Homoallyl-cyclopropylcarbinyl cation manifold. Trimethylsilyl versus aryl stabilization

A series of E- and Z-1-aryl-5-trimethylsilyl-3-buten-1-yl trifluoroacetates were solvolyzed in CD3CO2D, and rates of reaction as well as products derived from these reactions were determined. Hammett plots showed a break, which was indicative of a mechanistic change from a kC process when the most electron-donating substituents were attached to the aryl group to a kDelta process involving formation of cyclized beta-silyl carbocation intermediates for electron-withdrawing groups. In the case of p-CH3O substitution (a kC extreme), the cationic intermediate captures solvent (95%) or loses a proton (5%). In the case of m-CF3 substitution (a kDelta extreme), the beta-silyl cation intermediate desilylates to give vinylcyclopropane products. Substituents with intermediate electronic properties give more complex product mixtures. Solvolysis of pure Z-trifluoroacetate (p-CH3) gives small amounts of E-trifluoroacetate (p-CH3) along with the E-substitution product. This isomerization suggests that the cyclized beta-silyl cation can isomerize and then reopen to a classical aryl-stabilized cation. By way of contrast, B3LYP/6-31G* computational studies show only cyclized beta-silyl cations as energy minima. Open kC cations are higher-energy nonminimum energy structures.