Stereomutations of Atropisomers of Sterically Hindered Salophen Ligands

摘要

The Stereomutations in nonsymmetrical salophen ligands 1-4 were studied by means of dynamic NMR and HPLC methods.DNMR experiments showed that in DMSO-d_6 hindered ligands 2-4 exist in two chiral conformations,depending on whether the imine carbon atoms are in a cis or trans disposition with respect to the plane of the central omicron-phenylenediamine ring,the latter being more stable by 1.0 kcal mol~(-1).Owing to its higher dipole moment,in the apolar solvent C_6D_6 the cis conformer is destabilized with respect to the trans one,in agreement with the results of ab initio calculations.In DMSO-d_6 solution the two conformers are in equilibrium through the less hindered rotation about the C6-N7 bond aligned to the a_(6,7)axis,and the interconversion barriers range from 18.4 to 19.3 kcal mol~(-1).The enantiomerization process is a two step-process that implies sequential rotations around the C6-N7 and the C1-N8 bonds,so that the rate determining step is the slower rotation around the more hindered C1-N8 bond aligned to the a_(1,8)axis,and the energy barriers range from 21.4 to 21.9 kcal mol~(-1).These values compare well with those determined by chromatography on an enantioselective HPLC column at low temperature,thus confirming that DNMR and DHPLC can be conveniently employed as complementary techniques.