A remarkable difference in the conformational preference of the so-bond in 1,3-di-thiane-1-oxides and 1,3-oxathiane-3-oxides.

Conformational Energies in Organic Six-Membered Cyclic Sulfoxides

“…An X-ray crystallographic structure for 2-phenyl-1-oxo-1,3-dithiane has been reported. 9 NMR data have shown 28 the equatorial conformation being favored over the axial form by 2.7 kJ mol −1 , which is consistant with the ab initio results (3.0 kJ mol −1 ) given in Table II. One reason for an equatorial preference in this structure has been related to the closer proximity of two sites of high electron density, namely, the sulfinyl oxygen and the crossring sulfur atom (Scheme 3).…”

Synthesis and Stereochemistry of 1H,5H-Naphtho[1,8-ef][1,3]dithiocine 2-Oxides

“…In the 1 H NMR spectrum of IVa, the parameters of both AB quartets (δ 3.00, 3.59 and 3.22, 3.56 pm) are very similar. An analogous weak stereochemical dependence of the chemical shifts of the methylene protons in the SCH 2 SO fragment was observed previously for the chair-like conformers of 1,3-dithiane 1-oxide with axial and equatorial orientation of the sulfoxide group [8][9][10][11]. Thus we can conclude that the minor conformer of IVa has a boat structure with axial orientation of the sulfoxide oxygen atom.…”

Oxides of 1,3-dithiane and 1,3,5-trithiane. Diamagnetic anisotropy of carbon-sulfur bonds

“…The proton spectrum of 1,3,5-trithiane 1-oxide (7), which is almost unintelligible at 90 MHz, proved to be made up of two AB quartets when observed at 270 MHz (Figure 4).…”

“…If the dimer has the struc- H, ture given by 8, the 2-equatorial protons do reside in a much more shielded position than in the monomer. The preference of 1,3-dithiane cis-1,3-dioxide (7) to exist as a dimer, but of thiane 1-oxide (6) to exist as a monomer (see structures III and IV in ref 3), must result from the ability of 7 to form two attractive dipolar interactions in the dimer (8), whereas 6 can form only one. The more favorable enthalpy of dimer formation can then overcome the unfavorable entropy of formation.…”