Protonation of Trimethylsilyl-Substituted Carbon-Carbon Multiple Bonds in Aliphatic Systems. Conformational Dependence of the beta-Silyl Stabilization of Carbocations (CROSBI ID 120451)
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Gabelica, Vesna ; Kresge, Alexander Jerry
engleski
Protonation of Trimethylsilyl-Substituted Carbon-Carbon Multiple Bonds in Aliphatic Systems. Conformational Dependence of the beta-Silyl Stabilization of Carbocations
Rates of carbocation protonation to give carbocation products were measured in concentrated perchloric acid solutions for cyclohexene, propyne, 1-hexyne, and their trimethyl-silyl-substituted analogs. The trimethylsilyl substituent accelerated the reaction markedly and provided the following β -silyl carbocation stabilizing effects: δ Δ G≠ = 5.7 kcal/mol-1 for the cyclohexyl systems and δ Δ G≠ = 6.5 kcal/mol-1 for both acetylenic systems. These effects are substantially greater than δ Δ G≠ = 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 previously systems were attenuated by additional carbocation stabilization provided through their ethoxy and pheny groups. The present effects, on the other hand, fall far short of δ Δ G≠ = 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.
carbocation stabilization; β -silyl effect; conformational dependence
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