The collinearity of terminal <i>p</i>
orbitals of a diene with that of a dienophile is required for an effective
overlap to result in s bond formation during the Diels-Alder reaction.
The ease of the DA reaction of a cyclic diene with a given dienophile,
therefore, must also depend on the distance between the termini of the diene. A
distance larger than the unsaturated bond of the dienophile is expected to
raise the energy of activation. This scenario has been amply demonstrated from
the study of reactions of several dienes, some designed to serve the purpose,
with different dienophiles. The five-ring heterocycles pyrrole, furan,
thiophene and selenophene possess varying aromatic character for the varied resonance
participation of the heteroatom lone pair with ring p bonds. The aromaticity
decreases in the same order due to: (a) the increasing s<sub>C-X</sub> (X = heteroatom) bond
length lifts the bond uniformity required for ring current, hence aromaticity, such
as in benzene and (b) size-mismatch of the interacting lone pair orbital and
the ring <i>p</i> orbitals, especially in
thiophene and selenophene, both allowing poor overlap in the ground state
structures. It is demonstrated that increase alone in the activation energies
of the DA reactions of pyrrole, furan, thiophene and selenophene cannot be considered
a measure of relative aromaticity as often done and even theoretically attempted
in many ways to prove just that. The separation of the termini of the diene has
a much larger role in the determination of activation energy, especially in
thiophene and selenophene, than their aromaticity profile. There cannot be a
measure better than the relative intensity of heteroatom lone pair overlap with
ring p bonds, giving rise to a six-electron like
system in following Hückel’s 4n+2 rule, to assess the relative aromaticity.
A Computational Study of the Relative Aromaticity of Pyrrole, Furan, Thiophene and Selenophene, and Their Diels-Alder Stereoselectivity