Planarity of ethylene/linear polyene analogues focused on pi-electron holding ability of the components

摘要

Ethylene/polyene analogues composed of heavier group 14 elements, such as silicon and germanium, do not prefer a planar structure. In the repulsion dominant (RD) model of our previous study mainly focusing on the planarity of hexasilabenzene, it was demonstrated that electron repulsion promotes nonplanarization of heavy benzene analogues. In this study, we have investigated a correlation between intramolecular pi-electron transfers (polarization effect) and planarity in various linear unsaturated compounds in order to deepen the RD model. Herein, it was revealed that the ability to hold pi-electrons in the planar molecular structure is characteristic of each element. For example, carbon can hold more than one pi-electron, whereas silicon and germanium cannot tolerate even one pi-electron to keep the planar structure. Thus, pi-accepting substituents on the heavy atom were found to make the heavy ethylenes and linear polyenes planar by controlling the number of pi-electrons on each skeletal atom.