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Magazine Chapter 27: Q 38. (page 1101)
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
Expert verified
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
01
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.
02
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]
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