Chiroptical Properties of Imines Derived from R-(+)-Norbornenone: The Role of Electronegativity Differences.

To allow a comparison with the specific rotations of R-(+)-5-methylenenorbornene (1) and R-(+)-norbornenone (2) we performed calculations at the LC-wPBE/aug-cc-pVTZ level for the imines (5a and 5b) derived from norbornenone and also for their protonated derivative (6). In accord with our results for simpler systems, the specific rotations increase in the order of 1 < 5 < 2 ≈ 6. In addition, the specific rotation of the protonated ketone was calculated and found to be considerably larger than that for 2 or 6. These rotations were found to be linearly dependent on the Hirshfeld charges at the carbon of the exocyclic double bond. This leads to the conclusion that charge transfer from the endocyclic double bond to the π* MO of the exocyclic double bond is an important component of the process that leads to the optical activity of these compounds.