A facile oxidation of secondary amines to imines by iodosobenzene or by a terminal oxidant and manganese or iron porphyrins and manganese salen as the catalysts

Abstract

The oxidation of secondary amines to imines by iodosobenzene or catalysed by either manganese(III) or iron(III) porphyrins, or by a manganese(III) salen complex with iodosobenzene as the oxygen donor has been investigated. Both aromatic and aliphatic amines can be oxidized smoothly to the corresponding imines with iodosobenzene as the oxidant and the elimination of hydrogen takes place towards the least substituted carbon. Manganese(III) and iron(III) porphyrins and manganese(III) salen are found to catalyse the oxidation of secondary amines to imines with iodosobenzene as the terminal oxidant. Those amines that are less reactive when iodosobenzene is the oxidant, can be converted to the imines in higher yields if a catalyst is added, compared with the uncatalysed reaction. By-products such as carbonyls and nitrones are found in some of these reactions. The manganese(III) salen complex is found to be the best catalyst, followed by manganese(III) porphyrin and iron(III) porphyrin. On the basis of Hammett plots, isotopic labelling studies and other experimental investigations the mechanisms for the uncatalysed and catalysed amine-to-imine oxidations are discussed.