Oxidative aromatizations

This ladical-geneiating reaction has been used in synthetic apphcations, eg, aioyloxylation of olefins and aromatics, oxidation of alcohols to aldehydes, etc (52,187). Only alkyl radicals, R-, are produced from aliphatic diacyl peroxides, since decarboxylation occurs during or very shortiy after oxygen—oxygen bond scission in the transition state (187,188,199). For example, diacetyl peroxide is well known as a source of methyl radicals (206). 

Jones JP, Mysinger M, Korzekwa KR. Computational models for cytochrome P450 a predictive electronic model for aromatic oxidation and hydrogen abstraction. Drug Metab Dispos 2002 30 7-12. 

In addition to nonheme iron complexes also heme systems are able to catalyze the oxidation of benzene. For example, porphyrin-like phthalocyanine structures were employed to benzene oxidation (see also alkane hydroxylation) [129], Mechanistic investigations of this t3 pe of reactions were carried out amongst others by Nam and coworkers resulting in similar conclusions like in the nonheme case [130], More recently, Sorokin reported a remarkable biological aromatic oxidation, which occurred via formation of benzene oxide and involves an NIH shift. Here, phenol is obtained with a TON of 11 at r.t. with 0.24 mol% of the catalyst. 

Further work at EniTecnologies was conducted with Rhodococcus strains. Rhodococ-cus was selected for its metabolical versatility, easy availability in soils and water, and remarkable solvent tolerance. Its capabilities for catalyzing diverse transformation reactions of crude oils, such as sulfur removal, alkanes and aromatics oxidation and catabolism caught their attention. Hence, genetic tools for the engineering of Rhodococcus strains have been applied to improve its biotransformation performance and its tolerance to certain common contaminants of the crude oil, such as cadmium. The development of active biomolecules led to the isolation and characterization of plasmid vectors and promoters. Strains have been constructed in which the careful over-expression of selected components of the desulfurization pathway leads to the enhancement of the sulfur removal activity in model systems. Rhodococcus, Gordona, and Nocardia were transformed in this way trying to improve their catalytic performance in BDS. In a... 

Korzekwa K, Trager W, Gouterman M, et al. Cytochrome-P450 mediated aromatic oxidation—a theoretical-study. J Am Chem Soc 1985 07(14) 4273 4279. 

The expulsion of CO from ketocyclohexadienyl radical is also reasonable, not only in view of the data of flow-reactor results, but also in view of other pyrolysis studies [64], The expulsion indicates the early formation of CO in aromatic oxidation, whereas in aliphatic oxidation CO does not form until later in the reaction after the small olefins form (see Figs. 3.11 and 3.12). Since resonance makes the cyclopentadienyl radical very stable, its reaction with an 02 molecule has a large endothermicity. One feasible step is reaction with O atoms namely,... 

The retrosynthetic concept of the Nicolaou group is shown in Scheme 22. The target molecule 36 is disconnected via an IMDA cyclization of the diene quinone precursor 138, which would be generated from the tetraline derivative 139 using Wittig chemistry followed by aromatic oxidation. A Claisen-type rearrangement would provide access to 139 whereby the side chain required for the rearrangement of 140 would be introduced by 0-acylation. The core of 141 would be formed via an intermolecular Diels-Alder reaction between diene 142 andp-benzoquinone 130 [42]. 

Gopishetty, S.R., Heinemann, J., Deshpande, M. and Rosazza, J.P.N., Aromatic oxidations by Streptomyces griseus biotransformations of naphthalene to 4-hydroxy-1-tetralone. Enzyme Microbiol TechnoL, 2007, 40, 1622. 

In an aqueous solution (initial pH 5.0), 2,4-dinitrophenol (100 pM) reacted with Fenton s reagent (35 pM). After 13-min and 3-h, about 50 and 90% of the 2,4-dinitrophenol was destroyed, respectively. After 7 h, no aromatic oxidation products were detected. The pH of the solution decreased due to the formation of nitric acid (Lipczynska-Kochany, 1991). 

