Reactions of phosphines with acetylenes. Part VI. 2-Phosphoniaethanesulphonate betaines. The sulphonation of vinyl phosphonium salts

Abstract

Dimethyl acetylenedicarboxylate, triphenylphosphine, sulphur dioxide, and water react to give the betaine 1,2-dimethoxycarbonyl-2-triphenylphosphoniaethane sulphonate(4a). The n.m.r. spectra show that only the threodiastereoisomer crystallises from the reaction mixture, and retains a cyclic conformation in solution. The betaine readily fragments on heating and forms a stable phosphorane on treatment with alkali. Similar betaines (4b) and (4c) are formed using methyl propiolate and methyl phenylpropiolate. It has been shown that the betaines are formed via sulphonation of vinyl phosphonium salts. The previously reported 1,2-dimethoxycarbonylvinylphosphonium bromide was in error and has now been prepared.