"Syn-Effect" in the Base-Induced Isomerization of Vinylic Sulfones to Allylic Sulfones and the Related Various Reactions

Abstract

“Syn–effect” has been recognized as a major cause of stabilizing the syn–conformation at the transition state of a number of reactions against the steric hindrance. This account deals with the origin of the “syn–effect” on the basis of the stereochemical outcome in the isomerization of vinylic sulfones leading to the formation of allylic sulfones and in the related various reactions. We proposed that a σ→π* interaction between the σ–orbital(s) of the allylic C–H σ–bond(s) (or of the C–H σ–bond(s) at the α–position for carbonyl compounds) and the antibonding orbital (π*) of the C=C double bond (or of the C=O double bond for carbonyl compounds) is the most important and essential factor for the “syn–effect”, though the contribution of a 6π–electron homoaromaticity and/or of a hydrogen bonding cannot be entirely ruled out.