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.
Learn More :- How to Convert a Decimal to a Fraction Math.Tutorvista.com
Page No. :- 2/4