Diphenoquinone, formation

Oxidation of Mixtures of Monomers. The method most likely to yield random copolymers of DMP and DPP is the simultaneous oxidation of a mixture of the two phenols, although this procedure may present problems because of the great difference in reactivity of the two phenols. The production of high molecular weight homopolymer from DPP is reported to require both a very active catalyst, such as tetramethylbutane-diamine-cuprous bromide, and high temperature, conditions which favor carbon-carbon coupling and diphenoquinone formation (Reaction 2) from DMP (II). With the less active pyridine-cuprous chloride catalyst at 25 °C the rate of reaction of DMP, as measured by the rate of oxygen... 

A comparison of phenolic coupling on solid potassium permanganate and on potassium manganate surfaces has been made to emulate the natural biological process (ref.4). Diphenoquinone formation in more than 90% yield was observed with the oxidation of 2,6-dimethyl, 2,6-di-isopropyl, and 2,6-di-tert-butyl phenol (R = Me,i-Pr,t-Bu) in chloroform solution on (a) solid potassium permanganate whereas underthe same conditions (b) potassium manganate. 

The observation that quinone formation on a polymer-bound Co(NMesalpr) catalyst is favored against diphenoquinone formation by higher cobalt loadings [40] is indirect evidence for cobalt involvement after the H-atom abstraction step. Significantly, with... 

The catalytic activity of copper complexes in oxidative coupling of 2,6-DMP to PPO is significantly improved when polymer-bound 4-aminopyridine is used as ligand [92]. Basic copper-amine complexes also catalyze the oxidative coupling of 2,6-disubstituted phenols [82]. Depending on the size of substituents and the conditions, polymerization or diphenoquinone formation may predominate [83-86]. Small substituents like methyl favor the PPO product. 

The oxidation of dihydroxy aromatic compounds under the conditions used by Goldschmidt ususually leads to the formation of quinones rather than diradicals. For example, >,/> -dihydroxydiphenyl gives >-diphenoquinone. Several attempts have been made to oxidize o.o -dihydroxydiphenyl, but without success. The product would be of special interest because of the possible equilibrium among diradical, quinone, and peroxide isomers ... 

Poly(phenylene oxide) PPO, or poly(phenylene ether) PPE, is an engineering polymer developed by General Electric. It concerns the oxidative coupling of phenols discovered in 1956 by Allan S. Hay [21], Oxidative coupling leads to the formation of carbon-oxygen bonds between carbon atoms 2,4, and 6 and the phenolic oxygen atom. To avoid coupling with carbon atoms 2 and 6, alkyl substituents at these two positions were introduced. In addition to the polymer a 4,4 dimer is formed, named diphenoquinone (DPQ). The... 

Homogeneous catalysts have a very low selectivity for 2,6-dialkyl-p-benzoquinone (I), whereas zeolites combine high activities with high selectivities towards formation of (I). The formation of poly(2,6-dialkyl-l,4-phenylene ether) (III) is suppressed on zeolites by steric constraints. The formation of 3,3, 5,5 -tetraalkyl-4,4 -diphenoquinone (II) is suppressed in the supercages, but promoted by high concentrations of phenol. 

Polymer formation readily occuis if the substituent groups are relatively small and not too electro-negative. When the substituents are bulky, the predominant product is the diphenoquinone formed by a tail-to-tail coupling. No appreciable reaction occurs when 2,6-dinitrophenol is oxidized even at 100°C. 

The base promotes the formation of a phenolate ion, which undergoes a one-electron oxidation to form Cu(I) and a phenoxy radical. Two of these radicals combine to give the 4,4/-dihydroxybiphenyl compound, which can be further dehydrogenated to give the diphenoquinone. Within the detection limit of atomic absorption spectroscopy no Cu was observed in solution. Cu retention on the molecular sieve in this case is favored by the apolarity of the solvent, the absence of competing anions (e.g., acetate in solution), and the presence of base, with the latter promoting formation of copper hydroxides. 

Oxidation of 2-iodo-4,6-di(terr-butyl)phenol (702) with K3Fe(CN)e in aq. KOH (room temp., 20 min) also provided in 82% yield the corresponding benzoxete 703 probably through o-diphenoquinone. In contrast, similar oxidation of three 2-halo-4,6-di(ferf-butyl)phenols (704, 705 and 706) mainly afforded a dibenzofuran derivative 707 (81%), a mixture of 708 and 709 (23 and 53%, respectively) and a diaryl ether 710 (63%), respectively (Scheme 138). Clearly, iodine and bromine substituents promote the diaryl formation, while chlorine and fluorine substituents prefer to produce diaryl ethers. These results seem to be in good agreement with the ab initio calculations . 

