Synthesis of Conjugated Dienes from Propene or other Alkenes by the Ene‐type Reaction of Thionium Ions

Abstract

The known ene-type addition of thionium ions to alkenes has now been applied to the synthesis of homoallylic sulfides, from thionium ions RHC=S+Ar (R = alkyl, Ar = 4-chlorophenyl) and propene or other unactivated alkenes. Oxidation and thermal dehydrosulfenylation then lead to conjugated dienes. Because the reaction conditions are compatible with many functional groups, this method is simple and useful.a-Thio carbocations, ,2 also called "thionium" ions, are easily available and reactive intermediates which are analogous in structure and in reactivity to oxonium ions (a-alkoxy carbocations). Iminium ions (a-amino carbocations) are much less reactive. Ab initio calculations,3 as well as experimental results4 suggest that an alkylthio or arylthio group in the a position stabilises carbocations about as much as an alkoxy group does, and both are much less stabilising than a dialkylamino group.