General description of the Diels-Alder reaction

Diels-Alder reactions occur between a conjugated diene and an alkene, usually called the dienophile. Here are some examples first an open-chain diene with a simple unsaturated aldehyde as the dienophile. 

The mechanism is the same and a new six-membered ring is formed having one double bond. Now a reaction between a cyclic diene and a nitroalkene. 

The mechanism leads clearly to the first drawing of the product but this is a cage structure and the second drawing is better. The new six-membered ring is outlined in black in both diagrams. Now a more elaborate example to show that quite complex molecules can be quickly assembled with this wonderful reaction. 

The diene component in the Diels-Alder reaction can be open-chain or cyclic and it can have many different kinds of substituents. There is only one limitation it must be able to take up the conformation shown in the mechanism. Butadiene normally prefers the s-trans conformation with the two double bonds as far away from each other as possible for steric reasons. The barrier to rotation about the central c bond is small (about 30 kj mol-1 at room temperature see Chapter 18) and rotation to the less favourable but reactive s-cis conformation is rapid. 

The s in the terms s-cis and s-trans refers to a a bond and indicates that these are conformations about a single bond and not configurations about a double bond. 

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General description of the Diels-Alder reaction 879 Q Radical reactions 970... 

The application of the general selection rule does not require analyzing a concerted reaction in any particular way. All descriptions of a pericyclic reaction that predict the same stereochemistry in the product will lead to the same conclusion. For example, three descriptions of the Diels-Alder reaction are given in Figure 11.88. In Figure 11.88(a), the reaction is described as a [ 2j + 4J cycloaddition. In this process there is one (4n - - 2)g component and no (4r)a component, so the total (1) is an odd number, and the reaction is allowed. In Figure 11.88(b), each double bond of the diene is considered to be a separate unit, so the reaction is described as a - - 2 -I- 2j process. 

Since the initial demonstrations of the participation of substituted l,2,4,S-tetrazines ° and oxa-zoles ° ° in [4 + 2] cycloaddition reactions with alkene and alkyne dienophiles, the investigation and application of the Diels-Alder reactions of heteroaromatic systems possessing reactive azadienes have been pursued extensively. A number of general reviewshave treated the spectrum of heteroaromatic azadienes that participate in [4 + 2] cycloaddition reactions and many of the individual heteroaromatic systems have been reviewed separatcly. ° ° An extensive account was published recently and should be consulted for descriptions of the [4 + 2] cycloaddition reactions of the common heteroaromatic azadienes that have been observed to date. 

The dipole is a triad of atoms which has a n system of four electrons and for which a dipolar resonance form provides an important component of its bonding description. The requirements have also been described as atom A must have a sextet of outer electrons and C has an octet with at least one unshared pair ([119], p. 743). The substrate is an olefin and an alkyne or a carbonyl. The reaction therefore is of n4 + n2 type like the related Diels-Alder reaction and has proved to be very useful synthetically for the construction of 5-membered ring heterocycles [268, 269]. Evidence suggests the reaction is concerted and regioselective. Dipoles fall into two general categories [119] ... 

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