Phenolic oxidations

The cycloaddition of alkynes and alkenes to nitrile oxides has been used in the synthesis of functionalised azepine systems <96JHC259>, <96T5739>. The concomitantly formed isoxazole (dihydroisoxazole) ring is cleaved by reduction in the usual way. Other routes to 1-benzazepines include intramolecular amidoalkylation <96SC2241> and intramolecular palladium-catalysed aryl amination and aryl amidation <96T7525>. Spiro-substituted 2-benzazepines have been prepared by phenolic oxidation (Scheme 5) <96JOC5857> and the same method has been applied to the synthesis of dibenzazepines <96CC1481>. 

Hopkins GD, J Munakata, L Semprini, PL McCarty (1993a) Trichloroethylene concentration effects on pilot-scale in-situ groundwater bioremediation by phenol-oxidizing microorganisms. Environ Sci Technol 27 2542-2547. 

Hopkins GD, L Semprini, PL McCarty (1993b) Microcosm and in situ field studies of enhanced biotransformation of trichloroethylene by phenol-oxidizing microorganisms. Appl Environ Microbiol 59 2277-2285. 

These experiments clearly showed that it is a-oxygen participation that provides FeZSM-5 zeolites with such a remarkable catalytic performance in the reaction of benzene to phenol oxidation. Equations (1-3) written above are the main stages of the reaction mechanism. 

The nitrosophenol (10), which may be isolated, is oxidised very rapidly by nitric acid to yield the p-nitrophenol (11) and nitrous acid more nitrous acid is produced thereby and the process is progressively speeded up. No nitrous acid need be present initially in the nitric acid for a little of the latter attacks phenol oxidatively to yield HN02. The rate-determining step is again believed to be the formation of the intermediate (9). Some direct nitration of such reactive aromatic compounds by N02 also takes place simultaneously, the relative amount by the two routes depending on the conditions. 

At alkaline conditions (pH=l 1) no phenol, hydroquinone were detected. This is possibly due to the fact that the rate of phenol oxidation increases under alkaline conditions with optimum pH between 9.5 and 13 [17] and so once it is formed, it is readily oxidised. The absence of phenol and increased concentration of p-hydroxybenzoic acid could be also explained by reduced decarboxylation rates under conditions of high pH, which would result in the oxidation of p-hydroxybenzaldehyde to form p-hydroxybenzoic acid. 

The phenanthrene 1,2- and 3,4-diones are synthetically accessible from the related 8 phenols. Oxidation of 2-phenanthrol with either Fremy s salt ((KS0 )2N0) or phenylseleninic anhydride gave phenanthrene 1,2-dione directly (55). Unexpectedly, oxidation of 3-phenanthrol with (KSOg NO yielded 2,2-dihydroxybenz(e)indan-l,3-d-ione (Figure 10). However, phenanthrene 3,4-dione was readily obtained from 3-phenanthrol by Fieser s method entailing diazonium coupling, reduction, and oxidation of the resulting 4-amino-3-phen-anthrol with chromic acid (56). 

The monophenolase activity of PPO is generally defined as the first step in the melaniza-tion pathway and consists of the o-hydroxylation of the monophenol to odiphenol. This activity distinguishes PPO from other phenol-oxidizing enzymes, such as laccase and peroxidase, and is characterized by the following facts ... 

Zilly A, de Souza CGM, Barbosa-Tessmann IP, Peralta RM (2002) Decolourisation of industrial dyes by a Brazilian strain of Pleurotus pulmonarius producing laccase as the sole phenol-oxidizing enzyme. Folia Microbiol 47 315-319... 

Consequently, in an inert atmosphere/= 2(1 + k(lls/krcc) > 2. When phenoxyl radicals react only with peroxyl radicals, /= 2 and there is no regeneration. At low dioxygen pressures, phenoxyl radicals react with both peroxyl and alkyl radicals / ranges between 2 and 2(1 +kdis/krec) and increases with decreasing p02- In addition to this, the product of phenol oxidation, quinone, becomes the efficient alkyl radical acceptor at low dioxygen pressure (see earlier). 

Phenol oxidation over TS-1 in H2O and methanol mixture solvent... 

Vargaftik, M.N. et al., Catalysis with a Pd giant cluster phenol oxidative carbonylation to diphenul carbonate conjugated with reductive nitrobenzene conversion, J. Mol. Catal. A Chem., 108, 77, 1996. 

Subrahmanyam VV, O Brien PJ. 1985. Phenol oxidation products, formed by a peroxidase reaction, that bind to DNA. Xenobiotica 15 873-885. 

Incorporation studies have clarified the relative importance of the two routes that lead to the catechol metabolite of phenytoin, namely phenol oxidation and dihydrodiol dehydrogenation (Fig. 10.8, Reactions e and/, respectively) [80], Indeed, upon incubation of phenytoin in an 1802 atmosphere, two atoms of 180 are incorporated into the catechol when formed via phenol oxidation, but only one when formed via dihydrodiol dehydrogenation. This elegant study showed quite clearly that most of the catechol (75 10%) came from the phenol, indicating a comparatively slow dehydrogenation of the dihydrodiol. 

Para-Xylene Oxidation (wt.%) Phenol Oxidation (wt.%) TBHP Conv. (TOF,h ) H2O2 Conv. (TOF,h )... 

Diarylamides with arenes activated by electron-donating substituents can be converted to azacycles by anodic oxidation through phenolic oxidative coupling reactions that can be a key step in the synthesis of alkaloids (Schemes 16 and 17). According to the nature of substituents and the experimental conditions, either spiro compounds [22] or non-spiro compounds [23, 24] were obtained. 

Enzymatic browning. Phenol-oxidizing enzymes (such as tyrosinase and peroxidase) oxidize tyrosine residues into reactive quinone derivatives, which will condense into colored polymers (melanins). Melanins are rich in carboxyl groups and therefore have high affinity for divalent metal ions such as calcium. 

---