In the case of 2,6-disubstituted phenols, the nature of the bismuth reagent, the nature of the alkyl substituents and the reaction conditions determined the outcome of the reactions. 2 Thus, in the reactions of 2,6-dimethylphenol (19) with pentaphenylbismuth (4) or with tetraphenylbismuthonium derivatives under basic conditions, ortho C-phenylation resulted in the formation of 6-phenylcyclo-hexadienone (20) in good yield. On the other hand, oxidative dimerisation took place in the reaction of 2,6-dimethylphenol with triphenylbismuth carbonate to afford the diphenoquinone (21) quantitatively 25... 

The behaviour of the most sterically hindered 2,6-di-rerr-butylphenol (25) and its derivatives was very dependent upon the reaction conditions. In the presence of a base favouring electron-transfer (such as BTMG), oxidation reactions took place with triphenylbismuth dichloride or triphenylbismuth carbonate leading to the diphenoquinone (26). Para-phenylation with formation of (27) was observed for the first time in the reaction of the potassium phenolate of 2,6-di-re rr-butylphenol with triphenylbismuth dichloride and in the reaction of the phenol (25) with tetraphenylbismuthonium tosylate in the presence of BTMG. Even ortho- and para-phenylation with concomitant de-rerr-butylation occurred in the reaction of the potassium salt of 2,4,6-tri-re r/-butylphenol with triphenylbismuth dichloride. 2... 

This results in the formation of polyphenylene oxide (PPO) or diphenoquinone (4-(3,5-dimethyl-4-oxo-2,5-cyclohexadiene-l-ilidene-2,6-dimethyl-2,5-cyclohexadiene-1-one). 

The selectivity of 2,6-di-t-butylphenol (2,6-DTBP) oxidation catalyzed by Co(salen) derivatives depends on the nature of axially coordinated (5th) ligand as well as on the solvent used. The relevant data of Kothari and Tazuma [23] are listed in Table I. These authors have found that DMF is an effective and selective solvent for benzoquinone (BQ) formation. With Co(salen)py as catalyst, up to 100 % selectivities to BQ can be reached relative to diphenoquinone (DPQ) formation. Mn(salen) in DMF, however, steers the reaction with 99.9 % selectivity to DPQ. The proper choice of reaction conditions permits even monoalkylphenols (2-t-butylphenol and 2-methylphenol) and phenol itself to be oxidized, mostly to the corresponding BQ. Phenol oxidation occurs at 6 bar 0 and ca. 70 °C in DMF. A recent report describes a... 

Detection The phenols can be detected on layers containing a fluorescent indicator by inspection in short wave UV light. Visualisation is usually possible with diazonium salts (Rgt. No. 100). Ferricyanide-ferric chloride (Rgt. No. Ill) and chlorine-tolidine (Rgt. No. 42) may also be employed. The action of the latter reagent depends on the easy formation of polychlorocyclohexadienones from chlorination of the phenols the active chlorine in these compounds oxidises the o-tohdine to a dye of the diphenoquinone-diimine type (KI is not necessary) [171]. 

Once ArO is formed (Eq. 14.1), it can be entrapped by O producing peroxy radical ArO (Eq. 14.2), which abstracts H atom from another ArOH molecule to give ArO and hydroperoxide (HP) as the primary oxidation product (Eq. 14.3). At low oxygen pressure, chain termination via recombination of two ArO radicals results in the formation of a labile dimer that after enolization yields bisphenol (BP) that can be further oxidized to diphenoquinone (DPQ) (Eq. 14.4). Benzoquinone (BQ) can derive from chain termination steps (Eqs. 14.5 and 14.6) or appear in the conrse of HP transformations, for example, dehydration (Eq. 14.7), which is favored by the formation of conjugate C=0 and C=C bonds. 

The reaction of alkyl- and halo-substituted phenols with Cr02Cl2 results mainly in the formation of quinones and diphenoquinones. Phenoxyl radicals are involved as intermediates. The mechanism of oxidation of a-hydroxycar-boxylic acids by pyridinium chlorochromate involves a rate-limiting hydride transfer. In the reaction of HOCD2CO2H, a kinetic isotope effect (W d) = 5.80 has been determined. Spectroscopic evidence for Cr(IV) and Cr(V) species has been obtained in the oxidation of alkylaromatics by chromyl acetate in acetic anhydride.Stopped-flow and esr studies show two stages (equations 22, 23) in the reactions of RCH2Ph, with both rates being decreased on deuteration at... 

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