Effect of the medium on the equilibrium between stereoisomeric six- and seven-membered cyclic chair-like acetals. Role of nonspecific and specific interactions

Abstract

Principles for establishing the nature of solvation effects in stereoisomeric equilibria have been formulated. Using 1H NMR spectroscopy, the equilibrium constants have been determined in 12 solvents for the endo and exo isomers of 1,9,10,11,12,12-hexachloro-4,6-dioxatricyclo[7.2.1.02,8]dodec-10-ene which is characterized by a high barrier to stereoisomeric transformations. The results of correlation analysis have shown that solvation of the conformers with dissimilar orientations of the hexachloronorbornene fragment with respect to the chair-like acetal moiety is determined by the polarity and proton-acceptor properties of the medium. Comparison with the data on solvent effect on the equilibrium between 2-isopropyl-5-methoxy1,3-dioxane epimers suggests that the formation of H-complexes is controlled by electronic and conformational factors.