Phenoxide radicals

Figure 3.6 Resonance forms of phenoxide radicals generated in biosynthesis.

Figure 3.7 Formation of oligolignols by non-enzymatic coupling of phenoxide radicals.

Nishinaga and co-workers isolated a series of stable cobalt(III)-alkyl peroxide complexes such as (170) and (171) in high yields from the reaction of the pentacoordinated Co"-Schiff base complex with the corresponding phenol and 02 in CH2C12. Complex (170 R=Bu ) has been characterized by an X-ray structure. These alkyl peroxide complexes presumably result from the homolytic addition of the superoxo complex Co111—02 to the phenoxide radical obtained by hydrogen abstraction from the phenolic substrate by the CoUI-superoxo complex. The quinone product results from / -hydride elimination from the alkyl peroxide complex (172)561,56,565,566 The quinol (169) produced by equation (245) has been shown to result from the reduction of the CoIU-alkyl peroxide complex (170) by the solvent alcohol which is transformed into the corresponding carbonyl compound (equation 248).561... 

Under different reaction conditions, phenols can be oxidized to p-quinones (equations 272600-602 and 273603), but in the case of phenol itself, insufficient selectivity has prevented, as yet, the commercial application of this potentially important synthesis of p-benzoquinone and hydroquin-one. The selectivity of p-benzoquinone, or p-quinol formation can be increased at the expense of oxidative coupling products by using a large excess of the copper reagent [Cu4Cl402(MeCN)3 or CuCl + 02 in MeCN] with respect to the phenolic substrate.604 The suggested mechanism involves the oxidation of the phenoxide radical (189) by a copper(II)-hydroxo species to p-quinol (190) which can rearrange (for R2 = H) to hydroquinone (191 Scheme 14), which is readily oxidizable to p-quinone.6... 

Most phenols have one electron redox potential, which allows their oxidation by compound I and compound II to phenoxide radicals according to the following reactions ... 

Phenol, 29, 88-89, 239, 259, 334, 345, 454, 473, 503, 513, 649, 673, 828, 964, 1292 methyl ether, 72, 206 Phenol — cydohexanoi, 724 Phenol methyl ethers, 964 Phenols, 430 hydrogenation, 979 oxidation, 550-551 protective derivatives, 394 reduction ArOH ArH, 251-252 Phenothiazine, 1215 Phenoxide radicals, 640 Phenoxthin, 1120 a-Phenoxyacetophenone, 276 d-(p-Phenoxybenzoyl>propioiiic add, 435 1-Phenoxynaphthalene, 170 -(p-Phenoxyphenyl)-butyric acid, 435 Phenoxytriphenylphosphonium bromide, 1274... 

The oxidative coupling uses a copper-catalysed system and a base, usually an aliphatic or heterocyclic amine, and oxygen as the oxidizing agent. In broad terms, free-radical processes are involved to explain the polymerization pathway which involves formation of the phenoxide radical, and coupling of two radicals through the attack by an oxygen-centred radical at the para position of another phenolic molecule (Scheme 25). 

Note that the phenoxide radical has several resonance structures including those which contain the free radical inside the benzene ring. This radical may attack phenol to form a dimer which quickly tautomerizes to give 4,4 -di-hydroxybiphenyl, which in turn will repeat the previous sequence to form a more complex quinoid structure or it may react with the peroxide catalyst to form a two ringed quinone. Summarizing ... 

The first step of the reduction process involves adsorption of oxygen at the reduced Fe VCu center to form an Fe -02 superoxide adduct with subsequent formation of an intermediate comprised of oxidized Cu an Fe =0 fenyl radical, and a peripheral phenoxide radical (Fig. 7.7). The oxidized intermediate is then reduced directly to water by simultaneous transfer of four electrons [35],... 

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. 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

The situation is not as clearly solved in a positive or negative sense for arenediazo phenyl ethers. Here three alternatives have to be considered, namely an intramolecular rearrangement of the arenediazo phenyl ether (Scheme 12-11, A), and two types of intermolecular rearrangement, either by heterolytic dissociation into a diazonium ion and a phenoxide ion (B) or by homolytic dissociation into a radical pair or two free radicals (C). 

