3-t-Butyl-1-methylcyclobutyl Cation. Experimental vs Computational Insights into Tertiary Bicyclobutonium Cations

The stereochemistry of the substitution product formed when (1s,3s)-3-(tert-butyl)-1-methylcyclobutyl methanesulfonate, 7, reacts in trifluoroethanol is one of retention of configuration. This is consistent with the intermediacy of a nonclassical bicyclobutonium cation 8N. As the solvent becomes less highly ionizing, the product contains increasing amounts of the isomeric inverted product. This is indicative of the competitive involvement of a classical cation 8C that captures solvent from both sides. Solvolytic rate studies show no large rate enhancements due to the t-butyl group in 7. However, computational studies suggest that the combined stabilization of the bicyclobutonium cation 8N by C-C σ-interactions and the t-butyl group amounts to 10.8 kcal/mol in the gas phase. Computationally, the t-butyl group contributes to stabilization of 8N, but this stabilization is not revealed by kinetic studies. This discrepancy suggests caution when relating computational studies on carbocations to solvolytic studies in common solvents.