Ab initio study of positive charge stabilization by silicon in five-membered rings

Abstract

Angular constraints in a five-membered ring prevent the optimal overlap between positive charge on carbon and a β-C–Si bond that is possible in six-membered rings, ab initio methods have been used for the first time to study this phenomenon in secondary, common ring systems. Within the most stable conformer of the cyclopentyl cation (the twist form) introduction of a β-pseudo-axial C–Si bond (as SiH₃) stabilizes the system by 15.8 kcal mol^–1, whereas introduction of a β-pseudoequatorial C–Si bond stabilizes it by only 5.6 kcal mol^–1. The structure distorts in three ways to optimize the hyperconjugative interaction: by adjusting the dihedral angle between the C–Si bond and the empty p-orbital to as low a value as permissible by angle strain, by lowering the ⁺C_α–C_β–Si valence angle, and by shortening the C_α–C_β bond length. The calculated dihedral angles are in good agreement with solvolysis results. The maximum hyperconjugative and inductive contributions were calculated to be 15.8 and 0.9 kcal mol^–1, respectively.