“Syn-Effect” in the Isomerization of (E)-α-Fluorovinylic Sulfones to the Corresponding Allylic Sulfones under Basic Conditions

Abstract Toward the elucidation of the origin of the “syn-effect,” the stereochemistry of the isomerization of (E)-α-fluorovinylic sulfones to the corresponding allylic sulfones under mild basic conditions was investigated. The ratio of (Z)-isomers of the resulting allylic sulfones decreased in the following order for the γ-substituents of the starting vinylic sulfones: F- > EtO- > CH3- > BnS- > CH3CH2- > (CH3)2CH- > (CH3)3C-, C6H5-. The fluorine atom showed the highest “syn-effect,” which is herein defined as an effect that stabilizes the syn-conformation in the transition state against a steric or nonbonded repulsion; the rest of the series was in accord with previous results found in the conversion of α-unsubstituted (E)-vinylic sulfones to the corresponding allylic sulfones under similar conditions. These results were rationalized based on the σ→π* interaction.