Mild, green copper/4-dimethylaminopyridine catalysed aerobic oxidation of alcohols mediated by nitroxyl radicals in water

Abstract

A novel copper/4-dimethylaminopyridine catalyst system has been found to catalyse the aerobic oxidation of a variety of primary and secondary alcohols in water at room temperature, in the presence of a TEMPO or ABNO nitroxyl radical. A tetranuclear Cu II cluster was revealed to be a plausible reactive intermediate.

New copper(II) species from the copper/2,2′-bypyridine and copper/4-dimethylaminopyridine catalyzed aerobic alcohol oxidations

“…In continuation with our previous work on the identification of new copper species involved in the copper-catalyzed aerobic alcohol oxidation reactions [11][12][13][14][15] Yellow-brown crystals that were previously characterized were first observed within two days, which, however, disappeared upon elongated exposure to the air without isolation [11].…”

“…As it was revealed previously [11], the copper(II)-catalyzed oxidation of benzylic alcohol in CH 3 CN gave much inferior results than those in methanol or water solution. We questioned whether or not different copper(II) species could be involved in the catalysis.…”

“…X-ray diffraction analysis reveals a different structure of copper(II) complex 1 from the known tetranuclear species (Fig. 1c) [11]. Complex 1 adopts a mononuclear ionic structure and in the asymmetric unit there contains two protonated pyridinium cations associated with one tetrachlorocuprate counter anion (Fig.…”

“…Recently, we were interested to investigate the influence of various organic ligands in the catalytic reactivity of Cu I -or Cu II -catalyzed aerobic alcohol oxidations, with a focus on the structural elucidation of possible Cu I or Cu II complexes formed during the catalytic reactions [10][11][12][13][14][15]. We have observed that tetranuclear copper clusters derived from N,O-Schiff base ligands catalyzed aerobic oxidations of benzylic alcohols under aqueous basic conditions, featuring higher activity than its dinuclear analogue [10].…”

“…Subsequently, we reported an inexpensive copper/DMAP system for the aerobic oxidations of a broad range of primary, secondary and allylic alcohols in water under mild conditions, where a unique catalytically active Cu 4 (µ 4 -O)(DMAP) 4 Cl 6 cluster has been isolated and structurally characterized by X-ray structural analysis [11]. The continuing work revealed that a dinuclear Cu II complex displayed higher reactive activity than some analogues derived from simple monodentate nitrogen ligands structurally related to DMAP [12,13].…”