ELECTROSTATIC ACCELERATION OF THE 1,5-H SHIFTS IN CYCLOPENTADIENE AND IN PENTA-1,3-DIENE BY LI+ COMPLEXATION - AROMATICITY OF THE TRANSITION STRUCTURES

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 deltaLi+ shifts, the deshieiding of deltaH as well as the exalted magnetic susceptibilities and magnetic anisotropies computed using the IGLO method.