Oxidation of phenol

Phenols are rather easily oxidized despite the absence of a hydrogen atom on the hydroxylbearing carbon. Among the coloured products from the oxidation of phenol by chromic acid is the dicarbonyl compound p-benzoquinone (also known as 1,4-benzoquinone or simply quinone). Dihydroxybenzenes, hydroquinone (7.30) and catechol (7.32) are oxidized to p-benzoquinone (7.31) and o-benzoquinone (7.33), respectively, by milder oxidants such as Jones reagent. Fremy s radical (7.34) is an excellent and very specific oxidizing agent for the oxidation of phenols to o- or p-benzoquinones. (m-Quinones do not exist.) 

The oxidation of phenols with Fremy s radical, dipotassium nitrosodisulfonic acid [N0(S03K)2, 7.34], to hydroquinones is known as the Teuber reaction . The disodium salt of nitrosodisulfonic acid [N0(S03Na)2] is also used for the oxidation of phenols. 

For example, 3,4-dimethylphenol (7.35) is oxidized to 4,5-dimethyl-o-benzoquinone (7.36) with Fremy s radical (7.34) and sodium dihydrogen phosphate. The oxidation of the phenol 2-[(Z)heptadec-10 -enyl]-6-methoxyphenol (7.37) with Fremy s radical produced irisquinone (7.38) in 87% yield. 

A cobalt complex salcomine (7.39) oxidizes substituted phenols but unsubstituted at para position, such as 2,6-di-fert-butylphenol (7.40), to give the corresponding p-quinone, 2,6-di- tert-butyl-p-benzoquinone (7.41). 

The oxidation of phenol with potassium persulfate (K2S2O8) in an alkaline medium to p-hydroquinone is known as the Elbs persulfate oxidation. Elbs used ammonium persulfate for the oxidation of 2-nitrophenol (7.42) to 2-nitro-p-hydroquinone (7.43), but the potassium salt is commonly used for the hydroxylation of phenols. 

The aerobic oxidation of phenols in the presence of cobalt-Schiffs base complexes as catalysts is facilitated by (electron-donating) alkyl substituents in the ring and affords the corresponding p-quinones, e.g. the Vitamin E intermediate drawn in Fig. 4.87. When the para-position is occupied the reaction may be directed to the ortho-position [252, 253]. Copper compounds also mediate this type of oxidation, e.g. the Mitsubishi Gas process for the Vitamin E intermediate 

Process (catalyst) Rhone-Poulenc (HCI04, H3PO4) Brichima (Fe /Co ) Enichem (TS-1) 

Interestingly, the results of oxidation (and other reactions) provide some evidence that carbonyl compounds are in equilibrium with the hydroxybenzenes (phenols). Indeed, in extreme cases such as 13,5-trihydroxybenzene (phloroglu-cinol) (Rgure 8.9), some reactions occur as if the tricarbonyl compound were present as the major tautomer. However, it is generally the case that for phenols, as with enols, oxidation occurs either at the adjacent a (or ortho) position or at the more remotely conjugated y (or para) position. 

An adequate dietary intake of the particular quinones known as K vitamins ( K from the Danish word koagulation) is required for the synthesis (in the liver) of prothrombin (a propeptide that leads to the peptide [Chapter 12] thrombin) and other factors needed for blood clotting. The metabolic cycle of the K vitamins includes the corresponding hydroquinones. The oxidation of vitamin K3 hydroqui-none to vitamin K3 (menadione) and its reduction is shown in Rgure 8.11. (The 

Other K vitamins are substituted with long alkenyl chains a or ortho to the methyl group.) 

It is widely held that enzyme-catalyzed oxidative coupling of variously substituted phenols is a common occurrence in nature. The process is thought to involve 

Interestingly, despite the expenditure of a large amount of work, the pathway shown for this and other oxidative coupling reactions, common in nature, remains speculative. 

The 4-(tert-butyldioxy)-4-alkylcydohexadienones 72 thus obtained are versatile synthetic intermediates. The TiCl4-promoted transformation of 75, obtained from the oxidation of 3-methyl-4-isopropy]phenol gives 2,6-disubstituted quinone 76 (93%), which is derived from the rearrangement of i-Pr group of 75 (Eq. 3.85). 

Interestingly, sequential migration-Diels-Alder reactions of tert-butyldioxy die-none 77 in the presence of 1,3-cydohexadiene gave as-fused octahydroanthraqui-none 78 stereoselectively (78%) (Eq. 3.86). 

