Facile open air oxidation of benzylic alcohols in distilled water by in situ made copper(II) complexes

A highly efficient, selective and green catalytic protocol for open air oxidation of primary benzylic alcohols into aldehydes by in situ made N-isopropyl-3,5-di-tert-butylsalicylaldimine (HL3)-Cu(II) complexes and TEMPO (2,2,6,6-tetramethyl-piperidinyloxyl radical) is introduced herein. Distilled water as a solvent the reaction proceeds at 80 degrees C temperature without any auxiliarities such as base and/or co-solvent. For example, benzyl alcohol is quantitatively and selectively oxidized to benzaldehyde within a few hours under optimized reaction conditions [80 degrees C, open air, 0.3 mol% CuBr2, 2 mol% HL3, 3 mol% TEMPO and 5 mL of distilled water]. Under anaerobic conditions, the catalyst decomposes to a free ligand and Cu2O nanoparticles (diameter >50 nm) via anticipated Cu(I)-hydroxo complex. On the basis of these observations and deuteration studies, a plausible reaction mechanism is proposed for the catalytic system. (C) 2012 Elsevier B.V. All rights reserved.