"Demystifying Electrocyclic Reactions: A Guide to Conrotatory and Disrotatory Pathways, Stereoselectivity, and Reversibility"

 

"Demystifying Electrocyclic Reactions: A Guide to Conrotatory and Disrotatory Pathways, Stereoselectivity, and Reversibility"

Electrocyclic reactions are an important class of organic reactions that involve the rearrangement of a cyclic conjugated system. “An intramolecular reaction results in a cyclic product with one fewer bond than the reactant due to the rearranging of electrons”. Completely stereoselective, an electrocyclic process creates only one stereoisomer.

For instance, only the cis product results from an electrocyclic reaction involving (2E, 4Z, 6E)-octatriene under thermal conditions; only the trans product results from an electrocyclic reaction using (2E, 4Z, 6Z)-octatriene under thermal conditions. Remember that Z indicates that the highly prior substituents are on the same side of the double bond whereas E indicates that they are on opposing sides of the double bond.

The products, however, have the opposite configurations when the reactions are carried out under photochemical conditions: the compound that forms the cis isomer under thermal conditions also forms the trans isomer under photochemical conditions, and the reverse is true for the compound that forms the trans isomer under thermal conditions.

So, these reactions can occur under both photochemical and thermal conditions, and they can lead to the formation of either cis or trans isomers depending on the conditions. The reaction can be conrotatory or disrotatory, and the stereochemistry of the product can be either Entgegen (E) or Zusamen (Z). 

Another example is cyclization of (2E, 4Z)-hexadiene to cis-3,4-dimethylcyclobutene and (2E, 4E)-hexadiene to trans-3,4-dimethylcyclobutene occurs under thermal circumstances.

 Similarly, if the reaction is carried out in presence of photochemical conditions, the product will be obtained with opposite stereochemistry.

A new σ bond is created as the end consequence of an electrocyclic process. The p orbitals at the ends of the conjugated system must spin for the connection to form, head-to-head overlapping and rehybridize to sp³. Rotation can happen in one of two ways. 

1. Conrotatory electrocyclic reaction

2. Disrotatory electrocyclic reaction

1. Conrotatory Electrocyclic Reaction:

In a conrotatory electrocyclic reaction, the substituents on the reacting atoms move in the same direction, either clockwise or counterclockwise, during the course of the reaction. 

2. Disrotatory Electrocyclic Reaction:

In a disrotatory electrocyclic reaction, the substituents move in opposite directions. This can have a significant impact on the stereochemistry of the product.

Keep in mind two cases;

• If the end orbitals of the HOMO are symmetric, rotation must be disrotatory in order to obtain in-phase overlap. In other words, whereas conrotatory ring closure is prohibited by symmetry, disrotatory ring closure is permitted.
• To obtain inphase overlap when the HOMO is asymmetric, rotation must be conrotatory. In other words, whereas disrotatory ring closure is prohibited by symmetry, conrotatory ring closure is permitted.

We know that what is symmetry allowed pathway (in-phase overlap) and symmetry forbidden pathway (out-phase overlap). We also have idea that under what conditions symmetry allowed and symmetry forbidden reaction take place. Symmetry allowed follows the concerted way.  If a symmetry forbidden reaction takes place at all, it must do so by a nonconcerted mechanism.

Now, we are able to understand that why product's configuration modifies when reaction is conducted under photochemical conditions. A molecule having three π conjugated bonds, such as (2E,4Z,6E)-octatriene, has a symmetric ground-state HOMO (Ψ₃). This indicates that ring closure is disrotatory under heating circumstances. The methyl groups are both pushed up (or down) during the disrotatory ring closure of (2E,4Z,6E)-octatriene, which produces the cis product. 

One methyl group is pulled up and another is pushed down during the disrotatory ring closure of (2E,4Z,6Z)-octatriene, which produces the trans product.

We must take into account the excited-state HOMO rather than the ground-state HOMO if the reaction occurs under photochemical circumstances. A molecule with three π bonds has an asymmetric excited-state HOMO (Ψ₄). Due to the conrotatory ring closure that (2E,4Z,6Z)-octatriene goes through under photochemical conditions, both methyl groups are forced down (or up), and the cis product is created.

The stereochemistry of the product can also be affected by the initial geometry of the starting material. 

Steroselectivity:

A reaction in which there is a choice of pathway, but the product stereoisomer is formed due to its reaction pathway being more favorable than the others available. Electrocyclic reactions are stereoselective in nature.

Reversibility:

Another important aspect of electrocyclic reactions is their reversibility. Some electrocyclic reactions are reversible, meaning that the reaction can proceed in both the forward and reverse directions. This can be particularly useful in the design of molecular switches or other responsive materials.

Conclusion:

Conclusion include following keypoints;

·         Electrocyclic reactions are an important class of organic reactions that can lead to the formation of both cis and trans isomers.

·         The reaction can be conrotatory or disrotatory, and the stereochemistry of the product can be either entgegen or zusamen.

·         The reaction can be stereoselective and reversible, and the conditions under which the reaction is carried out can have a significant impact on the stereochemistry of the product.

·         Stereochemical outcome of an electrocyclic reaction depends on the symmetry of the HOMO of the compound undergoing ring closure.

1. The ground-state HOMO is symmetric—so disrotatory ring closure occurs
2. The excited-state HOMO is asymmetric—so conrotatory ring closure occurs.