Chiral cooperativity and solvent-induced tautomerism effects in electronic circular dichroism spectra of [2.2]paracyclophane ketimines.

[2.2]Paracyclophanes with chiral ketimine side chains constitute a class of highly versatile and enantioselective ligands for catalytic carbon-carbon bond forming reactions. Proper matching of the side chain and [2.2]paracyclophane configurations induces chiral cooperativity, which is key to high selectivities. Here we show that the absolute configuration of both chirotropic elements may be fully resolved by CD spectroscopy and time-dependent density functional calculations. Different ketimine side chain conformations of the diastereomers perturb the planar chiral [2.2]paracyclophane chromophore. This leads to characteristic changes in the measured CD spectra and the specific rotation allowing for the simultaneous assignment of the absolute configuration of both chiral elements. Our results give rise to a simple rule relating sign and magnitude of the specific rotation and the first band of the CD spectra to the absolute configuration of both chiral elements. We infer a tautomeric equilibrium between an ortho-hydroquinone-imine and an ortho-quinone-enamine from strong solvatochromism observed in the CD spectra.