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.
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.
mechanistically different, the Ene reaction can produce a result similar