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Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction

Cycloadditions: Scope of the Diels-Alder Reaction

In most cases, the dienophile carries electron-withdrawing substituents, thereby accelerating the reaction (also see Reactivity of Cycloadditions). Electron-poor alkines are also suitable dienophiles giving rise to 1,4-cyclohexadienes. Similarly reactive are electron-poor heterodienophiles such as nitroso compounds. Following are some examples illustrating the broad scope of the Diels-Alder reaction.

Fig.1
Scope of the Diels-Alder reaction

Using the first reaction as an example, Diels and Alder in 1928 discovered the principle of the reaction (Nobel prize in 1950). In this reaction, the dienophile benzoquinone with two electron-poor double bonds adds to two molecules of cyclopentadiene. The third example shows a reaction between an electron-rich dienophile (enolether) and an electron-poor diene (α, β-unsaturated ketone); this type of polarisation is called a Diels-Alder reaction with inverse electron requirements.

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