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The Claisen rearrangement is one of the most effective organic chemical processes for forming carbon-carbon bonds. Ludwig Claisen, a German chemist, discovered this approach in 1912. It’s important not to mix up Claisen condensation and Claisen rearrangement because they’re two completely separate processes. The mechanics and variations of Claisen Rearrangement reactions will be examined in depth in this article.
Claisen Rearrangement
Claisen rearrangement is a technique for creating carbon-carbon bonds. It’s an organic chemical reaction named after Ludwig Claisen, a German chemist who discovered it in 1912. In the presence of Lewis acid and heat, it is a rearrangement reaction in which allyl vinyl ether is transformed into a, γ,δ, unsaturated carbonyl molecule.
The Claisen rearrangement is a type of sigma tropic rearrangement’. It’s a reaction in which all of the bonds simultaneously break and form.
Mechanism of Claisen Rearrangement
Claisen rearrangement is a pericyclic and exothermic process. As energy is released, the arrangement reaction is transformed, and its intermediate transition state is cyclic. Polar solvent effects accelerate the reaction to a greater extent.
Sulfolane has a 10-fold greater rate constant than ethanol or water solvent combinations. The regioselectivity of this rearrangement is also affected by meta-substitution.
Allyl Vinyl Ethers
A transition state is formed when allyl vinyl ethers are heated. The following reaction occurs, resulting in the creation of the desired, γ,δ- unsaturated carbonyl molecule. The core C-C bond is broken, causing the two 3-carbon allyl fragments to rearrange through an intermediate state. The mechanism is as follows:
Allyl Phenyl Ethers
Electrons are pushed around the six-membered ring via an electrocyclic mechanism. Tautomerization of the resultant dienone yields a more stable aromatic phenol. As a result, the needed chemical is created.
The following is the mechanism:
An electrocyclic process pushes those electrons around the 6-membered ring:
Hetero-Claisens
Hetero-Claisen rearrangement is a form of traditional Claisen rearrangement that necessitates the presence of double bonds in α and β place on the substrate. Aromatic double bonds can be found in both double bonds.
In around a half-dozen known rearrangements, one or more carbon atoms of the initial allyl vinyl ether were exchanged by hetero-atoms such as sulphur, nitrogen, or phosphorus. Rearrangements of 3-oxa-1,5-hexadiene in which one or more carbon atoms are replaced by non-carbon atoms are feasible. Heterodoxa-Claisen-rearrangements are the name for these reactions (HOCR).
Chromium Oxidation
Chromium can oxidise allylic alcohols into alpha-beta unsaturated ketones on the opposite side of the unsaturated bond from the alcohol. The chromium atom has access to d- shell orbitals, allowing the reaction to take place in a larger range of configurations.
Chen-Mapp Reaction
The Staudinger reduction replaces an alcohol with a phosphite and converts it to an imine in the Chen–Mapp reaction, also known as the [3,3]-Phosphoramidate Rearrangement or Staudinger–Claisen Reaction. The Claisen is driven by the fact that a P=O double bond is more energetically favourable than a P=N double bond.
Overman Rearrangement
The Overman rearrangement, named after Larry Overman, is a Claisen rearrangement of allylic trichloroacetimidates to allylic trichloroacetimidates through an immediate intermediate.
Zwitterionic Claisen rearrangement
Zwitterionic Claisen rearrangements occur at or below room temperature, unlike typical Claisen rearrangements. The acyl ammonium ions are particularly selective for Z-enolates in mild conditions.
Claisen rearrangement in nature
The Claisen rearrangement of chorismate ion to prephenate ion, which is an essential step in the shikimic acid pathway, is catalysed by chorismate mutase (the biosynthetic pathway towards the synthesis of phenylalanine and tyrosine).
Kazmaier-Claisen arrangement
The Kazmaier-Claisen reaction occurs at –78 °C when an unsaturated amino acid ester reacts with a strong base (such as lithium diisopropylamide) and a metal salt to produce a chelated enolate as an intermediate. While other metal salts can be used to create the enolate, zinc chloride produces the highest yield and has the best stereospecificity. At –20 °C, the enolate species reorganises into an amino acid with an allylic side chain at the α -position. Uli Kazmaier first detailed this approach in 1993.
Aza-Claisen
One of the pi-bonded moieties in the rearrangement can be an iminium.
Conclusion
Claisen rearrangement is a technique for creating carbon-carbon bonds. It’s an organic chemical reaction named after Ludwig Claisen, a German chemist who discovered it in 1912. In the presence of Lewis acid and heat, it is a rearrangement reaction in which allyl vinyl ether is transformed into a, γ,δ, unsaturated carbonyl molecule.
Aza-Claisen
One of the pi-bonded moieties in the rearrangement can be an iminium.
