Oxidations of arenes

Included in this section are oxidations of benzene and phenyl rings, and in general the oxidation of aromatic and polycyclic aromatic compounds. The main catalyst for this type of reaction is RuO. The earliest example was the use of stoich. RuOy CCI4 for phenanthrene oxidation [239], while the first catalytic reagent was RuO / aq. Na(I04)/acetone for oxidation of pyrene [240]. Another early example was the conversion of diketo compounds to the nor-diketo acids, with concomitant destruction of the two phenyl rings by RuO /aq. NallO l/acetone (Fig. 3.18, 3.2.2.1) [206]. 

Other early oxidation of phenyl rings by RUCI3 or RnO /aq. Na(lO )/CCl /60°C were noted (Table 3.4) - cf. also large-scale preparations, 3.3.5 below [241]. 

Oxidations of alkylaromatic, phenol and hydroqninone snbstrates by macrocy-clic complexes containg the trans-Ru XO) unit have been reviewed, with emphasis on mechanistic aspects [242]. 

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Oxidation of Arenes and Enamines - Potassium Permanganate (KMn04)-Alumina... 

Table 3.4 Oxidation of arenes phenyl, furyl and other aromatic rings...

Oxidation to Quinones. Direct oxidation of arenes to quinones can be accom-plished by a number of reagents. Very little is known, however, about the mechanism of these oxidations. Benzene exhibits very low reactivity, and its alkyl-substituted derivatives undergo benzylic oxidation. Electrochemical methods appear to be promising in the production of p-benzoquinone.797 In contrast, polynuclear aromatic compounds are readily converted to the corresponding quinones. 

Some of the reagents used in olefin epoxidation can be applied in the direct oxidation of arenes to arene oxides. Benzene oxide, however, like other arene oxides, exists in equilibrium with oxepin, its valence tautomer, and has not been isolated. Existence of benzene oxides as intermediates can be concluded from observations like the NIH shift discussed above.752,753... 

T. Hudlicky, H. Luna, G. Barbieri, and L. D. Kwart, Enantioselective synthesis through microbial oxidation of arenes. 

Scheme 3.18. Oxidation of arenes via abstraction of single electrons [72].

Hydroxylation of arenes.1 Sodium perborate in combination with trifluoro-methanesulfonic acid is an attractive reagent for electrophilic oxidation of arenes to phenols. 

The direct oxidation of arenes to quinones is a reaction with a limited scope [41], Only substrates that form stable quinones give good yields. For example, oxidation of anthracene to stable 9,10-anthraquinone with chromic acid is practiced on industrial scale. Such oxidations are believed to proceed through a series of one-electron oxidation/solvolysis steps. Yields and selectivity may be improved by using a strong one-electron oxidant such as cerium ammonium nitrate (CAN), as in the oxidation of phenanthrene to phenanthrenequinones (Eq. 9) [42]. 

Y. Fujiwara, Acc. Chem. Res. 2001, 34, 633-639 (d) Y. Fujiwara, C. Jia, Palladium-Catalyzed Carboxylative Oxidation of Arenes, Alkanes, and Other Hydrocarbons, in Handbook of Organopalladium Chemistry for Organic Synthesis,... 

Direct Oxidation of Arenes to Phenols and Quinones 99 Vsevolod V. Rostovtsev... 

In the presence of oxygen, the formation of biaryls by Pd(II) oxidation of arenes can be made to be catalytic in palladium.576 For example, toluene with Pd(OAc)2 and 02 at 150°C for 16 hr afforded bitolyls in 20,600% yield based on palladium. It was concluded that biaryl formation in these systems occurs via free aryl radicals.576 The role of homolytic processes in these reactions is not clear, and further clarification of the mechanism is desirable. 

Moreover, it is difficult to visualize a simple mechanism for the proposed rearrangement of a o-aryl to a-benzyl species. More studies are clearly needed before any definite conclusions can be drawn about the mechanisms of Pd(II) oxidations of arenes. 

Figure 4.8 Oxidation of arenes in the presence of aqueous hydrogen peroxide and Sn-ZSM-12.

Oxidation of arenes (8, 80-81). This oxidation can be carried out in a two-phase system with sodium dodecyl sulfate as a micellar catalyst. CAS can be used in catalytic amounts if ammonium persulfate is used in excess to convert Ce(lll) as formed to Ce(lV). This oxidation is slow in the absence of a Ag(l) salt. This catalytic two-phase oxidation is very useful for preparation of polycyclic quinones from hydrocarbons, but is ineffective for other substrates. 

Tin(IV) oxide is used in various heterogeneous catalyst mixtures, e.g. Sn02/V20s for oxidation of arenes to carboxylic acids and anhydrides, and Sn02/Sb20s for selective oxidation and ammoxidation of propylene to acrolein, acrylic acid, and acrylonitrile. 

Although one-electron oxidation of arenes by thallium trifluoroacetate presumably does not proceed via formation of a discrete aiylthallium bond, some examples involving oxidative cyclization mediated by thallium trifluoroacetate will be considered here. Schwartz and Hudec employed thallium trifluoroacetate to effect an intramolecular cyclization of the amide (41) to give a key intermediate (42) in their projected synthesis of lycorine alkaloids (Scheme 16). Interestingly, when the bromine was replaced by a hyctogen the yield was poorer. 

Oxidation of arenes This oxidant is somewhat superior to CrOs for oxidation of arenes to quinones but less efficient than ceric ammonium sulfate (8,80-81). An example is the oxidation of naphthalene to 1,4-naphthoquinone in 75% yield. 

Microbial oxidation of arenes is a feasible process. An example is the conversion of benzene to cylohexa-3,5-diene-1,2-diol (14, R = H) by the bacterium Pseudomonas putida P. putida). The process is stereoselective and with substituted benzenes (14, R h) a single enantiomer is produced (Scheme 11.7). Such compounds are useful starting materials for natural product synthesis. 

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