Hydroxylation of Phenol by Hydrogen Peroxide Catalyzed by Copper(II) and Iron(III) Complexes: The Structure of the Ligand and the Selectivity of ortho-Hydroxylation

A number of complexes of copper(II) and iron(III) with different N,N and N,O ligands were tested as catalysts for the hydroxylation of phenol to dihydroxybenzene by hydrogen peroxide for the purpose of achieving a high catechol selectivity. Cu(II) complexes were demonstrated to give a high selectivity on catechol. The best selectivity was found for Cu(II) complex with 2,6-dihydroxypiridine. The best conditions for the selective formation of catechol were the reaction time of 15 min and a ratio of 2,6-dihydroxypiridine to copper greater than 3 (65 degrees C). The concentration of phenol and the reaction time had a dramatic influence on the catechol yield and selectivity for most selective catalysts. At high concentrations and reaction times, both the catechol yield and selectivity decreased, with tars being formed.