Computational Study of the Stereoselectivity Profiles of the Diels-Alder Cycloaddition Reactions of Cyclopentadiene and Butadiene with Cyclopropenes

The endo and exo stereoselectivities of the Diels-Alder (DA) cycloaddition reactions of 3,3-disubstituted cyclopropenes with butadiene and cyclopentadiene, the latter for the first time, were investigated by means of density functional and quantum chemical calculations for a comparison. To establish distinction between the selectivites, activation free energies were systematically estimated in the gas phase and also in solvents. The differential activation free energies clearly predict exclusive endo configuration of the products formed from the reaction of the unsubstituted cyclopropene with butadiene and cyclopentadiene. However, the results were found to be markedly different for the substituted cyclopropenes from available experimental selectivities. It was also discovered that butadiene and cyclopentadiene are markedly different in their respective stereospecific product yields, nevertheless the difference between the two was only a methylene group. The failure of the differential activation free energy approach to predict the experimental stereoselectivities of the DA reactions of several perhalocyclopropenes with cyclopentadiene is probably due to yet insufficient development of the various theoretical models dealing with the endo and exo DA preferences.