Zwitterionic silenes: interesting goals for synthesis?
Properties of silenes, as a function of increased reversal of the Si=C bond polarity, have been examined through quantum-chemical calculations. The aim of this study was to identify silenes that can be of general interest for organic synthesis. The calculations were carried out primarily with the B3LYP hybrid density functional method, but also with the CASSCF, MP2, MP4(SDQ), and CCSD(T) methods. The study was performed on Z(2)Si=CXY compounds which were divided into three sets that differ with regard to their Si substituents (Z), and with their C substituents (X and Y) varying from weakly to strongly pi-electron-donating groups. The charge at the Si atom (q(Si)) was used as a measure of the extent of reversed silicon-carbon bond polarity. For each of the three sets, the variation in silicon-carbon bond lengths (r(Si=C)) and extent of Si pyramidalization (SigmaSi) in relation to q(Si) follow three separate curves. Silenes with strongly pi-electron-donating X and Y groups are completely described by zwitterionic (reverse-polarized) resonance structures. Such zwitterionic silenes are singly (Si=C) rather than doubly bonded (Si=C), and have a distinctly pyramidal Si atom due to negative charge localization. These silenes also have much lower heats of dimerization than the parent silene. Finally, inversion barriers of zwitterionic silenes are increased by electron-withdrawing substituents, and this enables computational design of silenes with their Si atoms as chiral centers. It is hoped that such chiral zwitterionic silenes can find use in organic synthesis.