CONFORMATIONAL-ANALYSIS .23. A LANTHANIDE-INDUCED SHIFT INVESTIGATION OF SOME CYCLIC AND ACYCLIC SULFOXIDES

A lanthanide-induced shift (LIS) analysis of methyl phenyl sulfoxide 1, diphenyl sulfoxide 2, thiane 1-oxide 3, thiolane 1-oxide 4 and 1,3-dithiane 1-oxide 5 is reported. The appropriate lanthanum-sulfoxide complexation model was investigated using 1 and 2 as substrates. A three-site binding model with variable populations of the three sites using the LIRAS4 program gives the best agreement factor. This model is then used to determine the conformations and conformational equilibria in the five- and six-membered ring sulfoxides. In the multi-functional 1,3-dithiane 1-oxide the lanthanum complexes exclusively with the SO group, and there is no evidence of any complexation with the 3-sulfur atom. The percentage of O equatorial conformer in thiane 1-oxide and 1,3-dithiane 1-oxide was determined as 45 and 95% respectively, in general agreement with the results of low temperature NMR studies, which are in solvents of different polarity. The large solvent dependence of these conformational equilibria is given a quantitative explanation in terms of classical solvation theory. Molecular mechanics calculations on thiolane I-oxide predicted a novel envelope conformation of the five-membered ring which gives very good agreement for the LIS shifts. It is concluded that the application of the LIS method via LIRAS4 to sulfoxide conformations and structures is a promising novel development in this area.