Oxidation to Phenols

Benzoquinone [106-31-4J, (quinone) has been reported as a by-product of benzene oxidation at 410—430°C. Benzene can be oxidized to phenols... 

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. 

The selectivity issue has been related to multi-phase processing [31]. Nitrations include both organic and aqueous phases. Oxidation to phenol as one side reaction takes places in the organic phase, whereas all other reactions occur in the aqueous phase and are limited by organic solubility. For this reason, enhancing mass transfer by large specific interfaces is a key to affecting product selectivity. 

Aromatic amines 4 are metabolised in vivo by cytochrome P450 mediated oxidation to phenolic and hydroxylamine derivatives 5 and 6. Phase II conjugation of the latter with PAPS or acyl transferase results in formation of the sulfuric or acetic acid esters 7. Nitrogen conjugation to give the A-acetyl analogues is also possible (Scheme l).54 65... 

Fig. 10.1. Simplified mechanisms of the spontaneous and acid-catalyzed isomerization of arene oxides to phenols via a carbonium ion intermediate (benzene oxide (10.1) as the... 

The cyclic metabolite 11.169 was also a substrate in further biotransformations, being (V-demethylated to the corresponding endocyclic imine, and oxidized to phenolic metabolites. Very little if any of the secondary amine metabolite (11.168) appeared to undergo direct (V-demethylation to the primary amine, in contrast to many other tertiary amines, presumably due to very rapid cyclization of the secondary amine facilitated by steric and electronic factors. The possibility for the iminium cation (11.169 H+) to become deprotonated (a reaction impossible for the iminium 11.166 in Fig. 11.20) should also drive the cyclization reaction. 

It is well known that U.S. space vehicles obtain their auxiliaiy power in space by the use of fuel cells (Chapter 13), electrochemical devices in which the spontaneous tendency of hydrogen to combine with oxygen drives the cell and produces electricity, with water as a by-product (pure enough to drink). It stands to reason then, that one might think of producing substances more economically valuable than water in this electrogenerative way. Such work is into its first decade and Fig. 7.190 shows an example benzene is oxidized to phenol with electricity as a by-product Clearly, the economics of such a process depend on the cost of the H2 and whether one can sell the electricity. This gives rise to a speculation. 

Oxidation to Phenols. Direct hydroxylation of benzene to phenol can be achieved in a free-radical process with H202 or 02 as oxidants.739-744 Metal ions [Fe(II), Cu(II), Ti(HI)] may be used to catalyze oxidation with H202. Of these reactions, the so-called Fenton-type oxidation is the most widely studied process.742 Oxidation in the presence of iron(II) sulfate was reported in early studies to yield phenol. Since phenol exhibits higher reactivity than benzene, varying amounts of isomeric dihydroxybenzenes were also formed. 

Benzoquinone [106-51-4], C6H402 (quinone) has been reported as a by-product of benzene oxidation at 410—430°C. Benzene can be oxidized to phenols with hydrogen peroxide and reducing agents such as Fe(II) and Ti(II). Frequendy ferrous sulfate and hydrogen peroxide are used (Fenton s reagent), but yields are generally low (12) and the procedure is of limited utility. Benzene has also been oxidized in the vapor phase to phenol in low yield at 450—800°C in air without a catalyst (13). 

The H-ZSM-5 coatings were tested for the one-step oxidation of benzene by nitrous oxide to phenol. The grids had a total area of 9 cm2, a wire diameter of 250 pm and a mesh size of 800 pm. Fifteen grids formed a stack separated by steel rings. By acid pretreatment of the grids, defects were generated which are known to become crystallization centers during the synthesis of the zeolite. 

Let us consider the reaction of benzene oxidation with hydrogen peroxide in the Fenton system as the classical situation [30], In the absence of iron ions benzene does not in practice interact with H202. The addition of bivalent iron salt to the system C6H6-H202-H20 induces benzene oxidation to phenol and diphenyl according to the following mechanism ... 

Baeyer-Villiger oxidation.1 Aromatic aldehydes can be converted to phenols in generally high yield by oxidation with hydrogen peroxide (30%) activated by an areneseleninic acid. Polymethoxyacetophenones are also oxidized to phenols, but in lower yields. 

In 1983, Mimoun and co-workers reported that benzene can be oxidized to phenol stoichiometrically with hydrogen peroxide in 56% yield, using peroxo-vana-dium complex 1 (Eq. 2) [20]. Oxidation of toluene gave a mixture of ortho-, meta-and para-cresols with only traces of benzaldehyde. The catalytic version of the reaction was described by Shul pin[21] and Conte [22]. In both cases, conversion of benzene was low (0.3-2%) and catalyst turned over 200 and 25 times, respectively. The reaction is thought to proceed through a radical chain mechanism with an electrophilic oxygen-centered and vanadium-bound radical species [23]. 

Isomerization of arene oxides to phenols proceeds by complex and often multiple pathways660 668 that show a marked dependence on pH. Arene oxides as key intermediates provide the basis for explaining the ortho/meta/para isomer ratios observed in enzymatic hydroxylations.655a,b For example, the absence of m-cresol in the metabolites of toluene can be attributed to the fact that none of the three possible isomers of toluene oxide rearranges to this product. 

Animals. Rapidly absorbed, extensively metabolised and rapidly excreted. No evidence of accumulation, even on repeated dosing. Metabolism proceeds by oxidation to phenolic derivatives which are excreted as glucuronide or sulfate conjugates Plants. Little metabolism occurs in fruit Soil. Rapid degradation, DT50 7-54 days. Low potential for leaching... 

Benzene Oxidation to Phenol Making Phenolic Resins for Building... 

Many aromatic boron compounds are readily oxidized to phenols, as illustrated in equation (11) for the oxidation of aryldihydroxyboranes (arylboronic acids). This is a useful synthetic pathway as arylboronic acids can resist both alkaline permanganate and nitric acid, so that many derivatives are available. ... 

On the basis of the evidence discussed, the rearrangement of arene oxides to phenols clearly involves dienone intermediates. However, from kinetic results it is evident that several carbonium ion intermediates must also be involved. The evidence for involvement of carbonium ion intermediates is ... 

Figure 11. Pathways involved in the acid-catalyzed and spontaneous (k i isomerization of arene oxides to phenols.

The mechanism of aromatization of deuterated and otherwise substituted arene 1,2-oxides to phenolic products has been studied in detail (Figure 13). Pathways leading to phenols A and B correspond to substituent loss, whereas phenols C and D represent substituent migration. Type A phenols are most commonly formed and have been isolated from aromatization of a range of monosubstituted arene 1,2-oxides for example, 33 and 34, 35 and 36, 37, 38, 39, and... 

From Hydrocarbons.—In the presence of aluminium chloride (Friedel-Craft reagent) benzene may be oxidized to phenol. 

The oxidation of carbon-boron bond converts boranes into alkyl or aryl borates, which may be hydrolyzed subsequently to alcohols and boric acid [991], The oxidation is carried out with hydrogen peroxide [183,1201, 1202] or trimethyiamine oxide [991, 992]. Phenylboronic acids are oxidized to phenols biochemically [1034]. 

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