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.

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).

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.

Asymmetric Carbonyl-Ene Reaction Catalyzed by Chiral N,N′-Dioxide-Nickel(II) Complex: Remarkably Broad Substrate Scope
K. Zheng, J. Shi, X. Liu, X. Feng, J. Am. Chem. Soc., 2008, 130, 15770-15771.

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.

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.

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.

Direct Synthesis of β-Alkyl N-Aryl Aza Baylis-Hillman Adducts via Nitroso-Ene Reaction
S. Murru, A. A. Gallo, R. S. Srivastava, J. Org. Chem., 2012, 77, 7119-7123.

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.

2-Methylenetetrahydropyrans: Efficient Partners in the Carbonyl Ene Reaction
G. Liang, D. T. Sharu, T. Lam, N. I. Totah, Org. Lett., 2013, 15, 5974-5977.

Confined Acid-Catalyzed Asymmetric Carbonyl-Ene Cyclization
L. Liu, M. Leutzsch, Y. Zheng, M. W. Alachraf, W. Thiel, B. List, J. Am. Chem. Soc., 2015, 137, 13268-13271.

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