Cycloadditions: the Diels-Alder reaction (cont'd)

Maitland Jones §11.15 and §21.3

The formation of sigma bonds: concerted, but not always synchronous.

Animation on/off

The reactions shown so far are symmetrical: both reactants are symmetrical, and so is the reaction complex at all stages. The two new C-C bonds are being formed simultaneously. This is the general picture of the Diels-Alder reaction: the stereochemistry of the reactants is maintained in the product.
The animation on the left shows the reaction between cyclopentadiene and ethene. It is again seen that the two new bonds are formed simultaneously. It is a concerted reaction.

If the reactants are not symmetric, the two new bonds are not equivalent anymore, and very often it is found that one of them is formed slightly ahead of the other one. This can be observed in the reaction of 1-methoxycyclopentadiene with ethene.
Try to trap the structure in which one bond is drawn. Is it on the substituted or unsubstituted side of the molecule?

Chime decides to draw a bond between two atoms if the distance falls below a preset value. This doesn't mean that the physical bond is suddenly there, and before it wasn't. The increase in overlap between the orbitals, the formation of the sigma bond, is a gradual process.

In the transition state the bond to C4 (2.07 Å) is somewhat shorter than the bond to C1 (2.14 Å). The reaction is still concerted, even if the formation of the two bond is (slightly) asynchronous.
The common explanation for this phenomenon is that in the HOMO C4 has a larger coefficient (-0.5294) than C1 (+0.4704).

Remember from the first Diels-Alder page that substituents have an effect on the rate of the reaction: electron donors in the diene increase the HOMO energy level and thus lower the HOMO-LUMO gap. This results in a better interaction and a faster reaction.
Electron withdrawing substituents in the ethene part (dienophile) have the same effect by lowering the LUMO energy.

As shown above, substituents also affect the order in which the bonds are formed. The larger coefficients form the first bond.
If both diene and dienophile are substituted, the formation of several isomeric products is possible. On the next page we'll try to predict the outcome.