Catalytic chemical and electrochemical wet oxidation of phenol using new copper(II) tetraazamacrocycle complexes under homogeneous conditions

Two new macrocycle complexes [Cu{Me4Bzo2[14]aneN4}]Cl2 and [Cu{Me4Bzo2[14]aneN4}](C6H5O)2 (where Me4Bzo2[14]aneN4 = 5,7,12,14-tetramethyldibenzo[b,i]-1,4,8,11-tetraazacyclotetradecane) were synthesized and characterized using spectral and microanalytical data. The [Cu{Me4Bzo2[14]aneN4}]Cl2 was found to catalyze chemical oxidation of phenol by H2O2 to dihydroxy benzenes. The reaction products were analyzed using gas chromatograph. Catalysis was highly stereoselective and led to predominant formation of ortho-dihydroxy benzene (catechol, CAT) as a major product, 93.0% at 50 °C and the remainder as para-dihydroxy benzene (hydroquinone, HQ) with no appreciable detection of other oxidized phenol derivatives. Product selectivity decreased with increasing reaction temperature or increasing relative amount of the oxidant. In the electrochemical investigation the complex [Cu{Me4Bzo2[14]aneN4}](C6H5O)2 exhibited a new irreversible oxidation wave, particularly the presence of traces of water in acetonitrile, due to catalysed electro-oxidation of the phenoxide counter ions. Based on the coordination behaviour of the complex, a plausible mechanism for the catalytic process has been proposed. The chemical catalysis reported here is suitable for the stereoselective transformation of monohydroxy aromatic compounds to ortho-dihydroxy derivatives, particularly in the manufacture of fine chemicals and pharmaceuticals.

Graphical abstractSynthesis and characterization of new copper(II) macrocycle complexes and their role in the electrochemical and regioselective chemical oxidation of phenol are described. Effects of temperature, time, reagent and catalyst amounts and pH on the overall conversion as well as on product selectivity have been investigated.Figure optionsDownload full-size imageDownload as PowerPoint slide