Aerobic oxychlorination of phenols catalyzed by copper(II) chloride

Oxychlorination of phenol and electron-rich phenolic compounds catalyzed by CuCl2 under mild conditions has been developed. Chloride ions are used as halogenating agents and dioxygen as a final oxidant. The catalyst shows not only high regioselectivity for para- or ortho isomers but also a remarkable chemoselectivity for monochlorination, with no products of the oxidation of phenols being formed in detectable amounts. Nonphenolic aromatics and phenols with electron withdrawing substituents undergo no transformation at all under similar conditions. A free radical mechanism is suggested, which involves one-electron oxidation of phenol by CuCl2 to the corresponding phenoxy radical followed by a rapid reaction of the tautomeric cyclohexadienyl radical with CuCl2 resulting in chlorinated product and CuCl. Re-oxidation of CuCl by dioxygen completes a catalytic cycle. This simple, low cost and selective method might be applied for the synthesis of chlorophenols, which are widely used in pharmaceutical, agricultural and dye industries. (c) 2006 Elsevier B.V. All rights reserved.