Organic sulfur mechanisms. 21. The reaction of arylsulfenes with sulfur dioxide

Abstract

Phenylsulfene and diphenylsulfene (2), obtained by reaction of 2,6-lutidine with appropriate precursors, react with sulfur dioxide to give, respectively, benzaldehyde and benzophenone, in confirmation of the suggestion of Staudinger and Pfenninger that the benzophenone formed in the reaction of diphenyldiazomethane (1) with sulfur dioxide arises from reaction of the latter with the sulfene (2). The reaction of sulfenes with sulfur dioxide would appear to have only limited scope, all known examples occurring with a sulfene having at least one aryl group attached to the sulfene carbon atom; even with these sulfenes the reaction is apparently readily precluded in the presence of most other sulfene traps. 18O-Labelling experiments in the reaction of diphenylsulfene (2) with sulfur dioxide indicate that the oxygen atoms in about two-thirds of the benzophenone molecules come from the sulfur dioxide, whereas 2 itself supplies the oxygen atoms for the remaining third. Previously suggested mechanisms predict that the benzophenone oxygens come only from the sulfur dioxide. General processes that could lead to the observed labelling pattern are discussed and specific examples presented. The available evidence, including reported labelling studies on the related reaction of sulfur dioxide with sulfur trioxide, provides a basis for excluding certain possible mechanisms and for preferring some over others, but is not sufficient to establish a single mechanism for the reaction of arylsulfenes with sulfur dioxide.