Stereoelectronic interactions and their effects on conformational preference for 1,3-dithiane-1-oxide and 1,4-dithiane-1-oxide. A theoretical and experimental study

Conformational preferences and orbital interactions of 1,3-dithiane-1-oxide (1) and 1,4-dithiane-1-oxide (2) were analyzed using experimental NMR data and theoretical calculations (NBO analysis). The conformational equilibria of compounds 1 and 2 are between axial and equatorial forms, for 1 the most stable form is the equatorial, while, for 2, it is axial. The conformational preference for 1 is dictated by repulsive interaction between lone pairs on sulfur and oxygen to yield an axial conformer, while for 2 the preference is dictated by orbital interaction between LP S 1 → σ C 2 – C 3 ∗ .