Oxidation of imines to oxaziridines catalysed by transition metal complexes using molecular oxygen as the terminal oxidant

Abstract

The metal-catalysed oxidation of imines using molecular oxygen as the terminal oxidant and aldehydes as co-reductants has been studied. Different transition metal complexes have been tested as catalysts and it is found that cobalt complexes can catalyse the selective oxidation of imines to oxaziridines. The cobalt-catalysed oxidation of imines to oxaziridines is dependent on the aldehyde used as co-reductant; thus, the presence of aliphatic aldehydes leads to a smooth oxidation, whilst in the presence of aromatic aldehydes there was no reaction. The oxidation proceeds well for imines with aliphatic and aromatic substituents attached to the nitrogen and carbon atom, respectively. The electronic nature of the oxygen atom transferred in these reactions, investigated using thianthrene 5-oxide as a probe, was found to be electrophilic in character. The mechanistic aspects of the cobalt-catalysed oxidation of imines to oxaziridines, probably involving a peroxyacyl cobalt intermediate, are discussed.