Use FMOT to predict the mechanism and products for the following cycloaddition. If no product is favored, write “symmetry-disallowed” in place of the product.

• A. Symmetry-disallowed
• B.
• C.
• D.

Answer: (B)

Verified step by step guidance

1. Identify the type of cycloaddition reaction: The problem involves a 2π + 2π cycloaddition, specifically the dimerization of thymine, which is a photochemical reaction.
2. Understand the Frontier Molecular Orbital Theory (FMOT): In photochemical reactions, the excited state of the molecules is considered. The HOMO of one molecule interacts with the LUMO of the other.
3. Analyze the symmetry of the molecular orbitals: For a 2π + 2π cycloaddition to be symmetry-allowed, the interacting orbitals must have compatible symmetries in their excited states.
4. Evaluate the symmetry of the thymine dimerization: In the excited state, the symmetry of the π orbitals allows the reaction to proceed, making it symmetry-allowed under photochemical conditions.
5. Predict the product: The reaction leads to the formation of a cyclobutane ring between the two thymine molecules, resulting in a thymine dimer, as shown in the provided images.