Structure, modelling and dynamic behaviour of aza- and azaoxamacrocyclic ligands derived from (R,R)-1,2-diaminocyclohexane

Abstract

Investigations into the conformational behaviour of macrocyclic ligands 5 and 6 derived from (R,R)-1,2-diaminocyclohexane have been undertaken using molecular modelling, single crystal X-ray diffraction and variable temperature ¹H NMR spectroscopy. These have revealed that the lowest energy conformers in both cases do not possess the expected C₂-element of symmetry, which can only be accessed at higher temperatures. Instead both molecules exist as C₁-conformers at room temperature and in the solid state. In solution a range of dynamic exchange processes is observed which result, in part from the inherent strain in these fused bicyclic systems. An unexpected but characteristic feature of the C₁-symmetric conformers is highlighted by the presence of a signal at unexpectedly low field in their ¹H NMR spectra due to the interaction of two of the sulfonyl oxygen atoms with one of the bridgehead hydrogen atoms.