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Tutorial MenuePericyclic ReactionsLearning Unit 3 of 6

Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction

Cycloadditions: Exo/Endo-Selectivity

Suprafacial cycloadditions of two components can take place in two different ways: exo and endo.

Fig.1
Endo
Fig.2
Exo

In many cases it is not possible to decide wether the attack occured exo or endo based on the structure of the product, for example:

Fig.3
Interconversion of exo and endo products

An unambiguous assignment is possible if the diene component in the Diels-Alder reaction is cyclic causing the conformation of the product to be fixed.

Fig.4
Fixed conformation in bicyclic products

Maleic anhydride and cyclopentadiene yield the endo product in a Diels-Alder reaction though for steric reasons the exo product is thermally the more stable one. Only by heating to 200 °C is the endo isomer transformed into the more stable exo isomer. Generally, the "endo rule" applies to Diels-Alder reactions but it is less adhered to than facial selectivity.

Tab.1
Comparison between products: endo vs. exo
Endo product Exo product
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3D Animation of the endo Diels-Alder reaction

3D Animation of the exo Diels-Alder reaction

Fig.5
3D Animation of orbitals in the endo Diels-Alder reaction
Fig.6
ACID Animation of the Diels-Alder reaction
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