page_title: Ene reaction
section_title: Advantages and rationale
hierarchical_section_title: Ene reaction / Lewis acid – catalyzed ene reactions / Advantages and rationale
caption_attribution_description: English: ene reaction
context_page_description: The ene reaction is a chemical reaction between an alkene with an allylic hydrogen and a compound containing a multiple bond, 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. Many useful Lewis acid-catalyzed ene reactions have been also 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.
context_section_description: Thermal ene reactions have several drawbacks, such as the need for very high temperatures and the possibility of side reactions, like proton-catalyzed olefin polymerization or isomerization reactions. Since enophiles are electron-deficient, it was reasoned that their complexation with Lewis acids should accelerate the ene reaction, as it occurred for the reaction shown in Figure 8.
  Alkylaluminum halides are well known as proton scavengers, and their use as Lewis acid catalysts in ene reactions has greatly expanded the scope of these reactions and has allowed their study and development under significantly milder conditions.
  Since a Lewis acid can directly complex to a carbonyl oxygen, numerous trialkylaluminum catalysts have been developed for enophiles that contain a C=O bond. In particular, it was found that Me₂AlCl is a very useful catalyst for the ene reactions of α,β-unsaturated aldehydes and ketones, as well as of other aliphatic and aromatic aldehydes. The reason behind the success of this catalyst is the fact that the ene-adduct- Me₂AlCl complex can further react to afford methane and aluminum alkoxide, which can prevent proton-catalyzed rearrangements and solvolysis (Figure 9).
  In the case of directed carbonyl-ene reactions, high levels of regio- and stereo-selectivity have been observed upon addition of a Lewis acid, which can be explained through chair-like transition states. Some of these reactions (Figure 10) can run at very low temperatures and still afford very good yields of a single regioisomer.
caption: Figure 9: Me2AlCl-catalyzed carbonyl-ene reactions
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