Conformational studies of 7-membered cyclic sulfites. 3-oxo-2,4,3-dioxathiepin and 3-OxO-2,4,3- benzodioxathiepin

The conformational and dynamic properties of the 7-membered cyclic sulfites 1 and 2 were investigated by ¹H and ¹³CDNMR as well as by IR spectroscopy. Compound 1 showed two spectral modifications at low temperature for both ¹H (100 MHz) and ¹³C (22.6 MHz) spectra while for compound 2 a single ¹H spectral change has been observed and resolved at 300MHz. Analysis of the low temperature NMR spectra together with the IR frequencies for the stretching vibration of the SO bond indicate that 1 exists as a 1:1 mixture of C-a and TB conformations in CHF₂Cl. Less polar solvents increase the proportion of the C-a form.In contrast, compound 2 is found to exist very predominantly as the TB form in CHF₂Cl at ?160°, whereas in the less polar CCl₄ solvent a small amount of C-a form is found to be present at room temperature from the IR data. The conformational and dynamic properties determined for 1 and 2 are discussed in light of current theories relating to n→σ^* stereoelectronic lone pair donation and dipole-dipole interactions. Furthermore the results for 1 and 2 are used as the basis for a new conformational interpretation of published IR data for dimethyl sulfite. Finally, the differences in properties of 1 and 2 are discussed.