Diels Alder Reaction

The [4+2] cycloaddition reaction between a conjugated diene (4π- lectron system) and a compound containing a double or triple bond, called the dienophile (2π-electron  system) to form an adduct is known as Diels-Alder Reaction named after the two german chemists, who recieved the noble prize for chemistry in 1950. In this reaction a 1,3-diene reacts with a dienophile to form a six membered ring  with a double bond. A typical example is the reaction between butadiene and ethylene by heating both the components alone or in an inert solvent. Similarly, another simplest example is the reaction between butadiene and acetylene.
Diels Alder reaction In Diels Alder reaction two new -bonds are formed at the expense of two -bonds in the starting materials.

Stereochemistry
The reaction is highly stereospecific and used for the synthesis of a variety of products, having different types of structures. For example,dimethyl maleate (1, a cis dieenophile) reacts with butadiene to give cis-dimethyl 1,2,3,6-tetrahydrophthalate (2).However, dimethyl fumerate (3, a trans dienophile) on reaction with butadiene gives corresponding trans product, trans-dimethyl 1,2,3,6-tetrahydrophthalate,(4).
Diels Alder reaction

Dienes can exist in a cisoid and transoid conformation and only the cisoid form undergoes this reaction.if the diene is looked into the transoid conformation, as in (5), the reaction does not take place.
Diels Alder reaction

Mechanism
it is believed that the thermal Diels-Alder reaction is a concerted addition. in the cyclic transition state, the bond-formation and bond-breaking takes place simultaneously, but may not to the same extent. Such a mechanism is known as retarted concerted mechanism.
Diels Alder reaction


Some Other important Reactions

Cannizzaro Reaction

Clemmensen Reduction

Etard Reactions

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