Can acyclic conformational control be achieved via a sulfur–fluorine gauche effect?

Abstract

The gauche conformation of the 1,2-difluoroethane motif is known to involve stabilising hyperconjugative interactions between donor (bonding, σ_C–H) and acceptor (antibonding, σ^*_C–F) orbitals. This model rationalises the generic conformational preference of F–C_β–C_α–X systems (ϕ_FCCX ≈ 60°), where X is an electron deficient substituent containing a Period 2 atom. Little is known about the corresponding Period 3 systems, such as sulfur and phosphorus, where multiple oxidation states are possible. Conformational analyses of β-fluorosulfides, -sulfoxides and -sulfones are disclosed here, thus extending the scope of the fluorine gauche effect to the 3rd Period (F–C–C–S(O)_n; ϕ_FCCS ≈ 60°). Synergy between experiment and computation has revealed that the gauche effect is only pronounced in structures bearing an electropositive vicinal sulfur atom (S⁺–O⁻, SO₂).