Claisen Rearrangement mechanism steps examples and synthetic applications
The investigations on the stereochemistry elements of the Claisen reactions deserve special note. The Claisen rearrangement has generated a great deal of investigation due to its value in synthetic activities, especially in the production of natural compounds, and also its use as an instrument to explore the bond configurations of aromatic compounds and its inherent significance as a molecular rearrangement.
The Claisen rearrangement is a systematic, pericyclic, exothermic reaction that involves bond cleavage and rearrangement. Crossover studies rule out the likelihood of the rearrangement taking place through a mechanism involving an intermolecular interaction and are in line with an intramolecular action. It's crucial to remember that Claisen condensation and Claisen rearrangement are two very different phenomena. This article offers a broad outline of the main important subjects pertaining to the Claisen rearrangement.
What is Claisen Rearrangement?
Claisen rearrangement is known as an organic chemical process which provides a potent way to generate carbon-carbon bond formation. When heated or exposed to a Lewis acid, the initiator of this reaction, for example, allyl vinyl ether, transforms into an unsaturated carbonyl molecule.
The mechanism of Claisen rearrangement exhibits significant solvent influences, with polar solvents having a larger tendency to speed up the process. The maximum reaction rates were produced by hydrogen-bonding solvents. The process is more quickly accelerated by polar solvent influences.
Examples of Claisen Rearrangement
The reorganisation of allyl vinyl ether and allyl aryl ether, such as allyl phenyl ether, is accomplished through this Claisen rearrangement. The latter is employed to prepare allyl phenol. These are some of the Claisen rearrangement examples. A Lewis acid or heat can cause the initiator of this Claisen rearrangement mechanism, allyl vinyl ether, to change towards a gamma, delta-unsaturated carbonyl molecule.
Combining C-C π-Bonds
Formally, the Claisen rearrangement can be described as the intramolecular addition of an allylic ether, a sulphide, or an amine to a carbonyl enol (X = O), thiocarbonyl enol (X = S), or an enamine (X = N), accordingly, generating a carbon-carbon σ-bond. The mechanism, which is categorised as a 3, 3-sigmatropic shifting, includes concurrent π-bond movement. It is typically described as coordinated, even though in reality, a variety of processes may be in play. As a result of hetero- and polyhetero-Claisen rearrangements, heteroatoms may occupy additional places and the unsaturation level might well be greater than indicated.
CC 𝝅-Bond
Mechanism of Claisen Rearrangement
A particular kind of sigma tropic rearrangement is the Claisen rearrangement. It is a mechanism in which every bond concurrently forms and breaks. Allyl phenyl ethers can also be used to carry out the Claisen rearrangement mechanism. Allyl phenyl ether is rearranged into its aromatic enol form in Claisen rearrangement. The meta-substitution in this rearrangement has an impact on the regioselectivity. An intermediate is produced via the [3, 3]-sigmatropic rearrangement of the allyl phenyl ether. This intermediate is then subjected to tautomerization, producing a phenol that has an ortho substitution. The allyl phenyl ether Claisen rearrangement is given in below diagram:
Allyl Phenyl Ether Claisen Rearrangement
Variations
Aside from the standard allyl vinyl ether rearrangement, a number of variants have been created that enhance the Claisen rearrangement's synthetic significance in terms of the formation of the parental compounds, the reactive environment, and stereoselectivity. The different types of variations are listed beneath:
Aromatic Claisen rearrangement
Bellus Claisen rearrangement
Eschenmoser–Claisen rearrangement
Ireland–Claisen rearrangement
Johnson–Claisen rearrangement
Kazmaier–Claisen rearrangement
Photo-Claisen rearrangement
Interesting Facts
The Claisen rearrangement mechanism is called after its inventor, German chemist Rainer Ludwig Claisen, who made the discovery in 1912.
It's significant to note that sigmatropic rearrangement was the first [3, 3]- sigmatropic rearrangement process to ever be documented.
In contrast to usual Claisen rearrangements, zwitterionic Claisen rearrangements take place at or beneath room temperature. In milder circumstances, the acyl ammonium ions are especially selective for Z-enolates.
Key Features to Remember
The organic reaction mechanism known as the Claisen rearrangement provides a potent way to generate carbon-carbon bonds.
Claisen rearrangement is an exothermic, pericyclic reaction. When energy is released, the arranging reaction changes and its intermediary transitional phase is cyclic.
If heated or exposed to a Lewis acid, the source of this synthesis, allyl vinyl ether, transforms into a gamma, delta-unsaturated carbonyl molecule.
The conversion occurs intramolecularly, follows a well-organised cyclic transitional phase, and possesses first-order kinetics.
