Studies of the Scholl reaction: oxidative dehydrogenation involving 1-ethoxynaphthalene and related compounds

Abstract

The oxidation of 1-ethoxynaphthalene and related compounds to give biaryl coupling products under a variety of Scholl reaction conditions has been investigated and the reaction mechanisms are discussed. The results are interpretable in terms of two complementary mechanisms, one involving conventional ionic intermediates and the other involving radical cation intermediates. Thus, mono- and meta-di-substituted aryl ethers are more likely to react by the radical cation pathway than by the ionic process whereas ortho- and para-di-substituted aryl ethers are expected to participate more readily in the ionic process. The oxidation of 1-ethoxynaphthalene with benzil and aluminium chloride in anisole solution has been shown to proceed through the intermediate formation of α-(4-methoxyphenyl)benzoin which causes the oxidation of the naphthyl ether to 4,4′-diethoxy-1,1′-binaphthyl while it is itself reduced to α-(4-methoxyphenyl)deoxybenzoin. This oxidative behaviour is also shown by α-phenyl- and α-(1-naphthyl)-benzoins; however, the intermediate formation of α-(4-ethoxy-1-naphthyl)benzoin does not lead to the formation of the binaphthyl ether but to the cyclodehydration product, 5-ethoxy-1,2-diphenylnaphtho-[2,1-b]furan. The significance of electron transfer processes in electrophilic aromatic substitution reactions is also discussed.