Direct measurement of the anomeric effect in sulfoxides: conformational analysis and X-ray crystal structures of 2-bromo- and 2-chloro-1,3-dithiane trans-1,3-dioxides

Abstract

X-Ray analysis of the title compounds has shown that crystalline 2-chloro-1,3-dithiane 1,3-dioxide adopts the conformation in which the chlorine substituent is axial, while the analogous 2-bromo compound crystallises in the alternative conformation, with bromine equatorial. However, variable-temperature NMR experiments have revealed that the halogen substituents of both compounds exhibit a pronounced axial preference in dichloromethane solution. This axial preference has been attributed to a combination of anomeric and dipole–dipole interactions between the sulfoxides and the C–X bonds.