Polynuclear arenes, oxidation

The nitrosonium cation can serve effectively either as an oxidant or as an electrophile towards different aromatic substrates. Thus the electron-rich polynuclear arenes suffer electron transfer with NO+BF to afford stable arene cation radicals (Bandlish and Shine, 1977 Musker et al., 1978). Other activated aromatic compounds such as phenols, anilines and indoles undergo nuclear substitution with nitrosonium species that are usually generated in situ from the treatment of nitrites with acid. It is less well known, but nonetheless experimentally established (Hunziker et al., 1971 Brownstein et al., 1984), that NO+ forms intensely coloured charge-transfer complexes with a wide variety of common arenes (30). For example, benzene, toluene,... 

Nitration of arenes. Reaction of 1-naphthol with CAN in acetic acid results in 2,4- and 4,6-dinitro derivatives. The reaction with CAN absorbed on silica gel results in 2-nitro- and 4-nitro-l-naphthol in 42 and 38% yield, respectively. Polynuclear arenes are not oxidized to quinones by CAN supported on silica but are converted mainly into mononitro derivatives. Thus phenanthrene is converted into 2-nitrophenanthrene (45% yield) and 3-nitrophenanthrene (28% yield).1... 

Polynuclear aromatic compounds such as naphthalene or anthracene are oxidized by chromyl reagents mainly into quinones, with a significant NIH shift, providing evidence for the intermediacy of arene oxide intermediates.306,54... 

Quinone synthesis. The oxovandium complex is a catalyst for aerial oxidation of polynuclear arenes and aryl ethers. 

Photocyclizations of mono- and distyryl substituted polynuclear arenes in the synthesis of helicenes have been reported by El Abed et al. (Chapter 4). In this work, irradiation of toluene solutions of (17 R = H, OMe, CN, Me, OH) and (18 R, = H, OMe, Me) with iodine in the presence of propylene oxide... 

Reaction with polynuclear arenes. Naphthalene and phenanthrene arc un-rcactive to this reagent but higher arenes such as (1) undergo substitution and oxidation as formulated for one example. ... 

Polynuclear aromatic systems can be oxidized to quinones, but unsymmetrical substrates will often give a mixture of products. It has been reported that mononitro derivatives were formed by the oxidation of polynuclear arenes with CAN adsorbed in silica, whereas drnitro compoimds and quinones were obtained from oxidation in solution. 

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. 

The synthesis of quinones from arenes is an area which demands further research, despite the number of reagents presently available for this transformation. This is highlighted by the synthesis of the naphthoquinone (3). Direct oxidation of the dibromoarene (1) was unsatisfactory, and therefore Bruce and coworkers had to resort to a multistep sequence involving nitration, reduction, diazotization, displacement by hydroxide and finally oxidation of the phenol (2) with Fremy s salt (Scheme 1). Although there are examples of the oxidation of polynuclear aromatic hydrocarbons to quinones, the direct oxidation of an arene to a quinone is a process not encountered in the synthesis of more complex mt ecules. 

Perfluorobenzenium salt can be conveniently prepared from the reaction of 1,4-perfluorocyclohexadiene with SbFs. " In the presence of SbFs, this salt is able to react with three equivalents of pentafluorobenzene to yield perfluoro-1,3,5-triphenylbenzene (eq 3). " When 2,2 -di-Ff-octafluorobiphenyl is used as the substrate, perfluorotriphenylene is obtained in 50% yield (eq 4). Per-fluoronaphthlenenium ion also reacts with polyfluorinated arenes in a similar fashion. " The above reaction offers a facile approach for the preparation of these perfluorinated polynuclear aromatic compounds. Oxidation of perchlorobenzene with SbFs in the presence of pentafluorobenzene leads to the formation of coupling products. ... 

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