylperoxy radical ROO formed duririg the autoxidation of the substrate RH. During this process, an aryloxy radical (A ) is first generated. 

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). 

Quinone Ketal Redistribution. This mechanism suggests that in the coupling of two aryloxy radicals the oxygen atom of one attacks at the para position of the phenolic ring of the second to yield the unstable quinone ketal. This rapidly decomposes either to yield the aryloxy radicals from which it was formed or two different aryloxy radicals, as shown... 

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