In 2003, Velusamy and Punniyamurthy reported on a copper(II)-catalyzed C—H oxidation of alkylbenzenes and cyclohexane to the corresponding ketones with 30% hydrogen peroxide (Scheme 131). The reaction was catalyzed by the copper complex 192a depicted in Scheme 131 and yields were high in the case of alkylbenzenes (82-89%) whereas cyclohexanone was obtained with a low yield of 18%. Chemoselectivity was very high in every case neither aromatic oxidation nor oxidation at another position of the alkyl chain was observed. 

The incorporation of vanadium(V) into the framework positions of silicalite-2 has been reported by Hari Prasad Rao and Ramaswamy . With this heterogeneons oxidation catalyst the aromatic hydroxylation of benzene to phenol and to a mixtnre of hydroqninone and catechol conld be promoted. A heterogeneons ZrS-1 catalyst, which has been prepared by incorporation of zirconinm into a silicalite framework and which catalyzes the aromatic oxidation of benzene to phenol with hydrogen peroxide, is known as well in the literature. However, activity and selectivity were lower than observed with the analogous TS-1 catalyst. 

The purpose of the present paper is to offer a contribute to the understanding of the mechanisms of these reactions by using an IR spectroscopic method and well-characterized "monolayer" type vanadia-titania (anatase) as the catalyst. We will focus our paper in particular on the following subjects i) the nature of the activation step of the methyl-aromatic hydrocarbon ii) the mechanism of formation of maleic anhydride as a by-product of o-xylene synthesis iii) the main routes of formation of carbon oxides upon methyl-aromatic oxidation and ammoxidation iv) the nature of the first N-containing intermediates in the ammoxidation routes. 

Table I. Recommended Values for Bond Dissociation Energies and Radical Thermochemistry in Aromatic Oxidation...

Methyl nitrite (CH3ONO) MNIT High-molecular-weight aromatic oxidation ... 

Two mechanisms help protect the fetus from drugs in the maternal circulation (1) The placenta itself plays a role both as a semipermeable barrier and as a site of metabolism of some drugs passing through it. Several different types of aromatic oxidation reactions (eg, hydroxylation, /V-dealkylation, demethylation) have been shown to occur in placental tissue. Pentobarbital is oxidized in this way. Conversely, it is possible that the metabolic capacity of the placenta may lead to creation of toxic metabolites, and the placenta may therefore augment toxicity (eg, ethanol, benzpyrenes). (2) Drugs that have crossed the placenta enter the fetal circulation via the umbilical vein. 

There are a number of analogies between the oxidation of aromatic hydrocarbons and olefins. Two classes of aromatic oxidations are to be distinguished. 

With respect to the kinetics of aromatic oxidations, (extended) redox models are suitable, and often provide an adequate fit of the data. Not all authors agree on this point, and Langmuir—Hinshelwood models are proposed as well, particularly to describe inhibition effects. It may be noted once more that extended redox models also account for certain inhibition effects, for mixtures of components that are oxidized with different velocities. The steady state degree of reduction (surface coverage with oxygen) is mainly determined by the component that reacts the fastest. This component therefore inhibits the reaction of a slower one, which, on its own, would be in contact with surface richer in oxygen (see also the introduction to Sect. 2). 

Sachtler [270] notes that the 7r-allyl complex can be attached to a metal ion or to an oxygen anion but doubts that a 7r-allyl metal complex can be stable at the high temperatures normally used. He draws attention to the fact that, in the case of aromatic oxidations, benzoates, maleinates, etc. are observed spectroscopically, indicating that a carbon—metal bond is not formed. 

Under lean conditions the phenyl radical is largely consumed by reaction with O2. This reaction, which is a key step in aromatics oxidation and PAH growth, produces mainly phenoxy radicals at higher temperatures,... 

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