Diels-alder and melting point

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.

Some important facts

1. The Diels-Alder reaction (a $\text{4π}+\text{2π}$ 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.

Some common examples of dienes and dienophiles:

Dienes

Dienophiles

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 $\text{2π}-\text{bonds}$ into 2 new stronger $\sigma -\text{bonds}$ .

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.

Stereochemistry of d.a. reaction

1. Regioselectivity: In absence of solvent and catalytic effect, regioselectivity depends on presence of partial positive or negative character on the terminal carbon centers of dienes or dienophile.

In this case para will be major product. (Convince yourself of this before you continue)

2. Stereoselectivity: Addition of two molecules is syn on both components (bonds form from same species at the same time). This is shown by the examples below:

cis-dienophile gives cis-substituents in the product.

trans-dienophile gives trans-substituents in the product.

If both substituents on the diene are Z, then both end up on the same face of the product.

If substituents on the diene are E and Z, then they end up on opposite faces of the product.

A study of the HOMO and LUMO orbitals for the reactions would probably prove very useful in determining (a) The preference for the endo product (b) Substituent effects on the rate of reaction (c) Substituent effects on the regiochemistry of the reaction. Nucleophilic (diene) characterized by the HOMO and Electrophilic (dienophile) characterized by the LUMO.