γ-Trimethylsilylcyclobutyl Carbocation Stabilization

Abstract

A series of isomeric 3-trimethylsilyl-1-arylcyclobutyl carbocations, 10 and 11, where the cross-ring 3-trimethylsilyl group has the potential to interact with the cationic center, have been generated under solvolytic conditions. When the cationic center can interact with the rear lobe of the carbon-silicon bond, rate enhancements become progressively larger as the substituent on the aryl group becomes more electron-withdrawing. When the potential interaction with the trimethylsilyl group is via a front lobe interaction, there is minimal rate enhancement over the range of substituents. Computational studies have also been carried out on these cations 10 and 11. Calculated trimethylsilyl stabilization energies progressively increase with electron-withdrawing character of the aryl groups when the trimethylsilyl interaction is via the rear lobe. By way of contrast, there are minimal changes in stabilization energies when the potential trimethylsilyl interaction is via the front lobe of the carbon-silicon bond. These computational studies, along with the solvolytic studies, point to a significant rear lobe 3-trimethylsilyl stabilization of arylcyclobutyl cations. They also argue against any front lobe stabilization of the isomeric arylcyclobutyl cations.

[Cp2TiCH2CH2(SiEt2CHMe2)]+, an Alkyl-titanocene(IV) Complex Containing an Unconventional Ti←C(β)-Si Mode of Bonding

“…The molecule was therefore suggested to contain an agostic-like Zr←C­(β)-Si bonding interaction which was rationalized on the basis of a strong, trimethylsilyl-induced bonding interaction (γ-silyl effect) between the zirconium and the back lobe of the β-CH 2 group of the alkyl ligand as in Figure . This unusual mode of bonding was thought to be analogous to that postulated as a γ-silyl effect for analogous silyl carbenes and carbocations …”

“…The latter contains an unprecedented kind of Zr ← C­(β)-Si (β-C agostic?) structure which was rationalized on the basis of a strong, trimethylsilyl-induced (γ-silyl effect) bonding interaction between the zirconium and the back lobe of the β-CH 2 group of the alkyl ligand. Consistent with this hypothesis, the SiMe 3 group was found to lie in the O–Zr–C­(α)-C­(β) plane, suggesting the absence of significant agostic β-H interactions with the metal…”

3-t-Butyl-1-methylcyclobutyl Cation. Experimental vs Computational Insights into Tertiary Bicyclobutonium Cations

“…A number of years ago we carried out studies designed to evaluate the effect of a γ-trimethylsilyl group on a number of cyclobutyl cations of type 1 . It was determined that such cations formed at remarkably enhanced rates (10 5 faster) relative to desilylated analogs.…”

The cyclopropylcarbinyl route to γ-silyl carbocations

“…We have been interested in long-range interactions of silicon with both carbene [45–48] and carbocation centers [49–50]. Along these lines, γ-trimethylsilyl cations of general type 11 have been generated under stable-ion [51] as well as solvolytic conditions [52–54]. They are greatly stabilized by the “rear lobe” type of interaction shown involving the γ-trimethylsilyl group.…”