The catalytic oxidation of 4-aryloxyphenols

Abstract The mechanism of the Cu + /amine catalysed oxidation of a number of substituted 4-aryloxyphenols (dimeric phenols) has been studied by analysis of primary reaction products. Quinone ketals are postulated as intermediates which can be decompose in two ways, depending on the substitution pattern of the starting 4-aryloxyphenol: a homolytic fission mechanism (two dimeric phenols yielding monomer and trimer as primary products), or an intramolecular rearrangement mechanism (two dimeric molecules giving a tetramer as a primary product). The tetramers are not formed by direct coupling of 4-aryloxyradicals, the oxygen of one radical attacking the para position of the terminal ring of the other. Experimental evidence is presented for the mechanisms proposed.