Facile Preparation of Trimethylsulfonium Bromide

Abstract

Trimethylsulfonium bromide, utilized to make oxiranes and molten salts, has been reported to be prepared from dimethyl sulfoxide and ethyl bromoacetate, alkyl halide and dimethyl sulfide, and dimethyl sulfoxide and bromine. Herein, we report a very simple and convenient method of trimethylsulfonium bromide from a modified Kornblum oxidation condition. Typical Kornblum oxidation is referred to a reaction in which alkyl/aryl halide (tosylate) is reacted with dimethyl sulfoxide followed by a direct workup procedure in basic aqueous solution to prepare corresponding aldehyde (path 1 in Figure 1). However we have found that Kornblum oxidation in the absence of water and base afforded colorless crystalline trimethylsulfonium bromide in about 65% yields after recrystallization. The X-ray crystallographic and NMR data of trimethylsulfonium bromide were consistent with those of the published report. Several pathways can be envisaged for the formation of trimethylsulfonium bromide in our conditions. However we believe that a path 2 in Figure 1 could be a major route in our acidic conditions based on control experiments. We found out that volatile products (methyl bromide, dimethyl sulfide, and hydrogen bromide) were efficiently made in our conditions. That is, we were able to isolate a mixture of aniline hydrobromide and N-methylaniline which was formed in an aniline trap attached at the outlet of reflux condenser. In an another experiment we could observe and collect solid trimethylsulfonium bromide (about 1 gm) formed in a reservoir attached between the reaction flask and reflux condenser, which gave us an information that methyl bromide and dimethyl sulfide were obviously generating in an appreciable amount in our reaction conditions. Supposing a path 2 in Figure 1 to be a major route, then one might ask for the verification of benzyl methanesulfenate. We tried to get any evidence for the existence of benzyl methanesulfenate by both NMR and GC/MS, only to fail. We believe that benzyl methanesulfenate can not survive our acidic conditions. Instead we found out that benzyl methyl sulfide and dibenzyl sulfide were formed. In order to get more pieces of evidence related with this modified Kornblum oxidation reactions, reactions of benzyl bromide with diphenyl sulfoxide and methyl phenyl sulfoxide were carried out. As shown in Figure 2, reactions of benzyl bromide with either diphenyl sulfoxide and methyl phenyl sulfoxide turned out to be very clean by NMR. Trimethylsulfonium bromide was not formed at all in these reactions. Noteworthy was that further oxidation proceeded in reaction of diphenyl sulfoxide to give benzoic acid, instead of benzaldehyde obtained in the reaction of methyl phenyl Figure 1. Reaction of benzyl bromide and dimethyl sulfoxide.