AB-INITIO CALCULATIONS OF DIELS-ALDER TRANSITION STRUCTURES FOR HETERO-DIENOPHILE ADDITIONS TO CYCLOPENTADIENE

Transition states for the hetero-Diels-Alder addition of formaldehyde, formaldimine, diazene and nitrosyl hydride to cyclopentadiene were lo cated with ab initio molecular orbital calculations. Asynchronous tran sition structures were located for the asymmetric hetero-dienophiles, whereas the symmetrical hetero-dienophile, cis-diazene, produced a syn chronous transition structure. The relative reactivities are in good a greement with the LUMO energies of the hetero-dienophiles, and the tra nsition states with the aro oxygen or nitrogen lone pairs have lower e nergies than the corresponding endo lone electron pair transition stat es. Endo/exo energy differences are much higher than in the addition o f ethylene derivatives to eyclopentadiene.