Aromatic Hydroxylation by Peroxydisulfate

Abstract

In the presence of suitable metal ion oxidants, notably Cu2+, S2Og2- brings about ring hydroxylation of a variety of aromatic molecules in modest to high yields. Observations are consistent with a scheme by which SO4-· radicals oxidize the aromatic to a radical cation, which can undergo reduction back to starting material, side-chain fragmentation, or reversible hydration to (a mixture of) hydroxycyclohexadienyl radicals. These in turn are oxidized to phenols by Cu2+ or other oxidants. The isomer distributions obtained on aromatic hydroxylation with either HO· or SO4-· radicals vary markedly with reaction conditions. With Cu2+ and other unselective oxidants three limiting conditions are approached: with HO· at low acidity, kinetic control by the point of HO· addition; with SO4-· at low acidity, kinetic control by the point of hydration of the intermediate radical cation; with either reagent in strong acid, thermodynamic control via equilibration of hydroxycyclohexadienyl radicals through the radical cation. Over tenfold changes in ortho-para ratios are observed with chloro- and some alkylbenzenes. © 1978, American Chemical Society. All rights reserved.