Advances in Carbocation Chemistry: Carbocations in Ionic Liquids, beta-Silyl Cyclopropyl Carbocations, and beta- and gamma-silyl Cyclobutyl Carbocations

This dissertation describes studies encompassing a variety of areas of carbocation chemistry. Carbocation formation in ionic liquids, which are polar aprotic solvents, was studied. The chemistry of some unusual silicon substituted carbocations was also investigated. Reactions of a number of triflates, mesylates and trifluoroacetates in ionic liquids have been investigated. It was concluded that these substrates all react via carbocationic intermediates. In fact, some of the ionic liquids under investigation were found to have an ionizing power similar to that of CF3CH2OH, a commonly used solvent for carbocation formation. Evidence in support of this conclusion include the observation of carbocationic rearrangements in a variety of substrates, a Hammett-Brown rho+ value of Ì¢‰âÂ'7.1 for the solvolyses of 1-aryl-2,2,2-trifluoroethyl triflates, and the facile solvolysis of 1-adamantyl mesylate which necessarily proceeds via a carbocationic intermediate. Some unusual reactivity was seen in ionic liquids as well. For example, it was found that small concentrations of water in the ionic liquids have a significant impact on the rates of solvolyses of mesylates. The effect of water on triflate solvolyses however, was negligible. Additionally, a novel oxidation process of triflates involving loss of sulfinate was found to occur in certain ionic liquids. An investigation of the stabilizing effect of beta-silyl groups on cyclopropyl carbocations was also undertaken. The normally large effect of a beta-silyl group (which can cause solvolysis rate enhancements of up to 1012) was found to be greatly repressed in cyclopropyl systems. Computational studies agreed with this conclusion. The intermediate silyl-substituted cyclopropyl cations instead tended to undergo ring opening since silyl stabilization is reduced. A similar study was undertaken involving cyclobutyl carbocations. Incorporation of a beta-silyl group into cyclobutyl carbocations led again to a smaller than expected b-silyl stabilization. In fact, the effect was successfully quashed in a beta-silyl benzocyclobutenyl carbocation in which beta-silyl stabilization would have led to antiaromatic character in the carbocation. The gamma-silyl effect was also investigated in cyclobutyl systems. Experimental and computational studies showed that gamma-silyl stabilization can occur via two distinct modes. Either the front, or the rear lobe of the gamma-silyl group can donate electron density into the empty p-orbital of the carbocation. This effect was also found to be much larger than previously recorded.