Some Special Features of the Diels-Alder Reaction

Some Special Features of the Diels-Alder Reaction 

The Diels-Alder reaction is the oldest and best studied pericyclic reaction. Here we will consider some special features in the light of PMO theory. 

This represents a further deviation from the BEP principle. 

The reason for this can be seen from an examination of the transition states for endo (172) and exo (173) dimerization of 1,3-butadiene, 

All the interactions between the two molecules of butadiene [indicated by dotted lines in (172) and (173)] are in phase, so both transition states are of Hiickel type. They differ only by the possibility of an extra interaction in (172) (indicated by an asterisk). The AHs isoconjugate with (172) and (173) are(174a) and (174b), respectively, the extra interaction in (172) corresponding to the asterisked bond in (174). Since both rings in (174a) are six-membered, both are aromatic. The extra interaction is therefore energetically favorable and is sufficient to tip the balance in favor of (172). The predominance of endo-cis addition in 

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B. Some Special Features of the Diels-Alder Reaction... 

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