FAQs on Claisen Rearrangement in Organic Chemistry
1. What is the Claisen rearrangement in organic chemistry?
The Claisen rearrangement is a thermal [3,3]-sigmatropic rearrangement in which an allyl vinyl ether rearranges to form a γ,δ-unsaturated carbonyl compound. It is a pericyclic reaction that proceeds through a six-membered cyclic transition state without the need for a catalyst.
Key features:
- Involves migration of a σ-bond with simultaneous shift of π-bonds.
- Occurs under heat (typically 150–250 °C).
- Forms a new C–C bond and converts an enol ether into an aldehyde or ketone after tautomerization.
- Highly regioselective and stereospecific.
2. What is the general mechanism of the Claisen rearrangement?
The mechanism of the Claisen rearrangement is a concerted [3,3]-sigmatropic shift that occurs through a six-membered cyclic transition state. All bond-making and bond-breaking steps happen simultaneously in one step.
Mechanistic steps:
- Heating an allyl vinyl ether initiates rearrangement.
- A six-membered chair-like transition state forms.
- A new C–C σ-bond forms while the original C–O σ-bond breaks.
- The product initially forms as an enol, which rapidly tautomerizes to a carbonyl compound (aldehyde or ketone).
3. What is an example of a Claisen rearrangement reaction?
A classic example of the Claisen rearrangement is the thermal conversion of allyl phenyl ether into o-allylphenol. Upon heating, the allyl group migrates from oxygen to the ortho position of the aromatic ring.
Example reaction:
- Allyl phenyl ether → o-allylphenol (on heating)
4. Why is the Claisen rearrangement called a [3,3]-sigmatropic rearrangement?
The Claisen rearrangement is called a [3,3]-sigmatropic rearrangement because a σ-bond shifts across two π-systems, each involving three atoms. The numbers [3,3] indicate that three atoms are involved on each side of the migrating bond.
Explanation:
- The reaction involves a six-atom cyclic transition state.
- One σ-bond breaks and a new σ-bond forms between the terminal atoms.
- π-bonds shift simultaneously in a concerted process.
5. What are the types of Claisen rearrangement?
The main types of Claisen rearrangement include the classical, aromatic, and modified variants used in synthesis. Each type follows the same [3,3]-sigmatropic principle but differs in substrate structure.
Common types:
- Classical Claisen rearrangement – Allyl vinyl ethers form γ,δ-unsaturated carbonyl compounds.
- Aromatic Claisen rearrangement – Allyl aryl ethers rearrange to o-allylphenols.
- Johnson–Claisen rearrangement – Allyl alcohols react with orthoesters to give γ,δ-unsaturated esters.
- Eschenmoser–Claisen rearrangement – Uses allylic alcohols and amide acetals to form unsaturated amides.
6. What conditions are required for the Claisen rearrangement?
The Claisen rearrangement requires heat and typically proceeds without a catalyst under thermal conditions. Most reactions occur at elevated temperatures between 150–250 °C.
Typical conditions:
- Strong heating (thermal activation).
- No acid or base catalyst required.
- Often performed in high-boiling solvents or neat conditions.
7. What is the product of a Claisen rearrangement?
The product of a classical Claisen rearrangement is a γ,δ-unsaturated carbonyl compound formed after enol–keto tautomerization. The rearrangement creates a new carbon–carbon bond and shifts the position of double bonds.
Product characteristics:
- Initial formation of an enol intermediate.
- Rapid tautomerization to an aldehyde or ketone.
- Formation of a more stable C=O bond.
8. Is the Claisen rearrangement stereospecific?
Yes, the Claisen rearrangement is stereospecific because it proceeds through a concerted cyclic transition state. The stereochemistry of the starting material directly influences the stereochemistry of the product.
Important points:
- The reaction typically follows a chair-like transition state.
- Substituents prefer equatorial positions in the transition state.
- Cis/trans geometry of alkenes is preserved in a predictable manner.
9. What is the difference between Claisen rearrangement and Cope rearrangement?
The key difference is that the Claisen rearrangement involves an allyl vinyl ether forming a carbonyl compound, while the Cope rearrangement involves a 1,5-diene rearranging to another diene. Both are [3,3]-sigmatropic rearrangements but differ in functional groups.
Comparison:
- Claisen rearrangement: Contains an oxygen atom; forms aldehydes or ketones.
- Cope rearrangement: Hydrocarbon system; no heteroatoms involved.
- Both proceed via six-membered cyclic transition states.
10. Why is the Claisen rearrangement important in organic synthesis?
The Claisen rearrangement is important because it forms new carbon–carbon bonds in a predictable, stereospecific, and high-yielding manner. It is widely used to construct complex molecular frameworks.
Importance in synthesis:
- Creates C–C bonds without catalysts.
- High regioselectivity and stereoselectivity.
- Useful in the synthesis of natural products and pharmaceuticals.
- Compatible with various functional groups.
