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In general, Diels - Alder reaction can be represented as:


Stereoselectivity:


When the diene is cyclic, there are 2 possible ways in which addition can occur if the dienophile is not symmetrical. The larger side of the dienophile or more bulky side of the olefin may be under the ring (endo addition) or it may be the smaller side (exo addition).



Most of the time addition is predominently endo & this is probably true for open chain dienes also. However exceptions are known & in many cases mixtures of exo & endo addition products are found.




In the endo transition state, the substituent on the dienophile is oriented towards the diene π system, while in the exo it is oriented away from it.


For normal demand Diels-Alder scenarios, with electron-withdrawing substituents such as carbonyls attached to the dienophile, the endo transition state is typically preferred, despite often being more sterically congested. This preference is known as the Alder rule. Endo selectivity is typically higher for rigid dienophiles such as maleic anhydride and benzoquinone; for others, such as acrylates and crotonates, selectivity is not very pronounced.The most widely accepted explanation for the origin of this effect is a favorable interaction between the dienophile substituent's π system and the diene's (secondary orbital effects), though dipolar and van der Waals attractions may play a part as well.

Oftentimes, as with highly substituted dienes or very bulky dienophiles, steric effects can greatly influence endo/exo selectivity. Dienes with bulky terminal substituents (C1 and C4) decrease the rate of reaction, presumably by impeding the approach of the diene and dienophile.However, bulky substituents at the C2 or C3 position actually increase reaction rate by destabilizing the s-trans conformation and forcing the diene into the reactive s-cis conformation. 2-tert-butyl-1,3-butadiene, for example, is 27 times more reactive than simple butadiene.


Stereospecificity:

“Selective” implies that there are factors which favor one product over the other, while “specific” is usually a sign that there’s something inherent to the mechanism that leads to only one product.

Diels–Alder reactions, as concerted cycloadditions, are stereospecific, i.e. stereochemical information in the reactants is retained in the products. E- and Z-dienophiles, for example, give rise to the adducts with corresponding syn or anti-stereochemistry.

a cis-dienophile gives cis-substituents in the product

a trans-dienophile gives trans-substituents in the product.


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