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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.