Conformational impact of pentafluorosulfanylation on acyclic aliphatic molecules

The diastereotopic nature of geminal protons γ to the pentafluorosulfanyl (SF5) group was investigated by computational modeling and experimental methods. 1D and 2D NMR techniques were employed to determine the vicinal coupling constants used in the estimation of HCCH dihedral angles required for the approximation of the average solution conformation of SF5-substituted alkyl chains. Rotational energy barriers were calculated at the B3LYP/6-31++G (d,p) level in an effort to assess the relative steric demand of the SF5 group relative to a trifluoromethyl or tert-butyl group. The observed diastereotopicity is likely a result of hindered molecular rotation where one of the γ protons is trapped between two equatorial fluorines of the SF5 group.

The influence of the SF5 group on conformation was investigated by computational modeling and NMR methods. Vicinal coupling constants, 3J, were used to determine average conformations. Rotational energy barriers were calculated at the B3LYP/6-31++G (d,p) level in an effort to assess the relative steric demand of SF5 group relative to CF3 and (CH3)3C-groups.Figure optionsDownload as PowerPoint slideHighlights
► 3D structures of select SF5 compounds were determined using NMR coupling constants.
► Gamma proton wedged between equatorial fluorines in most stable conformer.
► Wedging hinders molecular rotation; gives diastereotopic signals for gamma protons.