Systematic Repression of β-Silyl Carbocation Stabilization

Abstract

Solvolysis of 1-(trimethylsilylmethyl)cyclopropyl mesylate in CD(3)CO(2)D gives ring-opened products as well as methylenecyclopropane. The rate enhancement due to the beta-trimethylsilyl group is a factor of about 10(6). The large stabilizing effect of a beta-silyl group (which can cause rate enhancements of up to 10(12)) on the intermediate cation has been repressed. B3LYP/6-31G* computational studies indicate a carbocation stabilization energy of 16.6 kcal/mol. Rates of solvolyses of 1-phenyl-2-trimethylsilylcyclopropyl chlorides are enhanced by a factor of 10(3)-10(4). The intermediate cyclopropyl cation undergoes substantial ring opening since beta-silyl stabilization is not large (calculated stabilization energy of 12 kcal/mol). Solvolysis rates of 2-trimethylsilylbenzocyclobutyl derivatives are not significantly enhanced by the beta-trimethylsilyl group. Beta-silyl stabilization of benzocyclobutenyl carbocations generated in solution has been effectively eliminated due to antiaromatic considerations (calculated stabilization energy of 3.7 kcal/mol when R = Ph). While computational studies parallel solvolytic rate studies, they overestimate the extent of beta-trimethylsilyl stabilization of solvolytically generated carbocations.

Hyperconjugation in Carbocations, a BLW Study with DFT approximation

“…Our results correspond roughly to Frenking's EDA evaluation of the relative stabilization energies between these two systems (33 kcal/mol) (Fernandez and Frenking, 2007), and similar results were reported by others for such silicon in β-position, for instance by isodesmic reactions 9 (Lambert, 1990; Creary and Kochly, 2009). The delocalization energy differences are similar, but the delocalization energies differ, sometimes significantly.…”

“…However, there are evidences that hyperconjugation can be large, and even of similar magnitude as conjugation (Daudey et al, 1980; Mullins, 2012; Wu and Schleyer, 2013). Large hyperconjugation effects (in silicon substituted species) are reported to lead to very significant rate enhancements (up to 10 12 times larger) (Lambert and Chelius, 1990; Creary and Kochly, 2009). They also have been isolated and an X-ray structure is even available 1 …”

The cyclopropylcarbinyl route to γ-silyl carbocations

“…The third type of carbocation that will be incorporated into this paper is the trimethylsilyl-substituted carbocation [36–44]. 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].…”

Access to Cyclic Amino Boronates via Rhodium-Catalyzed Functionalization of Alkyl MIDA Boronates

“…Therefore, substitution of electron-rich groups at the reactive site is typically beneficial . In addition to benzylic cation stabilization (for aryl substituents), the electron-rich sp 3 -boron center is proposed to stabilize the α- and β-cations via hyperconjugation (Figure A,B) . This is similarly invoked in the C–H amination of mannose-derived sulfamate esters (Figure B) as well as [1,2]-boryl migration of oxiranyl-MIDA boronates (Figure C). , These factors presumably contribute to the single regioisomer ( 3e ).…”