TEMPO‐Copper(II) Diimine‐Catalysed Oxidation of Benzylic Alcohols in Aqueous Media

Abstract

In situ generated copper(II)-diimine complexes combined with TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl radical) were studied in the oxidation of benzylic alcohols, the focus being on enviromentally benign reaction conditions. In this respect, reactions were studied in aqueous alkaline solutions and dioxygen was used as an end oxidant. This simple catalytic system turned out to be highly efficient and selective in the oxidation of primary and secondary benzylic alcohols to their corresponding carbonyl compounds. Under optimised reaction conditions [5 mol % of TEMPO, 3 mol % of copper (II) diimine, pH 12.6-13.5, 80 8C, 10 bar O 2 ] benzyl alcohol was quantitatively and selectively oxidised to benzaldehyde. According to ESI-MS studies, coordination of TEMPO, as well as deprotonated benzyl alcohol to the parent copper-diimine complex in aqueous solutions is feasible. Supported by these observations a plausible reaction mechanism is proposed for the oxidation reaction.

Solvent-Free Microwave-Assisted Peroxidative Oxidation of Alcohols Catalyzed by Iron(III)-TEMPO Catalytic Systems

“…The very similar catalytic activities of 1-3 are in accord with their very similar reduction potentials as detected by cyclic voltammetry (see above), which conceivably determine the iron promoted decomposition of TBHP (see mechanistic considerations below) [39,40].…”

Modern Oxidation of Alcohols Using Environmentally Benign Oxidants

“…More recently, Repo and coworkers [117] showed that the selective aerobic oxidation of benzyl alcohol to benzaldehyde could be performed with TEMPO in combination with Cu(II)/phenanthroline in aqueous alkaline with no added organic solvent. Finally, an interesting recent development in copper-catalyzed oxidation of alcohols is the use of copper nanoparticles on hydrotalcite as a heterogeneous catalyst for the liquid phase dehydrogenation of alcohols in the absence of an oxidant [118].…”

Room temperature aerobic oxidation of alcohols using CuBr2 with TEMPO and a tetradentate polymer based pyridyl-imine ligand

“…2-furylmethanol) under an oxygen atmosphere [38]. Repo et al recently reported a highly efficient catalytic system based on TEMPO/Copper diimine (10 bar oxygen) and 2-Narylpyrrolecarbaldimino (1 atm oxygen) systems for alcohol oxidation [39][40]. In recent studies, Koskinen et al and Stahl et al have described improved bipyridine-Cu-TEMPO catalytic systems for the effective oxidation of alcohols [41][42].…”