Persulphate oxidation of carboxylic acids. Part IV. Oxidation of o-arylphenoxyacetic acids

Abstract

Oxidation of o-phenylphenoxyacetic acids with persulphate generates ArOCH₂· radicals which rapidly cyclise at the C-2′ position to give dibenzo[b,d]pyrans in good yield and dimeric material. The cyclisation can be reversed by pyrolysis of the bis-cyclohexadiene dimer (4) which gives the parent pyran, the linear ArO·CH₂·CH₂·OAr dimer, and the bis-dibenzopyran (6). Oxidation of dibenzo[b,d]pyran with DDQ yields benzocoumarin and the bis-acetal (8) which is also formed in the persulphate oxidation of o-phenylphenoxyacetic acid. Oxidation of o-(1-naphthyl)phenoxyacetic acids gives largely spiro-dimers and a low yield of pyran. ArOCH₂⁺ cations produced by oxidation of o-phenylphenoxyacetic acid in the presence of an excess of Cu^II ions, and in other ways, do not cyclise but undergo solvolysis to give phenols.

Oxidation of 2,2′-biphenylenedioxyacetic acid results in double cyclisation to a pyranopyran to a slight extent, the main reaction product being a dimer of the spirocyclohexadienone (21) formed via cyclisation at C-1′. When heated, the dimer equilibrates with the monomer and three other isomeric dimers. The monomer (21) can be trapped by Diels–Alder addition with maleic anhydride but not with tetracyanoethylene which effects a rearrangement to 2,2′-methylenedioxybiphenyl.