Question

Question: What product is formed when each compound undergoes a photochemical electrocyclic reaction? Label each process as conrotatory or disrotatory and clearly indicate the stereochemistry around tetrahedral stereogenic centers and double bonds.

Short answer

Answer

The product formed from photochemical electrocyclic reaction:

a.

b.

Step-by-step solution

Electrocyclic reactions

Reactions in which conjugated polyenes close to form rings or rings open to form polyenes are known as electrocyclic reactions.

To predict whether the reaction is allowed or forbidden, the set of Woodward-Hoffmann rules are followed.

Product formed from the photochemical electrocyclic reaction

(a)

• It is an electrocyclic ring-opening reaction under photochemical conditions.

• In an electrocyclic ring-opening reaction, a new double bond is formed, and an old sigma bond is removed.

Electrocyclic reaction

Frontier orbital representation:

• The HOMO for this molecule under thermal conditions is .

• The molecular orbitals for HOMO and symmetry are correct for bond interaction to form the product.

• The opening of the sigma bond by con-rotation of orbitals takes place to form the product.

(b)

• It is a s-trans-1,3-dines.

• s-trans-1,3-dienes does not participate in the electrocyclic reactions.

• These molecules are first converted to the s-cis-1,3-diene to participate in the electrocyclic reactions.

Conversion from trans to cis

• It is an electrocyclic ring closure reaction of the compound with two conjugated double bonds, and the ring closure is dis-rotatory under photochemical conditions.

• From Woodward-Hoffmann's rules, the photochemical electrocyclic reaction involves (4n) electrons that are dis-rotatory.

Electrocyclic reaction

Therefore, the product formed is of a 2 electron system arising from dis-rotary cyclization.Frontier orbital representation:

• The HOMO for this molecule under thermal conditions is in which the end orbitals are symmetrical.

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

HOMO of the compound