PROTONATION OF TRIMETHYLSILYL-SUBSTITUTED CARBON-CARBON MULTIPLE BONDS IN ALIPHATIC SYSTEMS - CONFORMATIONAL DEPENDENCE OF THE BETA-SILYL STABILIZATION 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 trimethyl silyl substituent accelerated the reaction markedly and provided the f ollowing beta-silyl carbocation stabilizing effects: delta Delta G dou ble dagger = 5.7 kcal/mol(-1) for the cyclohexyl system and delta Delt a G double dagger = 6.5 kcal mol(-1) for both acetylenic systems. Thes e effects are substantially greater than delta Delta G double dagger = 2.9 and 3.4 kcal mol(-1) found previously for the protonation of ethy l vinyl ether and phenylacetylene, which suggests that the silyl effec ts in these previous systems were attenuated by additional carbocation stabilization provided through their ethoxy and phenyl groups. The pr esent effects, on the other hand, fall far short of delta Delta G doub le dagger = 16-17 kcal mol(-1) found for conformationally optimum syst ems. The influence of conformation on the magnitude of beta-silyl effe cts and how this impinges on the presently studied systems is discusse d.