Proximity effects in diaryl derivatives. Part 7. Mechanism of base-catalysed rearrangement of 2-(hydroxyamino)aryl phenyl sulphones to 2-hydroxy-2′-(phenylsulphonyl)azoxybenzenes

Abstract

Base-catalysed rearrangement of a 2-(hydroxyamino)aryl phenyl sulphone (1b) to the 2-hydroxy-2′-(phenylsulphonyl)azoxybenzene (4b) was shown by isolation and kinetic studies to be a rapid reaction requiring oxygen; in the absence of oxygen the sulphur-free azoxybenzene (3; R = Cl) was the only product isolated from the reaction of (1c). A mechanism for the formation of 2-hydroxyazoxybenzenes (4) is proposed (Scheme 2) involving dimerization of a nitrosoaryl radical anion (9) to the dianion of an NN-diol (10), and displacement of a phenylsulphonyl group by intramolecular transfer of oxygen from a nitrogen atom. A similar study of the base-catalysed reactions of a 2-(hydroxyamino)aryl phenyl sulphide (12) in the presence of oxygen showed that with a poorer leaving group the bis(phenylthio)azoxybenzene (11; R = SPh) is formed. An improved procedure for the preparation of N-arylhydroxylamines from nitrobenzenes is described.