On In Situ Prepared Cu–Phenanthroline Complexes in Aqueous Alkaline Solutions and Their Use in the Catalytic Oxidation of Veratryl Alcohol

Abstract

AbstractThe in situ prepared complex based on copper(II) sulfate and 1,10‐phenanthroline (phen) and its performance in the catalytic oxidation of veratryl alcohol (3,4‐dimethoxybenzyl alcohol) to veratraldehyde with O2 in alkaline aqueous solutions have been studied. In aqueous solution, the structure of Cu–phen complexes varies with reaction conditions; by changing either the copper‐to‐ligand molar ratio or the pH, the amount of coordinated 1,10‐phenanthroline and hydroxido ligands in the coordination sphere of copper can be altered in a controlled manner. The highest activities are achieved in the pH range 12.6–13.3, which correlates with the presence of the [Cu(phen)(OH)2] species according to complex distribution curves. According to the UV/Vis and EPR studies, the oxidation reaction is initiated by the reduction of [Cu(phen)(OH)2] to CuI species by veratryl alcohol, which can be seen as a disappearance of the Cu2+ EPR signal and as a shift of the UV/Vis absorbance maximum from about 690 nm to the 410 and 525 nm regions. When molecular oxygen is added, the CuI species is readily oxidised back to CuII species, and the catalytic formation of aldehyde is initiated. On the basis of these observations, a mechanism for the catalytic oxidation is proposed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)