1.4- Dimethylbenzene oxide, formation

Dihydrodiols have not been obtained from hydrolysis reactions of benzene oxides however, evidence for the formation of 1,4-dihydrodiols has been obtained from acid-catalysis studies on 1,4-dimethylbenzene oxide 63 ° and 8,9-indan oxide 7. The mechanism of acid-catalyzed rearrangement and hydrolysis of benzene oxide has been investigated theoretically using the semiempirical allvalence electron MINDO/3 method and the perturbational MO method, both from the viewpoint of product stabilities and reaction pathways. The aromatization reaction was found to be much more exothermic than the hydrolysis and thus would be the preferred reaction, as was found experimentally. 

Based on the add-catalyzed addition of methanol to 1,4-dimethylbenzene oxide which gave 4-methoxy-l,4-dimethyl-2,5-cyclohexadienol (173), Anderson and Faulkner (5) proposed that an analogous 1,4-addition of solvent to an arene oxide 286 could result in the formation of dienone 1 or the corresponding ketal 3 (5). Although arene oxides have been proposed as intermediates in the biosynthetic oxidation of aromatic compounds, there is no evidence of their existence as natural products. The coexistence of the lactone aeroplysinin-2 (18) with dienone 1 and the ketal 3 suggested that the imino-ether 287 should also be considered as a possible precursor of these alkaloids. 

Aromatic hydrocarbons which have methyl side chains mainly behave like toluene and form aldehydes, while combustion is stimulated and selective oxidation of the nucleus is repressed. The oxidation of methyl-naphthalene, for example, exhibits a low selectivity with respect to phtha-lic anhydride formation, combustion and maleic acid formation being the dominating reactions. Durene is a special case because it resembles o-xy-lene. The oxidation of durene over a V—W—O catalyst at 420° C is reported to produce pyromellitic dianhydride, phthalic and maleic anhydride, although combustion dominates (Geiman et al. [122]). 1,2,4-Trimethyl-benzene yields dimethylbenzene and trimellitic acid if oxidized on a Sn— V—O catalyst. Kinetic data have been measured by Balsubramanian and Viswanath [37]. 

Oxidation of aromatic systems containing alkyl side-chains results in the formation of a carboxylic acid, irrespective of the length of the side-chain. The usual oxidizing agents are potassium permanganate [potassium manganate(VII)] or chromic acid [chromium(VI) acid]. For example, 1,4-dimethylbenzene is oxidized to benzene-1,4-dicarboxylic acid (tereph-thalic acid, 9), an important building block for polyesters. The oxidation of isopropylbenzene (cumene) to phenol is an important industrial process and is discussed in Chapter 4. 

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