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 compound containing a multiple bond (the enophile), in order to form a new σ-bond with migration of the ene double bond and 1,5 hydrogen shift. The product is a substituted alkene with the double bond shifted to the allylic position.
This transformation is a group transfer pericyclic reaction, and therefore, usually requires highly activated substrates and/or high temperatures. Nonetheless, the reaction is compatible with a wide variety of functional groups that can be appended to the ene and enophile moieties. Also,many useful Lewis acid-catalyzed ene reactions have been developed which can afford high yields and selectivities at significantly lower temperatures, making the ene reaction a useful C–C forming tool for the synthesis of complex molecules and natural products.
Enes are π-bonded molecules that contain at least one active hydrogen atom at the allylic, propargylic, or α-position. Possible ene components include olefinic, acetylenic, allenic, aromatic, cyclopropyl, and carbon-hetero bonds. Usually, the allylic hydrogen of allenic components participates in ene reactions, but in the case of allenyl silanes, the allenic hydrogen atom α to the silicon substituent is the one transferred, affording a silylalkyne. Phenol can act as an ene component, for example in the reaction with dihydropyran, but high temperatures are required (150–170 °C). Nonetheless, strained enes and fused small ring systems undergo ene reactions at much lower temperatures. In addition, ene components containing C=O, C=N and C=S bonds have been reported, but such cases are rare.
Enophiles are π-bonded molecules which have electron-withdrawing substituents that lower significantly the LUMO of the π-bond. Possible enophiles contain carbon-carbon multiple bonds (olefins, acetylenes, benzynes), carbon-hetero multiple bonds (C=O in the case of carbonyl-ene reactions, C=N, C=S, C≡P), hetero-hetero multiple bonds (N=N, O=O, Si=Si, N=O, S=O), cumulene systems (N=S=O, N=S=N, C=C=O, C=C=S, SO2) and charged π systems (C=N+, C=S+, C≡O+, C≡N+).
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