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 °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 °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.

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► A novel water tolerant catalytic system for alcohol oxidation has been developed.
► Various benzylic alcohols converted to aldehydes with high isolated yields.
► Very low loading of catalyst (0.3 mol%) and TEMPO (3 mol%) is required for efficient oxidation.
► Utilization of distilled water and open air compose a green protocol for the reactions.