ABINITIO STUDY OF DIELS-ALDER REACTIONS OF CYCLOPENTADIENE WITH ETHYLENE, ISOPRENE, CYCLOPENTADIENE, ACRYLONITRILE, AND METHYL VINYL KETONE

Ab initio molecular orbital calculations have been used to investigate the structures and energetics for the reactants and transition states of Diels-Alder reactions between cyclopentadiene and ethylene, isoprene, cyclopentadiene, acrylonitrile, and methyl vinyl ketone (MVK). Geometry optimizations were performed with both RHF/3-21G and RHF/6-31G* calculations, and the energies were computed to the MP3/6-31G*//g-31G* level in each case. Calculations of the vibrational frequencies permitted computation of the enthalpies and entropies of activation. Though the structural results are very similar for the two basis sets, the computed activation enthalpies are sensitive to the treatment of the correlation energy. The reactivity order and the enthalpies of activation reach good accord with experimental results at the MP3 level. The stereochemical preferences of the reactions were investigated; for both isoprene and MVK, the endo-cis transition states are favored. The observed strong endo preference for cyclopentadiene dimerization is also reproduced as well as the lack of stereoselectivity for the addition of acrylonitrile.