Introduction to Diels-Alder Cycloaddition

In the Diels-Alder Cycloaddition Reaction the distinctive character of conjugated dienes apparent itself significantly. A cycloaddition reaction is the concerted bonding together of two free pi-electron systems to create a new ring of atoms. While this come into existence, two pi-bonds are converted to two sigma-bonds, the simple most illustration being the theoretical /hypothetical combination of two ethane molecules to give cyclobutane. In the normal conditions this does not occur but the cycloaddition of 1,3-butadiene to cyanoethene (acrylonitrile) does, and this is an instance of the Diels-Alder reaction. The figure demonstrates two cycloadditions and introduces various terms that are helpful in discussing reactions of this type.

In the theoretical /hypothetical ethylene dimerization on the left side, each reactant molecule has a pi-bond (colored orange in the diagram) occupied by two electrons. Cycloaddition converts these pi-bonds into new sigma-bonds (colored green in the diagram) and this transformation is then elected a [2+2] cycloaddition, to specify the reactant pi-electrons that change their bonding location. 
The Diels-Alder reaction is a significant and extensively used technique for making six-membered rings, as displayed on the right. The reactants used in this type of reactions are a conjugated diene, simply considered as the diene and a double or triple bond co-reactant called the dienophile, since it combines with (has an affinity for) the diene. The Diels-Alder cycloaddition is categorized as a [4+2] process, since in the reaction the diene has four pi-electrons that shift position and the dienophile has two. 
The Diels-Alder reaction is a single step process, so the diene component must adopt a cis-like conformation in order for the end carbon atoms to bond at the same time to the dienophile. Such type of conformations is called s-cis, the s is considering as the single bond connecting the two double bonds. The 1,3-butadiene's s-trans and s-cis conformers are displayed in the preceding figure. For several acyclic dienes than the s-cis conformer (because of steric crowding of the end groups) the s-trans conformer is more stable, but the two are usually in rapid equilibrium, permitting the make use of all but the most hindered dienes like reactants in Diels-Alder reactions. In its general form, the best dienophiles are electron poor and the diene component is electron rich because of electron withdrawing substituents like CN, C=O & NO₂. With the two new sigma-bonds being formed simultaneously the initial bonding interaction reflects this electron imbalance, but not essentially at equal rates.

Stereospecificity

We noticed earlier that addition reactions of alkenes frequently displayed Stereoselectivity, in that the reagent elements in few cases added syn and in other cases anti to the plane of the double bond. Both reactants inside the Diels-Alder reaction may illustrate stereoisomerism and when they do it is found that the relative configurations of the reactants are conserved in the product. For the reaction of 1,3-butadiene with (E)-dicyanoethene the following drawing demonstrates this issue. In the dienophile the trans relationship of the cyano groups, is preserved in the six-membered ring of adduct. Similarly, if the terminal carbons of the diene bear substituents, their relative configuration will be reserved in adduct. By using the previous terminology, we could say that bonding to both the dienophile and the diene is syn. Additional descriptions, though, consider to the planar nature of both reactants and terms the bonding in each case to be suprafacial (that is to or from similar face of each plane). This stereospecificity also verify the synchronous nature of 1,4-bonding that takes place.

The important features of the Diels-Alder cycloaddition reaction may be shortened as follows:

(i) The reaction all the time creates a new six-membered ring; when intramolecular, other ring may also be formed.
(ii) The diene component has to be able to assume an s-cis conformation.
(iii) On the dienophile the electron withdrawing groups facilitate reaction.
(iv) On the diene electron donating groups facilitate reaction.
(v) Steric hindrance at bonding sites may inhibit or prevent reaction.
(vi) The reaction is stereospecific regarding substituent configuration in both the diene and the dienophile.

These characteristics are demonstrated by the following eight examples, one of which does not offer a Diels-Alder cycloaddition. The creation of a new six-membered ring has to be evident in every case where reaction occurs.

Since this diene cannot adopt an s-cis orientation there is no reaction in case D. In illustrations B, C, F, G & H minimum one of the reactants is cyclic so that the product have more than one ring, but the ring that is formed newly is all the time six-membered. In the instance B the the same cyclic compound acts as both the diene (colored blue in the diagram) and the dienophile (colored red in the diagram). The adduct has three rings, two out of them are the five-membered rings exist in the reactant, and the remaining third one is the new six-membered ring (shaded light yellow). Illustration C has an alkyne as a dienophile (colored red in the diagram), therefore, the adduct keeps a double bond at that location. That double bond could still serve as a dienophile, but in the current case the diene is suitably hindered to retard a second cycloaddition. In reaction F the quinone dienophile has two dienophilic double bonds. Though, the double bond with two methyl substituents is less reactive than the unsubstituted dienophile due in part to the electron donating properties of the methyl groups and in part to steric hindrance. The Stereospecificity of Diels-Alder reaction is illustrated by instances A, E & H. In A & H the stereogenic centers resides on the dienophile, while in E these centers are on the diene. In all examples, the configuration of the reactant is conserved in adduct.

Cyclic dienes, like those in instances B, C & G, give bridged bicyclic adducts for which an additional configurational characteristic must be designated. As displayed in the diagram, of this type, there are two possible configurations for compounds. If a substituent (colored magenta in diagram) is oriented cis to the longest or more unsaturated bridge (colored blue in diagram), it is said to be as endo. When directed trans to the bridge it is exo. When the Diels-Alder reaction forms bridged bicyclic adducts and an unsaturated substituent is located on this bicyclic structure (as in B & G), the major product is generally the endo isomer "Alder's Endo Rule". Instance C does not merit such type of a nomenclature, because stereoisomeric orientations of the substituent are not possible.

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