A series of 3-trimethylsilyl-1-substituted cyclobutyl trifluoroacetates have been prepared and reacted in CD(3)CO(2)D. Rate data indicate that the substrates with the trimethylsilyl group cis to the leaving group react with assistance due to gamma-silyl participation. Rate enhancements range from a factor of 209 for alpha-phenyl-substituted cations to 4.6 x 10(4) for alpha-methyl-substituted cations to >10(10) for the unsubstituted gamma-trimethylsilylcyclobutyl cation. Acetate substitution products are formed with net retention of stereochemistry. These experimental studies, as well as B3LYP/6-31G* computational studies, are consistent with the involvement of carbocations where the rear lobe of the gamma-Si-C bond interacts strongly with the developing cationic center. Solvolytic rate studies, as well as computational studies, suggest that the secondary gamma-trimethylsilylcyclobutyl cation is even more stable than the beta-trimethylsilylcyclobutyl cation, i.e., the gamma-silyl effect actually outweighs the potent beta-silyl effect. Although computational studies suggest the existence of certain isomeric cations, where the front lobe of the Si-C bond interacts with the cationic center, solvolytic evidence for the involvement of these front lobe stabilized cations is less compelling.

Solvolysis of 1-(trimethylsilylmethyl)cyclopropyl mesylate in CD(3)CO(2)D gives ring-opened products as well as methylenecyclopropane. The rate enhancement due to the beta-trimethylsilyl group is a factor of about 10(6). The large stabilizing effect of a beta-silyl group (which can cause rate enhancements of up to 10(12)) on the intermediate cation has been repressed. B3LYP/6-31G* computational studies indicate a carbocation stabilization energy of 16.6 kcal/mol. Rates of solvolyses of 1-phenyl-2-trimethylsilylcyclopropyl chlorides are enhanced by a factor of 10(3)-10(4). The intermediate cyclopropyl cation undergoes substantial ring opening since beta-silyl stabilization is not large (calculated stabilization energy of 12 kcal/mol). Solvolysis rates of 2-trimethylsilylbenzocyclobutyl derivatives are not significantly enhanced by the beta-trimethylsilyl group. Beta-silyl stabilization of benzocyclobutenyl carbocations generated in solution has been effectively eliminated due to antiaromatic considerations (calculated stabilization energy of 3.7 kcal/mol when R = Ph). While computational studies parallel solvolytic rate studies, they overestimate the extent of beta-trimethylsilyl stabilization of solvolytically generated carbocations.

No abstract

Abstract:A study was undertaken of the solvolysis of pivaloyl triflate in a variety of ionic liquid:alcohol solvent mixtures. The solvolysis is a k ∆ process (i.e., a process in which ionization occurs with rearrangement), and the resulting rearranged carbocation intermediate reacts with the alcohol cosolvent via two competing pathways: nucleophilic attack or elimination of a proton. Five different ionic liquids and three different alcohol cosolvents were investigated to give a total of fifteen dual solvent systems. 1 H-NMR analysis was used to determine relative amounts of elimination and substitution products. It was found, not surprisingly, that increasing the bulkiness of alcohol cosolvent led to increased elimination product. The change in the amount of elimination product with increasing ionic liquid concentration, however, varied greatly between ionic liquids. These differences correlate strongly, though not completely, to the Kamlet-Taft solvatochromic parameters of the hydrogen bond donating and accepting ability of the solvent systems. An additional factor playing into these differences is the bulkiness of the ionic liquid anion.

Solvolysis studies of pivaloyl triflate were carried out in the ionic liquid 1-butyl-3-methylimidazolium bistrifluoromethanesulfonamide with various cosolvents. Reaction solutions were analyzed by 1 H NMR and the relative amounts of substitution and elimination products measured. It was found that regardless of cosolvent, increasing the ionic liquid:cosolvent ratio leads to increased elimination product. The rate of increase, however, is dependent on the identity of the cosolvent. Kamlet-Taft solvatochromic parameters of hydrogen bond accepting and donating ability were invoked to explain this phenomenon.

In Scheme 2, the value of ΔE = 16.6 kcal/mol for the first isodesmic reaction is incorrect. The correct value (shown below) is 31.9 kcal/mol and refers to the constrained cation 21. The value of 16.6 kcal/mol originally reported is the value for the unconstrained 1-methylcyclopropyl cation, which opens without barrier to the allylic cation.

No abstract