UCLA

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).