Selective aerobic oxidation of alcohols catalyzed by iron chloride hexahydrate/TEMPO in the presence of silica gel

An environmentally friendly and efficient process whereby FeCl3 center dot 6H(2)O/2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO)-catalyzed oxidation of alcohols to the corresponding aldehydes and ketones is accomplished in the presence of silica gel using molecular oxygen or air as the terminal oxidant. The electron-deficient benzyl alcohol was smoothly oxidized to the corresponding aldehydes with up to 99% isolated yield. It was found that silica gel not only could enhance the catalytic reaction rate but also increase the selectivity for the product. The high performance of FeCl3 center dot 6H(2)O/TEMPO catalyst system in the presence of silica gel might be attributed to the surface silanol groups. UVvisible spectra analysis showed that the Fe (III)TEMPO complex could serve as the active intermediate species in the present catalytic system. A plausible mechanism of the catalytic system is proposed. Copyright (C) 2012 John Wiley & Sons, Ltd.