Direct synthesis of phenols by iron-catalyzed biphasic oxidation of aromatic hydrocarbons with hydrogen peroxide

The hydroxylation of a series of aromatic hydrocarbons with hydrogen peroxide, catalyzed by iron complexes with pyrazine-3-carboxylic acid N-oxide, was investigated, operating in a biphasic reaction medium. The new catalyst showed a high selectivity to the corresponding phenols, in minimizing the over-oxidation reactions to polyoxygenated derivatives and tars which, along with dimers formation, are the major limitations of the classical Fenton's reagent for a practical synthetic application. In the case of alkylbenzenes, the competitive side chain oxidation at the benzylic positions also occurred. Electron rich substrates' such as anisole, were oxidized with very poor selectivity. The reactions were carried out in a biphasic system that allows a convenient recovery and recycling of the catalyst by phase, separation techniques. The catalyst showed a complete retention of activity after six consecutive reaction cycles. The new catalyst appears as a promising tool for the direct synthesis of phenols, in alternative to the conventional multi-step methods. (C) 2003 Elsevier Science B.V. All rights reserved.