orbital overlap between the ene and enophile is largely controlled by the geometry of the approach of components. Lewis acid – catalyzed ene reactions Advantages and rationale 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. Reaction conditions As long as the nucleophilicity of the alkyl group does not lead to side reactions, catalytic amounts of Lewis acid are sufficient for many ene reactions with reactive enophiles. Nonetheless, the amount of Lewis acid can widely vary, as it largely depends on the relative basicity of the enophile and the ene adduct. In terms of solvent choice for the reactions, the highest rates are usually achieved using halocarbons as solvents; polar solvents such as ethers are not suitable, as they would complex to the Lewis acid, rendering the catalyst inactive. Reactivity of enes While steric effects are still important in determining the outcome of a Lewis acid catalyzed ene reaction, electronic effects are also significant, since in such a reaction, there will be a considerable positive charge developed at the central carbon of the ene. As a result,