THE CIEPLAK EFFECT - HYPERCONJUGATIVE INTERACTIONS IN DIELS-ALDER REACTIONS

Molecular orbital calculations of the transition states for the reactions of the dienes la-e with ethylene using the AM1 Hamiltonian show the diene C-X bond length for anti addition of the dienophile is longer than the comparable bond length when the X-substituent is syn to the approaching dienophile. The bond lengthening is consistent with hyperconjugative stabilization involving the donation of electron density from the sigma-bond of the anti substituent to the forming sigma*-orbital.