Question

Question: What type of cycloaddition occurs in Reaction [1]? Draw the product of a similar process in Reaction [2]. Would you predict that these reactions occur under thermal or photochemical conditions?

Short answer

Answer

• Reaction [1] is a cycloaddition reaction that is thermally allowed.
• Product for reaction [2]:

Reaction [2] occurs under thermal conditions.

Step-by-step solution

Stereochemical requirement for cycloaddition

Cycloaddition reaction is a concerted process in which the transition state determines the stereochemistry of the product.

The transition state depends on the following requirements:

• s-cis conformation of diene.
• Stereochemistry of diene and dienophile is retained in the product.
• Syn-addition takes place.
• The electron-withdrawing group occupies the endo position.

Reaction mechanism

• Reaction [1] and [2] are cycloaddition reactions that are thermally allowed.

• The endo product is favorable because of a favorable interaction between the orbitals even though no bonds are formed.

Considering HOMO of the triene and LUMO of the diene:

• HOMO and LUMO have the same symmetry

• The electron-deficient triene has low energy LUMO, and the electron-rich diene has high energy HOMO. So the combination gives a better overlap in the transition state.

Reaction [1]

Reaction [2]