Chirality Induced by the Interaction of C═C and C═X Bonds (X═CH, NH, NH, O, and S) Separated by a Methylene Group.

The chirality derived from the interaction between two double bonds (C═C and C═X where X ═ CH, NH, NH, O and S) across a methylene group has been examined in some detail at the B3LYP/aug-cc-pVTZ level. The rotamers of these formally achiral molecules that lead to large calculated specific rotations are those that allow overlap between the pair of double bonds, and the rotation is increased when the distance between them is decreased. Although the large specific rotation of unconjugated enones such as norbornenone is usually attributed to a magnetic transition dipole arising from an n-π* interaction of the carbonyl group, it has been found that large rotations may also be found with some conformations of unconjugated dienes such as 1,4-pentadiene. In the relative orientations of the double bonds that lead to large optical rotations, the pair of p-π orbitals are oriented so that they can interact leading to an electric transition dipole, while at the same time they are somewhat twisted with respect to each other allowing a significant magnetic transition dipole Other information was obtained by studying 2-propene-1-imine and its conjugate acid. The former gives only a small increase in optical rotation as compared to pentadiene. However, protonation at N leads to a large increase in optical rotation. This suggests that lone pairs have little effect on rotation, whereas the change in electronegativity has a large effect.