Absolute Configuration of Chiral [2.2]Paracyclophanes with Intramolecular Charge-Transfer Interaction. Failure of the Exciton Chirality Method and Use of the Sector Rule Applied to the Cotton Effect of the CT Transition

Abstract

Optically active 4,7-dicyano-12,15-dimethoxy[2.2]paracyclophanes have been separated by chiral HPLC and their absolute configurations determined by comparison of the experimental and the theoretical VCD spectra. X-ray crystallographic structures for both diastereomers are also reported. The electronic circular dichroism spectra of these enantiomeric pairs, as chiral intramolecular charge-transfer complexes, have been obtained for the first time. The exciton coupling method, usually used for determining the absolute configuration of chiral molecules, however, did not give a correct prediction for the present CT−paracyclophane system. Instead, empirical sector rules for the signs of the Cotton effects of the CT transition can be applied for the assignment of the absolute configuration