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The Ene reaction (also known as the Alder-ene reaction) is a chemical reaction between an alkene with an allylic hydrogen and a compound containing a multiple bond.^[1] The product is a substituted alkene with the double bond shifted to the allylic position.

These reactions are promoted by heat and/or Lewis acids and often require highly activated substrates and/or high temperatures. Ene reaction products are also often found as unintended byproducts of other chemical reactions.

The convention for describing an ene reaction is to describe it as a reaction between an ene and an enophile. The ene is the alkene having an allylic hydrogen, while the enophile is the compound containing the multiple bond. This can be confusing, since the enophile is very often itself an alkene. For most known ene reactions, the ene is electron-rich, and the enophile is electron-deficient. Unlike the Diels-Alder reaction, only a handful of exotic inverse-electron demand ene reactions (electron-deficient ene, electron-rich eneophile) are known.

Maleic anhydride is known to participate in many ene reactions; this is due to the combined electron-widthdrawing power of the two carbonyl groups pendant to the alkene.^[2]

Contents 1 Reaction mechanism 2 Variations 2.1 Carbonyl ene reaction 2.2 Conia reaction 2.3 Singlet-oxygen ene reaction 2.4 Miscellaneous 3 References 4 See also

[edit] Reaction mechanism

Much like with Diels-Alder reactions, Lewis acids such as boron trifluoride or aluminium chloride can participate in metal-catalyzed ene reactions. These reactions are still pericyclic as they operate by generating the reactive complex which undergoes a concerted electrocyclic reaction.

[edit] Variations

[edit] Carbonyl ene reaction

When a carbonyl is the enophile, the reaction is often called the carbonyl ene reaction.^[3]

For example, Citronellal undergoes an intramolecular ring-closure with a range of Lewis acid catalysts such as niobium pentachloride.^[4]

[edit] Conia reaction

The intramolecular Ene reaction of unsaturated ketones in which enol-tautomer of the ketone serves as the ene are called the Conia reaction or Conia ene reaction.^[5][6]

[edit] Singlet-oxygen ene reaction

In the Singlet-oxygen ene reaction, singlet-oxygen reacts with alkenes to form allylic peroxides.^[7][8] This reaction was first described in 1945 by Schenck and is sometimes called the Schenck ene reaction.^[9]

[edit] Miscellaneous

• Selenium dioxide oxidation is also an ene reaction coupled to a retro-[3 + 2] reaction.

[edit] References

1. ↑  Alder, K. et al. Ber. 1943, 76, 27.
2. ↑  Succinic acid, cinnamyl Christian S. Rondestvedt, Jr. Organic Syntheses, Coll. Vol. 4, p.766 (1963); Vol. 31, p.85 (1951). (Article)
3. ↑  Snider, B. B. Comp. Org. Syn. 1991, 2, 527-561. (Review)
4. ↑  Intramolecular ene reactions catalyzed by NbCl₅, TaCl₅ and InCl₃ Carlos Kleber Z. Andrade; Otilie E. Vercillo; Juliana P. Rodrigues; Denise P. Silveira J. Braz. Chem. Soc. vol.15 no.6 São Paulo Nov./Dec. 2004 (Article)
5. ↑  Rouessac, F. Tetrahedron Lett. 1965, 3319.
6. ↑  Conia, J. M., Le Perchec, P. Synthesis 1975, 1-19. (Review)
7. ↑  Wasserman, H. H.; Ives, J. L. Tetrahedron 1981, 37, 1825-1852. (Review: DOI:10.1016/S0040-4020(01)97932-3)
8. ↑  Clennan, E. L. Tetrahedron 2000, 56, 9151-9179. (Review: DOI:10.1016/S0040-4020(00)00794-8)
9. ↑  Schenck, G. O. Naturwissenschaften 1948, 35, 28-29.

[edit] See also

• Diels-Alder reaction
• Certain isotoluenes isomerize by an ene mechanismja:エン反応