Question

Write a general rule that can be used to predict the major product of a Diels–Alder reaction between an alkene with an electron-withdrawing substituent and a diene with a substituent that can donate electrons by resonance depending on the location of the substituent on the diene.

Short answer

1. EDG in C-1 of diene produces ortho product.
2. EWG in C-2 of diene produces para product.
3. EDG in alkene increases in reaction rate.

Step-by-step solution

Definition of Diels Alder reaction

Diels Alder reaction is a pericyclic reaction where a diene and dienophile reacts to form a cyclic entity. This reaction is a reversible reaction. At high temperature, the cyclic compound breaks to get back the reactants.

General rule

In a general manner, either HOMO of alkene and LUMO of dienophile or HOMO of dienophile and LUMO of alkene combination carry out the reaction.

For a faster reaction, energy of HOMO should be higher and LUMO should be lower. So, an electron donating group on HOMO and electron withdrawing group on LUMO can increase the reaction rate.

If energetically diene is HOMO and dienophile or alkene is LUMO, electron withdrawing group (EWG) on alkene and electron donating group (EDG) on diene can increase the reaction and the position of the group on diene and dienophile can predict the product.

When the EDG is in C-1 position of diene:

In this condition, ortho product will be formed

When the EDG is in C-2 position of diene:

In this condition, para product will be formed.

In case of alkene, EWG increases the reaction rate by decreasing the energy of LUMO and the attack will be on that carbon where EWG is attached. The above two reactions has EWG in alkene in EDG in diene.