"Percyclic Reactions in Hexatriene: Understanding Woodward Rules and Photochemical vs. Thermal Electrocyclization for Symmetric and Asymmetric Products"

 

"Percyclic Reactions in Hexatriene: Understanding Woodward Rules and Photochemical vs. Thermal Electrocyclization for Symmetric and Asymmetric Products"

Pericyclic reactions refer to a type of chemical reaction where a cyclic molecule is formed from a linear molecule through a series of concerted bond-breaking and bond-forming steps. This type of reaction is widely studied in organic chemistry, and the molecular orbitals of the reactants play a significant role in determining the reaction pathway and product.

One classic example of a pericyclic reaction is the electrocyclization of hexatriene. This reaction can occur through either a photochemical or thermal pathway, and the resulting product can be either symmetric or asymmetric. The reaction mechanism is governed by Woodward's rules, which describe the allowed and forbidden cyclic transitions based on the molecular orbitals of the reactants.

The structure of hexatriene contains 16 covalent bonds (8 C-H σ, 5 C-C σ, 3 C-C π)

Molecular orbitals for the hexatriene are six named as  Ψ₁ , Ψ_{2 ,} Ψ_{3 ,} Ψ_{4 ,} Ψ_{5 ,} Ψ₆ . Among all, first three are bonding molecular orbital (Ψ₁, Ψ_2, Ψ₃) and the remaining are antibonding (Ψ_4, Ψ_5, Ψ₆) in nature.

The reaction utilized for the conversion  hexatriene of  1,3-Cyclohexadiene by thermal and photochemical pathway is mentioned below;

Notice that, there are two symmetry elements followed by hexatriene;

• Mirror plane (m) symmetry is maintained in thermal conversion of 1,3,5-hexatriene into 1,3-cyclohexadiene
• C₂-axis of symmetry is maintained in photochemical conversion of 1,3-cyclohexadiene into 1,3,5-hexatriene or vice versa.

Thermal Electrocyclization of Hexatriene:

The thermal electrocyclization reaction of hexatriene is initiated by heating the molecule to a high temperature. The reaction is also governed by Woodward's rules, but the allowed and forbidden cyclic transitions are different from the photochemical pathway. In particular, thermal electrocyclization tends to favor the formation of symmetric products, while photochemical electrocyclization can result in either symmetric or asymmetric products. This generates a diradical intermediate, which can undergo a series of concerted bond-breaking and bond-forming steps to form the cyclic product. 

Photochemical Electrocyclization of Hexatriene:

In the photochemical electrocyclization of hexatriene, the reaction is initiated by absorption of a photon, which promotes an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). This follows the same mechanism  in which a series of concerted bond-breaking and bond-forming steps are involved to form the cyclic product. The resulting product can be either symmetric or asymmetric, depending on the orientation of the substituents on the hexatriene molecule. 

Thermal vs Photochemical Electrocyclization:

Woodward Hoffman Rules for Electrocyclization:

We've shown that the mode of ring closure, which in turn depends on the amount of conjugated bonds present in the reactant and whether the reaction takes place under thermal or photochemical circumstances, determines the stereochemistry of an electrocyclic reaction. Our understanding of electrocyclic processes can be simplified by the selection rules stated in Table. The Woodward-Hoffmann rules for electrocyclic reactions are another name for these.

For evaluating if a specific electrocyclic reaction is "allowed or forbidden by orbital symmetry," see Table mentioned below. 

I know this is bit tricky to remember these rules. So, I would suggest you to use acronym "TE-AC" to remember all of the regulations, which can be fairly taxing.

• If TE (Thermal/Even) describes the reaction, the outcome is given by AC (Antarafacial or Conrotatory).
• If one of the letters of TE is incorrect (it is not Thermal/Even but is Thermal/Odd or Photochemical/Even), the outcome is not given by AC (the outcome is Suprafacial or Disrotatory).
• If both of the letters of TE are incorrect (Photochemical/Odd), the outcome is given by AC (Antarafacial or Conrotatory)—“two negatives make a positive.

In both the photochemical and thermal pathways, the reaction mechanism is highly dependent on the molecular orbitals of the reactants. The HOMO and LUMO of hexatriene play a critical role in determining the allowed and forbidden cyclic transitions, as well as the orientation of the substituents in the final product. The Woodward rules provide a useful framework for understanding the reaction mechanism and predicting the outcome of the reaction.

Conclusion:

In conclusion, the electrocyclization of hexatriene is a classic example of a percyclic reaction, which can occur through either a photochemical or thermal pathway. The resulting product can be either symmetric or asymmetric, depending on the orientation of the substituents on the hexatriene molecule. Woodward's rules provide a useful framework for understanding the reaction mechanism and predicting the outcome of the reaction.