Reduction by sulphoxides : The mechanism of deoxygenation of heterocyclic N-oxides and tertiary amine N-oxide

Abstract

Heterocyclic and tertiary amine N-oxides undergo an acid-catalysed deoxygenation by sulphoxides at elevated temperatures. The reductions are rationalized by postulating the formation of an initial coordination complex formed by attack of the nucleophilic N-oxide oxygen atom on the sulphur atom of the sulphoxide or its conjugate base. This complex may undergo an internal redox reaction to give a sulphone and amine or alternatively be attacked by a nucleophilic species (presumably sulphoxide oxygen) to produce a sulphinate and amine. Both decomposition pathways were observed. Deoxygenation of heterocyclic N-oxides, however, proceeds primarily by the second route and that of aliphatic amine oxides by the first decomposition process. These observations are rationalized mechanistically. The effect of varying the N-oxide substrates and the sulphoxides has also been examined.