Introductory lecture. Electrostatic acceleration of the 1,5-H shifts in cyclopentadiene and in penta-1,3-diene by Li+ complexation: aromaticity of the transition structures

Abstract

High-level ab initio molecular orbital calculations reproduce the experimental activation parameters for the 1,5-H shifts in cyclopentadiene and in penta-1,3-diene excellently and predict a remarkable electrostatic acceleration of both 1,5-H shifts by Li⁺ complexation. These catalytic effects (8.0 and 5.2 kcal mol^–1, respectively) are electrostatic in origin: the transition states are more stabilized by Li⁺ than the ground states. Replacement of Li⁺ by a positive charge gives similar results. The aromaticity of the transition states was evidenced by various criteria: by the large energies of concert, by C—C bond-length equalization and by the ring-current effects: upfield δLi⁺ shifts, the deshielding of δH as well as the exalted magnetic susceptibilities and magnetic anisotropies computed using the IGLO method.