Alder-Ene Reaction
Ene Reaction

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.

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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).

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Recent Literature

Formaldehyde Encapsulated in Zeolite: A Long-Lived, Highly Activated One-Carbon Electrophile to Carbonyl-Ene Reactions
T. Okachi, M. Onaka, J. Am. Chem. Soc., 2004, 126, 2306-2307.

Ytterbium(III) Triflate/TMSCl: Efficient Catalyst for Imino Ene Reaction
M. Yamanaka, A. Nishida, M. Nakagawa, Org. Lett., 2000, 2, 159-161.

Highly Enantioselective Carbonyl-ene Reactions Catalyzed by a Hindered Silyl-Salen-Cobalt Complex
G. E. Hutson, A. H. Dave, V. H. Rawal, Org. Lett., 2007, 9, 3869-3872.

Steric acceleration of an uncatalysed ene reaction at room temperature
N. Choony, P. G. Sammes, G. Smith, R. Ward, Chem. Commun., 2001, 2062-2063.

Iron(III) Chloride Catalysis of the Acetal-Ene Reaction
A. Ladépêche, E. Tam, J.-E. Ancel, L. Ghosez, Synthesis, 2004, 1375-1380.

Ene Reaction of Arynes with Alkynes
T. T. Jayanth, M. Jeganmohan, M.-J. Cheng, S.-Y. Chu, C.-H. Cheng, J. Am. Chem. Soc., 2006, 128, 2232-2233.