Sigmatropic rearrangements belong to the class of Pericyclic Reactions where a sigma (σ) bond moves from one position to another within a pi(π)-electron system. This movement occurs while preserving the overall molecular connectivity. Atoms shift their positions while maintaining a continuous pi-system. These reactions proceed in a single step without forming intermediates. They occur smoothly with heat (thermal) or light (photochemical) without the need for catalysts. ^[1-4]

Nomenclature and Classification ^[1-4]

The [m, n] notation is used to represent a sigmatropic rearrangement or shift. It describes the number of atoms involved in the migration of a sigma bond. Here, m represents the initial position of the sigma bond, and n represents its new position after rearrangement. ^[1-4]

To illustrate this concept, consider the sigmatropic rearrangement of 2,4-pentadiene. Depending on the reaction conditions, this compound undergoes different types of sigma bond shifts.

[1,3] Sigmatropic Shift

Under the influence of light (hv), 2,4-pentadiene undergoes a [1,3] sigmatropic shift, where the hydrogen atom moves from position 1 to 3. It occurs due to the excitation of electrons into an excited state, altering the electronic structure and allowing for a concerted migration of the hydrogen atom. The process follows Woodward–Hoffmann rules, which dictate that photochemical conditions allow for suprafacial [1,3] shifts, meaning that the hydrogen remains on the same face of the π-system.

[1,5] Sigmatropic Shift

When heat (Δ) is applied, the system undergoes a [1,5] sigmatropic shift, in which the hydrogen atom migrates from position 1 to 5. It occurs through a concerted reaction mechanism, preserving the conjugation of the diene system. Unlike the photochemical shift, the thermal shift follows the orbital symmetry rules, predicting that a [1,5] hydrogen shift is thermally allowed in a suprafacial manner.

In addition to [1,3] and [1,5] sigmatropic shifts, there is another important category known as the [3,3] sigmatropic shift, which plays a significant role in pericyclic reactions.

[3,3] Sigmatropic Shift

In a [3,3] sigmatropic shift, a sigma bond migrates within a conjugated system through a six-membered cyclic transition state. This rearrangement results in the reorganization of both sigma and pi bonds while maintaining the overall molecular framework. Two well-known examples of [3,3] shifts include the Cope rearrangement and Claisen rearrangement.

Cope Rearrangement

In the Cope rearrangement, a 1,5-diene undergoes structural reorganization under thermal conditions, producing a new diene with altered connectivity. This reaction follows the principles of orbital symmetry and proceeds through a six-membered cyclic transition state.

For example, 3-methylhexa-1,5-diene undergoes thermal rearrangement at 300 °C to form hepta-1,5-diene:

CH₂​=CH−CH(CH₃​)−CH₂−CH=CH₂​ → CH₂​=CH−CH₂​−CH₂−CH=CH−CH₃

Claisen Rearrangement

The Claisen rearrangement involves the thermal conversion of an allyl vinyl ether into a γ,δ-unsaturated carbonyl compound. This transformation occurs through a six-membered cyclic transition state, resulting in a more stable product.

For instance, allyl vinyl ether rearranges into 4-pentenal:

CH₂​=CH−O−CH₂​−CH=CH₂ → ​​CH₂​=CH−CH₂​−CH_2​−CHO

A variation of this reaction, known as the Aromatic Claisen Rearrangement, involves allyl aryl ethers, leading to the formation of hydroxyaryl ketones. For example, allyl phenyl ether rearranges to allyl phenol upon heating:

C₆​H₅​-O-CH₂​CH=CH₂​ → ​HO-C₆​H₄​-CH₂​CH=CH₂​

Applications  ^[1-4]

• Synthesis of New Molecules – Used in the laboratory synthesis of alcohols, ketones, and esters, which are essential in the pharmaceutical and fragrance industries.
• Biosynthesis of Natural Compounds – Plays a key role in the formation of essential oils, herbal compounds, and plant-based medicines.
• Drug Development – Modifies drug molecules to enhance absorption and therapeutic effectiveness.
• Advanced Materials – Applied in the synthesis of high-performance plastics and flexible polymers.
• Fragrance and Flavor Industry – Used to synthesize aromatic compounds for perfumes and food additives.