The reaction can be rationalized by assuming the mechanism which involves an oxo-ruthenium complex (Eq. (7.74)). Hydrogen abstraction with oxo-ruthenium species gives the phenoxyl radical 58, which undergoes fast electron transfer to the ruthenium to give a cationic intermediate, 59. Nucleophilic reaction with the second molecule of t-BuOOH gives the product, 57. 

Ruthenium-catalyzed oxygenation of catechols gives muconic acid anhydride and 2H-pyran-2-one [123]. 

Oxidation of naphthalene derivatives gave naphthoquinones (Eq. (7.78)) [126]. Oxidative coupling can be carried out with supported ruthenium catalyst, Ru(OH)2/ AI2O3 (Eq. (7.79)) [127]. 

As indicated in the A Word About... Quinones and the Bombardier Beetle box on this page 225, phenols are easily oxidized. Samples that stand exposed to air for some time often become highly colored due to the formation of oxidation products. With hydroqui-none (1,4-dihydroxybenzene), the reaction is easily controlled to give 1,4-benzoquinone (commonly called quinone). 

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Phenols are antioxidants, preventing oxidation of substances sensitive to 

Hydroquinone and related compounds are used in photographic developers. They reduce silver ion that has not been exposed to light to metallic silver (and, in turn, they are oxidized to quinones). The oxidation of hydroquinones to quinones is reversible this interconversion plays an important role in several biological oxidation-reduction reactions. 

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Other Methods. A variety of other methods have been studied, including phenol hydroxylation by N2O with HZSM-5 as catalyst (69), selective access to resorcinol from 5-methyloxohexanoate in the presence of Pd/C (70), cyclotrimerization of carbon monoxide and ethylene to form hydroquinone in the presence of rhodium catalysts (71), the electrochemical oxidation of benzene to hydroquinone and -benzoquinone (72), the air oxidation of phenol to catechol in the presence of a stoichiometric CuCl and Cu(0) catalyst (73), and the isomerization of dihydroxybenzenes on HZSM-5 catalysts (74). 

Product identification does not distinguish OH versus hole oxidation, because the products are identical. For example, the products identified in the photo oxidation of phenol (qv) (Fig. 7) may originate either by OH radical attack of the phenol ring, or by direct hole oxidation to give the cation radical which subsequendy undergoes hydration in solvent water. 

Titanium Silicates. A number of titanium siUcate minerals are known (160) examples are Hsted in Table 19. In most cases, it is convenient to classify these on the basis of the connectivity of the SiO building blocks, eg, isolated tetrahedra, chains, and rings, that are typical of siUcates in general. In some cases, the SiO units may be replaced, even if only to a limited extent by TiO. For example, up to 6% of the SiO in the garnet schorlomite can be replaced by TiO. In general, replacement of SiO by TiO bull ding blocks increases the refractive indices of these minerals. Ti has also replaced Si in the framework of various zeofltes. In addition, the catalytic activity of both titanium-substituted ZSM-5 (TS-1) and ZSM-11 (TS-2) has received attention (161), eg, the selective oxidation of phenol, with hydrogen peroxide, to hydroquinone and catechol over TS-1 has been operated at the 10,000 t/yr scale in Italy (162). 

There are numerous stmctures that are similar to 2eofites, such as aluminophosphate molecular sieves, AlPOs, but these have not found catalytic apphcations. Zeofites can be modified by incorporation of cations in the crystalline lattice which are not exchangeable ions, but can play catalytic roles. For example, sificahte, which has the stmcture of ZSM-5 but without Al, incorpora ting Ti in the lattice is a commercial catalyst for oxidation of phenol with H2O2 to give diphenols the catalytic sites may be isolated Ti cations (85). 

Oxidation of phenols or anilines to quinones by means of potassium nitrosodlsuHanate (Fremy s salt). 

Muconic acid has been obtained in a variety of ways. The procedures that seem most important from a preparative point of view are by treatment of ethyl o ,5-dibromoadipate with alcoholic potassium hydroxide, by condensation of glyoxal (as the sodium bisulfite addition product) with malonic acid, by heating ethyl l-acetoxy-l,4-dihydromuconate (obtained by condensing ethyl oxalate and ethyl crotonate, acetylating, and reducing),and by oxidation of phenol with peracetic acid. ... 

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. ... 

Oxidation of phenols with chlorine dioxide or chlorine produces chlorinated aromatic intermediates before ring rupture. Oxidation of phenols with either chlorine dioxide or ozone produces oxidized aromatic compounds as intermediates which undergo ring rupture upon treatment with more oxidant and/or longer reaction times. In many cases, the same nonchlorinated, ringruptured aliphatic products are produced using ozone or chlorine dioxide. 

