Question

Question: What cycloaddition products are formed in each reaction? Indicate the stereochemistry of each product.

Short answer

Answer

Cycloaddition products

(a)

(b)

Step-by-step solution

Cycloaddition reaction

In a cycloaddition reaction, two unsaturated molecules undergo an addition reaction to form a cyclic product.

Cycloaddition products

• When substituted ethylene is irradiated with UV-light, an electron is promoted from a (ground state) to(excited state) orbital.

• Photochemical cycloaddition takes place between the excited HOMO of one molecule and the ground state LUMO of another molecule.

• The result is a mixture of the ground state and excited state of the ethylene molecule.

HOMO and LUMO interaction

• If there is stereochemistry in the dienophile, then it is reproduced in the product.

Thus, cis and trans dienophiles give different diastereomers of the product.

a. Cis dienophile

cis substituted cyclobutene

b. Trans dienophile

trans substituted cyclobutene