Enes and Enophiles

The overall reaction includes allylic transposition of a double bond, migration of the allylic hydrogen and formation of a bond between ene and enophile. Experimental findings suggest a concerted mechanism. Alternatively a diradical species 4 might be formed as intermediate however such a species should also give rise to formation of a cyclobutane derivative 5 as a side-product. If such a by-product is not observed, one might exclude the diradical pathway ... 

Regioselectivity. Caibon-carbon bonds are preferentially formed between the proximal sites of ene and enophile in the type I process and between the more (Mg-ene reaction) or less substituted (Pd-ene reaction) ene terminal and the proximal enophile site in the type II version. 

This isolated report encouraged preparation of a host of chiral Lewis acids throughout the world to test their crqracity to induce asyrmnetric Diels-Alder reactions catalytically. However, until very recently vide injra) only disrqrpointing face selectivities (<50%) were found on [4 + 2] cycloadditions of alkenic ene and enophile paitners. 

Cyclopropenes have been found to act both as the ene and enophile in reactions with other alkenes, e.g. formation of l,302 2,303 3304 and 4.305... 

Sol 2. Thermal pericychc ene reactions proceed in a supra—supra manner with respect to the ene and enophile. The substituents on the enophile can be endo or exo. In terms of the diastereoselection with respect to the newly created chiral centers, an endo preference has been qualitatively observed, but steric effects can easily modify this preference. For example, the reaction between (Z)-but-2-ene and maleic anhydride gave erythro- and threo-(l-methylalkyl)-succinic anhydride in a kinetically controlled ratio of 1 4. The major product in this case arises through the endo transition state. The exo transition state leading to erythro isomer is unstable due to the eclipsing steric interactions between the methyl and the carbonyl group. 

Intramolecular ene reactions have lower activation energy than intermolecular because of entropic advantage. Intramolecular ene reactions are often classified into three main types. These types differ only in the position of the attachment of the tether that connects the ene and enophile components (Figure 6.11). 

Regioselectivity of the ene reaction of an unsymmetrical enophile is governed by the orbital coefficient of carbons in allylic alkene and enophile. For example, in the reaction of propylene with methyl acrylate, the major and minor products are obtained as per orbital interactions of ene and enophile [4] ... 

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