Pyramidalized Alkenes: Theory and Experiment

Structural X-ray data on highly strained anti-Bredt olefins (see, for instance, J. Am. Chem. Soc. 1990 112 8627) prompted us to perform model ab initio calculations with the 6-31G* basis set for ethylene and its simple derivatives. A variety of modes of distortion for ethylene were examined: deformations in plane, pyramidalization, and twist. Olefinic atoms are found to be spontaneously pyramidalized either (1) under the influence of certain substituents that break the original symmetry of a molecule, or (2) when a distortion is introduced in a molecule that simulates the cyclization. Furthermore, pyramidalization is enhanced when a bond angle deformation takes place. The stereochemistry of C=C and Si=Si is briefly compared and discussed.