Synthesis of p-quinones, derivatives of fused systems including heterocyclic fragments through oxidation of phenol derivatives 98OPP603. 

The main product of the Elbs reaction is the 1,4-dihydroxybenzene (hydro-quinone). If the para position is already occupied by a substituent, the reaction occurs at an ortho position, leading to a catechol derivative although the yields are not as good as for a hydroquinone. Better yields of catechols 7 can be obtained by a copper-catalyzed oxidation of phenols with molecular oxygen ... 

Intramolecular Friedel-Crafts acylation of diaryl ketones Oxidation of phenols or aromatic amines Oxidation of aromatic hydrocarbons... 

PO performs vitally important functions in the plant cell and is mainly associated with the oxidation of phenolic compounds and with the formation and strengthening of the cell wall (Passardi et al., 2004). PO is involved in the oxidative transformation of molecules in growth-regulating or signalling activities and - as a result - can also perform regulatory functions in the cell. Plant POs are represented by genetically different proteins with the same enzymatic activity (Welinder et al., 2002). 

Carulite (Mn02/Cu0 on alumina) has shown exceptional performance for the complete rapid oxidation of phenol and other difficult substrates at temperatures just above T. The first full-scale SCWO plant has been commercialized by Huntsman, and it is expected that the technology will now become more mainstream as the value of different kinds of supercritical fluid technology becomes generally more widely appreciated and cost effective. 

Previous studies by Sorokin with iron phthalocyanine catalysts made use of oxone in the oxidation of 2,3,6-trimethylphenol [134]. Here, 4 equiv. KHSO5 were necessary to achieve full conversion. Otherwise, a hexamethyl-biphenol is observed as minor side-product. Covalently supported iron phthalocyanine complexes also showed activity in the oxidation of phenols bearing functional groups (alcohols, double bonds, benzylic, and allylic positions) [135]. Besides, silica-supported iron phthalocyanine catalysts were reported in the synthesis of menadione [136]. 

The permanganate oxidation of phenols is complicated by the intervention of lower oxidation states of manganese, (c/. the oxidation of toluene, p. 298). For example, the oxidation of 2,6-dinitrophenol in weakly acidic solution displays an induction period, following second-order kinetics thereafter. However, addition of potassium fluoride inhibits reaction almost completely, but manganous ions strongly accelerate it. 

The oxidation of phenol in alcoholic media by a morpholine complex of Cu(II) (as a model for tyrosinase) to give 4,5-dimorpholino-orr/jo-benzoquinone in 64 %... 

Hyman MR, AW Sansome-Smith, JH Shears, PM Wood (1985) A kinetic study of benzene oxidation to phenol by whole cells of Nitrosomonas europaea and evidence for the further oxidation of phenol to hydroqui-none. Arch Microbiol 143 302-306. 

New materials are also finding application in the area of catalysis reiated to the Chemicals industry. For example, microporous [10] materials which have titanium incorporated into the framework structure (e.g. so-calied TS-1) show selective oxidation behaviour with aqueous hydrogen peroxide as oxidizing agent (Figure 5). Two processes based on these new catalytic materials have now been developed and commercialized by ENl. These include the selective oxidation of phenol to catechol and hydroquinone and the ammoxidation of cyclohexanone to e-caproiactam. 

Table 3 Oxidation of phenols (TON ) with Oj over copper acetate-based catalysts at 298 K...

The oxidation of phenol, ortho/meta cresols and tyrosine with Oj over copper acetate-based catalysts at 298 K is shown in Table 3 [7]. In all the cases, the main product was the ortho hydroxylated diphenol product (and the corresponding orthoquinones). Again, the catalytic efficiency (turnover numbers) of the copper atoms are higher in the encapsulated state compared to that in the "neat" copper acetate. From a linear correlation observed [7] between the concentration of the copper acetate dimers in the molecular sieves (from ESR spectroscopic data) and the conversion of various phenols (Fig. 5), we had postulated [8] that dimeric copper atoms are the active sites in the activation of dioxygen in zeolite catalysts containing encapsulated copper acetate complexes. The high substratespecificity (for mono-... 

Table 6 Oxidation of phenol with over phthalocyanines... 

Table Oxidation of phenol over CuClg Pc-Na-Y (0.26) InOuenceof HjOjConcentration...

High potential Oj reduction to H O coupled to oxidation of phenols... 

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