Diels-alder Reaction Diels-alder Reaction The Diels–Alder reaction is an organic chemical reaction (specifically, a cycloaddition) between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon. Some of the Diels–Alder reactions are reversible; the decomposition reaction of the cyclic system is then called the retro-Diels–Alder. For example, retro-Diels–Alder compounds are commonly observed when a Diels–Alder product is analyzed via mass spectrometry. Otto Paul Hermann Diels and Kurt Alder first documented the novel reaction in 1928 for which they were awarded the Nobel Prize in Chemistry in 1950 for their work on the eponymous reaction. The Diels–Alder reaction is generally considered one of the more useful reactions in organic chemistry since it requires very little energy to create a cyclohexene ring, which is useful in many other organic reactions.
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Reaction mechanism :- The reaction occurs via a single transition state, which has a smaller volume than either the starting materials or the product. It is an associative type of reaction, and it is sped up by very high pressures. Diels–Alder is an example of a pericyclic reaction. Some free-radical versions of this reaction have been observed, though these are not Diels–Alder reactions since the stereochemistry at the carbons is scrambled. The diene :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 exist in the s-cis conformation. Butadiene itself normally prefers the s-trans conformation, with the two double bonds as far away from each other as possible. If there are substituents larger than hydrogen then steric hindrance may influence the relative stabilities of the conformations. For simple cases, the barrier to rotation about the central bond is small and rotation to the less favourable but reactive s-cis conformation is rapid. Cyclic dienes that are permanently in the s-cis conformation are exceptionally reactive in Diels–Alder reactions (cyclopentadiene is a classic example), while cyclic dienes that are permanently in the s-trans conformation and cannot adopt the s-cis conformation will not undergo the Diels–Alder reaction at all. An especially reactive diene is Danishefsky's diene. Dendralenes are a new class of experimental DA dienes. Unstable dienes, such as o-quinodimethane, can be generated in situ. Aromatic stabilization in the product of a DA reaction using such a diene is, in some cases, the reason behind the very high reactivity and lack of stability of such diene. The use of such unstable dienes is advantageous, despite the trouble, in that the products will contain newly formed aromatic six-membered rings.
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Benzenoid compounds rarely undergo DA reactions and often require very reactive dienophiles. One example of such rare reaction is the Wagner-Jauregg reaction Cyclopentadiene does not react with cyclohexenone in ethyl acetate unless the Lewis acid is present. The yield improves when reaction temperature is lowered to −78°C because polymerization side reactions are prevented. Niobium pentachloride catalysis gives only the endo conformer. The same reaction with aluminium chloride results in an endo and exo mixture. Many of these Lewis acids are not good catalysts for the reaction of α,β-unsaturated carbonyls, this is because the carbonyl oxygen binds too tightly to the metal centre. A far better catalyst for such a system is a combination of silver perchlorate and Lawesson's reagent in cold dichloromethane.
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