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.