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Ene reaction

The Ene reaction (also known as the Alder-ene reaction) is a chemical reaction between an alkene with an allylic hydrogen (the ene) and a multiple bond (the enophile).^[1] It is a group transfer reaction. 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 of referring to the reactive components of ene reaction as the "ene" and "enophile" can be confusing, as 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 ene reactions are known for the inverse of electron demand, with an electron-deficient ene and an electron-rich eneophile.)

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

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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.

Variations

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]

Conia reaction

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

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]

Miscellaneous

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

References

^ Alder, K. et al. Ber. 1943, 76, 27.
^ Succinic acid, cinnamyl Christian S. Rondestvedt, Jr. Organic Syntheses, Coll. Vol. 4, p.766 (1963); Vol. 31, p.85 (1951). (Article)
^ Snider, B. B. Comp. Org. Syn. 1991, 2, 527-561. (Review)
^ 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)
^ Rouessac, F. Tetrahedron Lett. 1965, 3319.
^ Conia, J. M., Le Perchec, P. Synthesis 1975, 1-19. (Review)
^ Wasserman, H. H.; Ives, J. L. Tetrahedron 1981, 37, 1825-1852. (Review: doi:10.1016/S0040-4020(01)97932-3)
^ Clennan, E. L. Tetrahedron 2000, 56, 9151-9179. (Review: doi:10.1016/S0040-4020(00)00794-8)
^ Schenck, G. O. Naturwissenschaften 1948, 35, 28-29.