| Abstract
| - The stereomutations in nonsymmetrical salophen ligands 1−4 were studied by means of dynamicNMR and HPLC methods. DNMR experiments showed that in DMSO-d6 hindered ligands 2−4exist in two chiral conformations, depending on whether the imine carbon atoms are in a cis ortrans disposition with respect to the plane of the central o-phenylenediamine ring, the latter beingmore stable by 1.0 kcal mol-1. Owing to its higher dipole moment, in the apolar solvent C6D6 thecis conformer is destabilized with respect to the trans one, in agreement with the results of abinitio calculations. In DMSO-d6 solution the two conformers are in equilibrium through the lesshindered rotation about the C6−N7 bond aligned to the a6,7 axis, and the interconversion barriersrange from 18.4 to 19.3 kcal mol-1. The enantiomerization process is a two step-process that impliessequential rotations around the C6−N7 and the C1−N8 bonds, so that the rate determining stepis the slower rotation around the more hindered C1−N8 bond aligned to the a1,8 axis, and theenergy barriers range from 21.4 to 21.9 kcal mol-1. These values compare well with those determinedby chromatography on an enantioselective HPLC column at low temperature, thus confirming thatDNMR and DHPLC can be conveniently employed as complementary techniques.
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