Iron Porphyrin Catalyzed Air Oxidation of Aldehydes and Alkenes

Abstract

Propionaldehyde undergoes autoxidation to propionic acid in benzene solution in the presence of millimolar quantitites of iron(III) tetraphenylporphyrin complexes. The iron prophyrin catalyst promotes oxidation of some 200 equivalents of aldehyde prior to prophyrin ring destruction that takes place over a period of hours. Addition of cyclohexene to the initial reaction mixture serves to diminish the yield of propionic acid, and produce cyclohexene oxide, 2-cyclohexene-l-ol, and 2-cyclohexen-l-one as major oxidation products. Yields of the oxide approximate those of propionic acid, and are higher than the allylic cyclohexene oxidation products. A mechanism is suggested that involves significant metal-oxo group transfer, rather than total radical chain oxidation. The manganese(III) tetraphenylporphyrin analogue also serves as an efficient catalyst.