Conformational Analysis. 47. The Stereochemistry of Sulfur Organic Compounds. 21. The Conformation of Oxanes with Sulfur Substituents

Article abstract of DOI:10.1021/jo00227a028

The energy difference between equatorial and axial sulfur functions at C(3) in oxane has been determined in various solvents by low-temperature 13C NMR spectroscopy.The values in CD2Cl2 are as follows: MeS, -1.21 +/- 0.11 kcal/mol; SOMe, more polar isomer, -0.10 +/- 0.05 kcal/mol, less polar isomer, -0.43 +/- 0.13 kcal/mol; SO2Me, ca. -1.45 kcal/mol; Me2S⁺, 0.55 kcal/mol; HS, -1.1 +/- 0.1 kcal/mol (negative values meaning preference for the equatorial conformer).The configurations of the two sulfoxides were determined by a combination of X-ray crystallography and proton and 13C NMR spectroscopy.In the case of MeS, -ΔG⁰ is close to the average of the corresponding conformational free energy differences in cyclohexane and dioxane (at C-5).For MeSO the same is true for the mean -ΔG⁰ of the two sulfoxides but not for the individual values.In the case of MeSO2, the experimental value of -ΔG⁰ is much larger than the above average, supporting an earlier hypothesis that in 1,3-dioxan-5-yl methyl sulfone the methyl group is turned into the ring.In the methyl 3-tetrahydropyranyl sulfone, this would produce serious steric compression with the axial hydrogen at C(5).