THE REACTION I LIKE MOST

Diels-Alder Reaction:

The Diels-Alder reaction is essentially a cycloaddition reaction ( Cycloaddition reaction is one example of a special class of reactions called pericyclic reaction. Other examples of pericyclic reactions are electrocyclic reactions, sigmatropic reactions ) in which an alkene adds to a 1,3-diene to form a 6-membered ring. The reaction is synthetically very useful, due to the formation of cyclic product

MECHANISM:

The Diels-Alder reaction mechanism still remains the topic of much debate because of the uncertainty as to whether the mechanism is stepwise, concerted, or free-radical. For the time being arrow-pushing concerted mechanism is preferred on order to better understand the beauty of this reaction.

 Important Points to Remember:

1. The Diels-Alder reaction (a

4π+2π4π+2π size 12{"4π"+"2π"} {} cycloaddition) is a very important synthetic method for the preparation of cyclohexanes, mainly because the stereochemistries of the products are so well controlled.

2. Normal Diels-Alder reaction is favoured by electron withdrawing groups (EWG) on the electrophilic dienophile and by electron donating groups on the nucleophilic diene.

3. The diene must be cisoid (this refers to the conformation about the single bond). Example: trans-butene does not undergo DA reaction where cis-butene does.

4. The reaction is usually thermodynamically favored due to the conversion of

2 π−bonds2 π−bonds size 12{"2 π" - "bonds"} {}into 2 new stronger

σ−bondsσ−bonds size 12{σ - "bonds"} {}.

5. The Diels-Alder reaction is stereospecific with respect to both the diene and the dienophile.

6. Under the reaction conditions, the product can be broken down to its starting materials known as retro Diels-Alder reaction.

7. To predict the relative rates of reaction of dienes, use these rules.

(a) Only conjugated dienes react as dienes in Diels-Alder processes. Isolated and

cumulated dienes may react as dienophiles, but they cannot react as dienes.

(b) Dienes in the s-trans conformation do not react in Diels-Alder reactions. Acyclic

dienes may rotate around a single bond to become s-cis, but some dienes are locked in the

s-trans conformation in cyclic structures and cannot react.

(c) Dienes containing electron-releasing groups (usually alkyl groups) react faster,

especially with dienophiles that contain electron-withdrawing groups (almost always groups that contain electronegative atoms). The opposite combination also gives relatively fast reactions, but the starting materials are unusual.