3-Trimethylsilylcyclobutylidene. The γ-effect of silicon on carbenes.

3-Trimethylsilylcyclobutylidene was generated by pyrolysis of the sodium salt of the tosylhydrazone derivative of 3-trimethylsilylcyclobutanone. This carbene converts to 1-trimethylsilylbicyclobutane as the major product. A labeling study shows that this intramolecular rearrangement product comes from 1,3-hydrogen migration to the carbenic center and not 1,3-silyl migration. Computational studies show two carbene minimum energy conformations, with the lower energy conformation displaying a large stabilizing interaction of the carbene center with the rear lobe of the C3-Si bond. In this conformation, the trimethylsilyl group cannot migrate to the carbene center, and the most favorable process is 1,3-hydrogen migration. When the carbene is generated photochemically in methanol, it reacts by a protonation mechanism giving the highly stabilized 3-trimethylsilylcyclobutyl carbocation as an intermediate. When generated in dimethylamine as solvent, the carbene undergoes preferred attack of this nucleophilic solvent from the back of this C-Si rear lobe stabilized carbene.