Effects of Bridgehead Metalloidal Substituents (MMe₃, M = Si and Sn) on the Stability of the 1-Norbornyl Cation

Abstract

4-Metalloidal (MMe3, M = Si and Sn)-substituted bicyclo[2.2.1]hept-1-yl triflates (1, Y = OSO2CF3) have been synthesized and their solvolyses in aqueous ethanol and trifluoroethanol have been examined together with those of the parent and methyl derivatives (1,Y = OSO2CF3; X = H and Me, respectively). Large rate enhancement factors are observed for the metalloidal substituents. A comparison of these accelerative effects with the corresponding values in the bicyclo[2.2.2]octane and adamantane ring systems (2, Y = OSO2Me, and 3, Y = Br, respectively) indicates the substrate order of these effects is 1 > 2 ≫ 3. Qualitative considerations lead to the pronounced effects in 1 being ascribed to “back-lobe” through-space effects (homohyperconjugation). A more quantitative picture of the stabilizing effects of SiMe3 and SnMe3 on the 1-norbornyl cation (5) is painted by high-level ab initio (MP2/6-31G(d)[DZP]) molecular orbital calculations. The calculations provide strong support for the homohyperconjugative stabilizing effects of these groups but indicate that the weak accelerative effect of 4-methyl in 1 (Y = OSO2CF3, X = Me) is more likely a manifestation of dominant destabilizing ground-state effects. Furthermore, the high-level calculations expose that the usual practice of theoretical modeling of SiMe3 by use of SiH3 to reduce the complexity of the computational problem can be seriously inadequate.

γ-Trimethylsilylcyclobutyl Carbocation Stabilization

“…This carbocation forms in solvolysis reactions at a moderately enhanced rate that is attributed to an interaction of the cationic center with the rear lobe of the carbon−silicon σ-bond. Additional studies by Grob and Sawlewicz 3 on cation 3 and Adcock et al 4 on cation 4 support this suggestion. The rate enhancement in formation of cation 3 is a modest factor of 8.6, whereas the rate enhancement in formation of 4 is a more substantial value of 1300.…”

On the Question of Cyclopropylidene Intermediates in Cyclopropene-to-Allene Rearrangements − Tetrakis(trimethylsilyl)cyclopropene, 3-Alkenyl-1,2,3-tris(trimethylsilyl)cyclopropenes, and Related Model Compounds

“…The steric requirements are greater in TS4a(Si) , where the migrating group closely approaches the gem ‐dimethyl group at C 3 , than in TS4a(H) (leading to the 1‐silyl‐substituted butyne 53a ) and this effect may reverse their relative stabilities, especially in view of the fact that the calculated energy difference between the transition states leading to the isomeric 54a and 53a is only 1.5 kcal mol −1 . Adcock et al have recently pointed out that the common practice of theoretical modeling of Me 3 Si by the use of H 3 Si to reduce the complexity of the computational problem can be inadequate 42…”

The cyclopropylcarbinyl route to γ-silyl carbocations

“…They are greatly stabilized by the “rear lobe” type of interaction shown involving the γ-trimethylsilyl group. A number of related cations are also stabilized by analogous γ-silyl interactions [55–59], which have also been termed ”percaudal” interactions [56]. Certain carbenes can also be stabilized in a similar fashion [60–61].…”