pathways in aromatic oxidation mechanistic on the reactions of aromatic substrate with periodic acid

Abstract

The thesis entitled "Pathways in Aromatic Oxidation – Mechanistic Studies on the Reactions of Aromatic Substrates with Periodic Acid" is divided into six chapters. The first chapter discusses the mechanisms of oxidation of cis-glycols, ?-hydroxyketones, phenols, and aromatic hydrocarbons with periodic acid described in the literature. The second chapter starts with a brief review on the results of oxidation of phenols by periodic acid. The results of oxidation of 2,2?-dihydroxy-1,1?-dinaphthyl, 2-hydroxy-2?-methoxy-1,1?-dinaphthyl, and 1,5-dihydroxynaphthalene with periodic acid are presented. Oxidation of 2,2?-dihydroxy-1,1?-dinaphthyl (I) in aqueous acetonitrile medium produced a nine-membered ketolactone (III). An intermediate (II) has been isolated in the oxidation of (I) to (III). Oxidation of (I) with periodic acid in absolute methanol medium formed a product (IV). The structure and stereochemistry of the product (IV) are discussed. 2-Hydroxy-2?-methoxy-1,1?-dinaphthyl (V) on oxidation with periodic acid in absolute methanol medium produced two products (VI) and (VII). Electron spin resonance (e.s.r.) experiments on the reactions of phenols with periodic acid showed one-line e.s.r. signals. On the basis of these results and by the analysis of the literature data, a general single electron transfer mechanism involving the aromatic cation radicals is postulated. The products of oxidation of these phenols by periodic acid and a typical one-electron oxidant like ceric ammonium nitrate (CAN) are found to be the same. Hence, it is concluded that reactive intermediates in the oxidation of phenols by periodic acid and ceric ammonium nitrate are the same. 2,2?-Dihydroxy-1,1?-dinaphthyl (I) on oxidation with sodium periodate in aqueous methanol medium furnished a product (VIII) which on heating above 175°C rearranged to the product (IX). The third chapter deals with the reactions of aromatic ethers with periodic acid. A brief survey of the methods for the direct aromatic iodination using iodine and various oxidising agents is made. Results of the reactions of aromatic ethers with periodic acid in absolute methanol medium are presented. Most of the aromatic ethers formed iodinated products in moderate to good yields. The ethers having methyl substituents were oxidised in the side chain to the corresponding aldehydes when 2 moles of periodic acid were used. From the results of the e.s.r. experiments and other evidences, a single electron transfer mechanism forming the aromatic cation radicals is put forward. In the fourth chapter, results of the oxidation of naphthalene, anthracene, and phenanthrene in methanol medium are presented. Naphthalene formed 1-methoxy-2-iodonaphthalene, anthracene produced 10-methoxyanthrone, and phenanthrene furnished 9,10-phenanthraquinone. The results of e.s.r. experiments of these aromatic hydrocarbons in aqueous fluoroacetic acid medium suggested the intermediacy of the aromatic cation radicals. On the basis of these data and by an analysis of the literature data, a general single electron transfer mechanism as a pathway in the reaction of polynuclear hydrocarbons is proposed. The fifth chapter deals with product studies in the oxidation of benzyl alcohol and phenethyl alcohol by periodic acid. Benzyl alcohol produced a mixture of benzaldehyde and 4-iodobenzaldehyde. Phenethyl alcohol formed the mixture of benzaldehyde, 4-iodobenzaldehyde, and 4?-iodophenethyl alcohol. On the basis of these data, a single electron transfer mechanism which operates in the case of phenols, aromatic ethers, and aromatic hydrocarbons is proposed. Conclusions are presented in chapter six: In the literature, oxidation of phenols by periodic acid has been described by the mechanism involving the periodate ester of phenol and its heterolytic cleavage to form the phenoxenium ion. Oxidation of aromatic hydrocarbons is rationalised by a mechanism which involves an electrophilic attack of periodic acid on the aromatic ring. The results of the present work are inconsistent with these mechanisms in the case of phenols and aromatic hydrocarbons. A single common mechanism involving a single electron transfer from aromatic substrate to periodic acid leading to the formation of aromatic cation radicals is proposed.