The authors formulate the mechanism in two steps, first an electron transfer from phenoxide ion to diazonium ion forming a radical pair, followed by attack of the diazenyl radical at the 4-position of the phenoxy radical and a concerted proton release, i. e., without involving the o-complex. Admittedly, there is no experimental evidence against such a concerted process, but also none for it It seems that those authors wanted only to demonstrate the occurrence of radical intermediates, but did not consider the question of the mechanism of the proton release. 

Radical cyclization to triple bonds is used as the key step for the synthesis of oxygen heterocycles. This methodology can benefit from a Lewis acid, such as aluminum fns(2,6-diphenyl phenoxide) (ATPH), which forms a complex with the... 

Some of the reactions in this chapter operate by still other mechanisms, among them an addition-elimination mechanism (see 13-15). A new mechanism has been reported in aromatic chemistry, a reductively activated polar nucleophilic aromatic substitution. The reaction of phenoxide with p-dinitrobenzene in DMF shows radical features that cannot be attributed to a radical anion, and it is not Srn2. The new designation was proposed to account for these results. 

Pendent arm 1,4,7-triazacyclononane macrocycles (91) and (92) have been used to stabilize the zinc-to-phenoxyl bond allowing characterization of these compounds.477 The interest in the zinc complexes comes from the wide potential range in which it is redox stable allowing observation of the ligand-based redox processes, this allows study of the radical by EPR and the electronic spectra is unperturbed by d-d transitions. Macrocycles of the type l,4,7-tris(2-hydroxybenzyl)-1,4,7-triazacylononane form a bound phenoxyl radical in a reversible one-electron oxidation of the ligand. The EPR, resonance Raman, electronic spectra, and crystal structure of the phenoxide complexes were reported. This compound can be compared to a zinc complex with a non-coordinated phenoxyl radical as a pendent from the ligand.735... 

We might well expect the resultant phenoxy radical to attack— through the unpaired electron on its O, or on its o- or p-C, atom—a further molecule of phenol or phenoxide anion. Such homolytic substitution on a non-radical aromatic substrate has been observed where the overall reaction is intramolecular (all within the single molecule of a complex phenol), but it is usually found to involve dimerisation (coupling) through attack on another phenoxy radical ... 

The synthesis of oxygen heterocycles in which cyclization onto a pendant alkyne is a key step has also been achieved. Reaction (7.36) shows an example of iodoacetal 29 cyclization at low temperature that afforded the expected furanic derivative in moderate Z selectivity [47]. A nice example of Lewis acid complexation which assists the radical cyclization is given by aluminium tris(2,6-diphenyl phenoxide) (ATPH) [48]. The (3-iodoether 30 can be com-plexed by 2 equiv of ATPH, which has a very important template effect, facilitating the subsequent radical intramolecular addition and orienting the (TMS)3SiH approach from one face. The result is the formation of cyclization products with Z selectivity and in quantitative yield (Reaction 7.37). 

Neutral organic molecules can also be one-electron donors. For example, tetracyano-quinodimethane gives rise to anion-radical on reduction with 10-vinylphenothiazine or N,N,N, N -tetramethyl-p-phenylenediamine. Sometimes, alkoxide or phenoxide anions hnd their applications as one-electron donors. There is a certain dependence between carbanion basicity and their ability to be one-electron donors (Bordwell and Clemens 1981). 

One postulate which does resolve many of the inconsistencies in the present data base is that there is a strong tendency towards alternation in these polymers. Such alternation could result from a combination of an intrinsically greater reactivity of the ortho-relative to para-positions of coordinated phenoxide, coupled with an incibility of radicals of the type 5 to attack coordinated phenoxide at the ortho-position due to steric blocking. (An examination of models shows this to be a plausible assumption). ... 

In the case of trichlorophenoxide such a model would give rise to an alternating 1,2-/1,4-sequence (which we call syndioregic by analogy with syndiotactic). In the case of 4-bromo-2,6-dichloro-phenoxide the much greater reactivity of the bromine relative to chlorine overides the factor which favoured ortho over para reactivity in the trichlorophenoxide case to such an extent that 1,4 catenation results. In the case of 2-bromo-4,6-dichlorophenoxide the 2-bromo position should be very reactive, but the radicals produced by such reaction (type 5) are not able to attack again at the ortho position of coordinated phenoxide. 

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. 

Steenken, S. and Neta, P., Electron transfer rates and equilibria between substituted phenoxide ions and phenoxyl radicals, J. Phys. Chem., 83, 1134, 1979. 

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