Protonation of Trimethylsilyl-Substituted Carbon−Carbon Multiple Bonds in Aliphatic Systems. Conformational Dependence of the β-Silyl Stabilizaton of Carbocations

Rates of carbon protonation to give carbocation products were measured in concentrated perchloric acid solutions for cyclohexene, propyne, 1-hexyne, and their 1-trimethylsilyl-substituted analogs. The trimethylsilyl substituent accelerated the reaction markedly and provided the following β-silyl carbocation stabilizing effects: δΔG⧧ = 5.7 kcal/mol-1 for the cyclohexyl system and δΔG⧧ = 6.5 kcal mol-1 for both acetylenic systems. These effects are substantially greater than δΔG⧧ = 2.9 and 3.4 kcal mol-1 found previously for the protonation of ethyl vinyl ether and phenylacetylene, which suggests that the silyl effects in these previous systems were attenuated by additional carbocation stabilization provided through their ethoxy and phenyl groups. The present effects, on the other hand, fall far short of δΔG⧧ = 16−17 kcal mol-1 found for conformationally optimum systems. The influence of conformation on the magnitude of β-silyl effects and how this impinges on the presently studied systems is discussed.