Oxidation of Phenols and Catechols

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). 

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. 

Figure 13.8 Products of phenol and catechol produced by the metabolic oxidation of benzene.

Serpone, N. Terzian, R. Colarusso, P. Minero, C. Pelizzetti, E. Hidaka, H. Sonochemical oxidation of phenol and three of its intermediate products in aqueous media Catechol, hydroquinone, and benzoquinone. Kinetic and mechanistic aspects, Res. Chem. Intermed. 1992, 18, 183. 

In phenol hydroxylation, remarkable selectivities to single products have been achieved using vanadium heteropolyacid catalysts.485 The use of the ZSM-5 titanium silicalite (TS-1)483 permits the oxidation of phenol to catechol and hydroquinone to be carried out on an industrial scale with a higher selectivity at a greater conversion of substrate that was not previously possible with strong acid catalysts. 

Dihydrodiols are seldom observed, as are catechol metabolites produced by their dehydrogenation, catalyzed by dihydrodiol dehydrogenase. The further oxidation of phenols and phenolic metabolites to a catechol or hydro-quinone is also possible, the rate of reaction and the nature of products depending on the ring and on the nature and position of its substituents. In a few cases, catechols and hydroquinones have been found to undergo further oxidation to quinones by two single-electron steps. The intermediate in this reaction is a semiquinone. Both quinones and semiquinones are reactive, in particular toward biomolecules, and have been implicated in many toxitication reactions. For example, the high toxicity of benzene in bone marrow is believed to be due to the oxidation of catechol and hydroquinone catalyzed by myeloperoxidase. 

The oxidation of phenols to catechols or hydroquinones by tyrosinase enzymes has been developed for biocatalysis. For example, the ortho-hydroxylation of L-tyrosine 162 (and also substituted variants) to give l-DOPA 163 has been extensively studied due to the importance of l-DOPA in the treatment of Parkinson s disease [92, 93]. An arene hydroxy lating enzyme having a broad substrate scope is 2-hydroxybiphenyl 3-monooxygenase from Pseudomonas azelaica, which is able to oxidize many ortho-substituted phenols 68 to the corresponding catechols 127 [94], as shown in Scheme 32.19. A notable example of an industrial biocatalytic arene hydroxylation that has been employed on very large scale (lOOm fermentation) is the pora-hydroxylation of R)-2-phenoxypropionic acid 164 by whole cells of Beauveria bassiana Lu 700 to give (R)-2-(4-hydroxyphenoxy)propionic acid 165, an important intermediate in herbicide manufacture [95]. 

In 1978, Caltech s W. A. Goddard proposed a different mechanism for the hydroxylation of phenolic compounds and attempted to show how flavin coenzymes carry out such oxidations. It is a theoretical proposal based on wave functions and quantum mechanics using generalized valence bond theory, applied to biological problems (297). An example is shown in Fig. 7.7 for the oxidation of phenol to catechol. 

Tetrahedrally-coordinated cations are also encountered in the lattice of zeolites (Figure 3.8). The well-defined nature of the lattice, the variety of microchannel dimensions, and the many possibilities for isomorphous substitution make them ideal catalysts for the production of fine chemicals. A recent application is the oxidation of phenol to catechol by hydrogen peroxide using a titanium-silicalite catalyst with Ti ions in tetrahedral sites. 

Monllor-Satoca, D. Gomez, R. A photoelectrochemical and spectroscopic study of phenol and catechol oxidation on titanium dioxide nanoporous electrodes. Electrochim Acta 2010, 55, 4661—4668. 

Also catalytic efficiency of peroxidase and tyrosinase from various sources were investigated. The actrvily of biocatalysts in reactions of phenol and catechol oxidation to melanin-type polymers was found as a change of optical density of reaction mixture at 440 nm. Besides, to determine the kinetic parameters of the catalysts the chronometric method was used [5]. 

Starting from Benzene. In the direct oxidation of benzene [71-43-2] to phenol, formation of hydroquinone and catechol is observed (64). Ways to favor the formation of dihydroxybenzenes have been explored, hence CuCl in aqueous sulfuric acid medium catalyzes the hydroxylation of benzene to phenol (24%) and hydroquinone (8%) (65). The same effect can also be observed with Cu(II)—Cu(0) as a catalytic system (66). Efforts are now directed toward the use of Pd° on a support and Cu in aqueous acid and in the presence of a reducing agent such as CO, H2, or ethylene (67). Aromatic... 

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). 

Polar functional groups such as alcohols or phenols 11 or trimethylsilanol 4 are transformed by monofunctional silylating reagents Me3SiX 12 into their hpophilic and often volatile trimethylsilyl ethers 13 whereas water is converted into persilyl-ated water (=Me3SiOSiMe3, hexamethyldisiloxane, HMDSO, 7, b.p. 100 °C). The persilylation of phenols and, in particular, catechol (or hydroquinone) systems (Scheme 2.1) protects them efficiently against air oxidation even at temperatures of up to 180 °C. (cf, e.g., the silylation-amination of purine nucleosides with dopamine hydrochloride in Section 4.2.4)... 

Waldmann et al. used tyrosinase which is obtained from Agaricus bisporus for the oxidation of phenols to give ortho-quinones via the corresponding catechols in the presence of oxygen (scheme 33).1881 A combination of this enzymatic-initiated domino process with a Diels-Alder reaction yields the functionalized bicyclic components 164 and 165 as a 33 1 mixture starting from simple p-methyl-phenol 160 in the presence of ethyl vinyl ether 163 as an electron rich dienophile via the intermediates 161 and 162 in an overall yield of 77%. 

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