Effect of aryl substitution at ring carbons on the reactivity of thiirenium ions
Abstract
Methyl(bismethylthio)sulphonium (5) tetrafluoroborate or tetrachloroborate in liquid sulphur dioxide adds to 1-(4-methoxyphenyl)propyne to give methylthio-[E-1-(4-methoxyphenyl)-2-methylthioprop-1-enyl]methylsulphoium and methylthio-[E-1-(4-methoxyphenyl)-1-methylthioprop-2-enyl]methylsulphonium ions. The intermediacy of 1,2-dimethyl-3-(4-methoxyphenyl)thiirenium ion is postulated. The sulphonium salts react with pyridine or triethylamine to give E-1,2-bisalkylthio-1-(4-methoxyphenyl)propene, which interconverts partially to the Z-isomer. Similarly the addition of (5) to 1-phenylpropyne and to 1-phenylbutyne gives, after demethylthiolation, E- and Z-1,2-bismethylthio-1-phenylpropene and E- and Z-1,2-bismethylthio-1-phenylbut-1-ene.