Oxidation aromatic ring systems

Amine oxides, known as A[-oxides of tertiary amines, are classified as aromatic or aliphatic, depending on whether the nitrogen is part of an aromatic ring system or not. This stmctural difference accounts for the difference in chemical and physical properties between the two types. 

Table 23.2 lists the oxidation states of the elements along with representative examples of their compounds. Coordination numbers as high as 12 can be attained, but those over 7 in the case of Cr and 9 in the cases of Mo and W involve the presence of the peroxo ligand or rr-bonded aromatic rings systems such as rj -CsHs or rj -C(,H(,. 

The benzofuroxtin [benzofurazan oxide, 3,4-benzo-l,2,6-oxa-diazole-2-oxide, or 2,1,3-benzoxadiazole-l-oxide (1)] ring system has been reviewed briefly on several occasions, notably by Kaufman and Picard,Boyer, and Behr. The mqst recent of these covers the literature until 1959, and since that date there have been many advances in the subject. This, we feel, justifies the field being covered once more, and its separation from the monocyclic 1,2,5-oxadiazole oxides—the furoxans. We consider also other furoxano-fused compounds in this chapter, subject to the limitation that the ring adjacent to the furoxan is aromatic and six-membered. 

Molecules that are involved in CL reactions are generally reduced species that can be easily oxidized, such as molecules containing amino and hydroxy groups and polycyclic aromatic ring systems. The solvent in which the experiment is carried out has a dramatic effect on the efficiency of the reaction. Solva-... 

Oxidation reactions can be divided into two kinds those in which oxygen is incorporated into the drug molecule, and those in which primary oxidation causes part of the molecule to be lost The former include hydroxylations, epoxidations, and sulfoxidations. Hydroxylations may involve alkyl substituents (e.g., pentobarbital) or aromatic ring systems (e.g propranolol). In both cases, products are formed that are conjugated to an organic acid residue, e.g., glucuronic add, in a subsequent Phase 11 reaction. 

The oxidized ligand(s) have not been well characterized in most systems, but from the limited evidence available (2, 7), introduction of hydroxy groups in the aromatic ring systems according to the generalized stoichiometry... 

Flavin coenzymes are usually bound tightly to proteins and cycle between reduced and oxidized states while attached to the same protein molecule. In a free unbound coenzyme the redox potential is determined by the structures of the oxidized and reduced forms of the couple. Both riboflavin and the pyridine nucleotides contain aromatic ring systems that are stabilized by resonance. Part of this resonance stabilization is lost upon reduction. The value of E° depends in part upon the varying amounts of resonance in the oxidized and reduced forms. The structures of the coenzymes have apparently evolved to provide values of E° appropriate for their biological functions. 

Nitration of aromatic hydrocarbons is usually carried out with the above mixed acid reagent at comparatively low temperatures (e.g. about 50 °C, as used in the preparation of nitrobenzene and 1-nitronaphthalene, Expt 6.17). Unnecessarily high temperatures should be avoided since polynitration is then more likely and oxidative breakdown of the aromatic ring system may occur. 

Fig. 8. Aromatic ring systems in Murchison polymer (Hayatsu et al., 1977, 1980a). Gentle oxidation converts substituents to COOH groups, but leaves ring systems intact. In addition to the ring systems shown, methyl naphthalene and methyl phenanthrene were also identified (Hayatsu et al., 1980a)...

Many organic compounds react with carboxylic acids, acyl halides, or anhydrides in the presence of certain metallic halides, metallic oxides, iodine, or inorganic acids to form carbonyl compounds. The reaction is generally applicable to aromatic hydrocarbons. Benzene, alkylbenzenes, biphenyl, fluorene, naphthalene, anthracene, acenaphthene, phenanthrene, higher aromatic ring systems, and many derivatives undergo the reaction. 

In addition to the temperature, solvent, and substituent effects, a preference for either the arene oxide or oxepin form may be achieved by localization of one double bond as part of an aromatic ring system. Thus the reluctance to form a cyclobutadiene ring causes 10 to exist preponderantly as its oxide form. Naphthalene 1,2-oxide 11 is the simplest arene-oxide member in the polycyclic aromatic hydrocarbon (PAH) series and exists exclusively in that tautomeric form. In contrast, naphthalene 2,3-oxide exists exclusively as the oxepin form 12 since the C4-CS bond in the oxepin ring forms part of an aromatic ring. ... 

---