Conformational przferknce of the S→O bond. ¹H and ¹³C NMR studies of the mono-S-oxides of 1,2-, 1,3- and 1,4-dithianes

The ¹H and ¹³C NMR behavior of the monosulfoxides of 1,2-, 1,3-and 1,4-dithianes (1-3) were studied in order to determine the conformational preference of the S→O bond in these heterocycles. From the results of variable temperature, double irradiation, solvent effects and shift reagent experiments, It is concluded that the axial conformers dominate the conformational equilibria of 1 and 3. On the other hand, 2-equatorial is more stable than 2-axial by 0.64 kcal/mol (ΔG°) at -80°, in CD₃OD. This value is essentially identical with the one determined in CHClF₂, and the lack of a solvent effect appears to indicate that dipole/dipole interactions do not control this equilibrium. AΔG_c^‡ = 11.0 kcal/mol was determined for the inversion process of 2. Complete ¹H and ¹³C NMR assignments for 1-3 are presented.