The four-electron system including an alkene π-bond and an allylic C-H σ-bond can participate in a pericyclic reaction in which the double bond shifts and new C-H and C-C σ-bonds are formed. This allylic system reacts similarly to a diene in a Diels-Alder Reaction, while in this case the other partner is called an enophile, analogous to the dienophile in the Diels-Alder. The Alder-Ene Reaction requires higher temperatures because of the higher activation energy and stereoelectronic requirement of breaking the allylic C-H σ-bond.
The enophile can also be an aldehyde, ketone or imine, in which case β-hydroxy- or β-aminoolefins are obtained. These compounds may be unstable under the reaction conditions, so that at elevated temperature (>400°C) the reverse reaction takes place - the Retro-Ene Reaction.
While mechanistically different, the Ene reaction can produce a result similar to the Prins Reaction.
Mechanism of the Alder-Ene Reaction
Also like the Diels-Alder, some Ene Reactions can be catalyzed by Lewis Acids. Lewis-Acid catalyzed Ene Reactions are not necessarily concerted (for example: Iron(III) Chloride Catalysis of the Acetal-Ene Reaction).
Highly Enantioselective and Anti-Diastereoselective Catalytic Intermolecular Glyoxylate-Ene Reactions: Effect of the Geometrical Isomers of Alkenes
X. Zhang, M. Wang, R. Ding, Y.-H. Xu, T.-P. Loh, Org. Lett., 2015, 17, 2736-2739.