Stereochemical and mechanistic aspects of sulphoxide, epoxide, arene oxide, and phenol formation by photochemical oxygen atom transfer from aza-aromatic N-oxides

Abstract

Stereoselectivity and relative yields are determined for the sulphoxide formation resulting from the u.v. irradiation of a range of aza-aromatic N-oxides in the presence of cyclic thioethers. A comparison is made with the results of oxidation by oxaziridines and by mono-oxygenase enzymes present in the fungus Aspergillus niger. The photochemical oxidation results are consistent with a transition state involving an oxaziridine intermediate where partial bonding of the oxygen atom to the ring nitrogen atom is maintained during the oxygen transfer process.

Photolysis of aza-aromatic N-oxides in the presence of cis- and trans-olefins yields epoxides. cis-4-Methylpent-2-ene yielded both cis and trans-epoxides in almost equal proportions indicating that the oxygen atom addition to a carbon–carbon bond in this system is non-concerted.

The photochemically induced oxygenation of perdeuteriated aromatic substrates provides no evidence for direct insertion of an oxygen atom into an aromatic carbon–hydrogen bond. Addition of an oxygen atom to form an epoxide (arene oxide) intermediate in this system is evidenced by the NIH shift in a wide range of aromatic substrates, and by the detection of arene oxide intermediates (and their isomeric phenols) from naphthalene and phenanthrene.