Question

Question: What cyclic product is formed when each decatetraene undergoes photochemical electrocyclic ring closure?

Short answer

Answer

Formed cyclic product:

a.

b.

Step-by-step solution

Woodward-Hoffmann rule for photochemical electrocyclic reaction

A thermal reaction with an even number of electron pairs is conrotatory

• If one of the reaction conditions changes — thermal to photochemical or even to odd — the stereochemistry of the reaction changes to disrotatory

• If both reaction conditions change— thermal to photochemical reaction and even to odd — the stereochemistry does not change

Product formation

• It is an electrocyclic ring closure reaction of a compound with four conjugated double bonds, and the ring closure is dis-rotary.

• A disrotatory process forms the product under photochemical conditions.

• From the Woodward-Hoffmann rules, photochemical electrocyclic reactions involving (4n) electrons are dis-rotary.

• Therefore, the product formed is of a 4+2 electron system arising from disrotatory cyclization.

• Stereochemistry of products - Frontier orbital representation

• The HOMO is in which the end orbitals are symmetrical.

• The symmetry is correct for bond formation, so the orbitals rotate in opposite directions to form a new sigma bond.

a. The product formed is cis in configuration.

Cis product

b. The product formed is trans in configuration.

Trans product