Saturday 18 October 2014

CLAISEN R.......REACTION AND MECHANISM

sigmatropic reaction in organic chemistry is a pericyclic reaction wherein the net result is one σ-bond is changed to another σ-bond in an uncatalyzed intramolecularprocess.[1] The name sigmatropic is the result of a compounding of the long-established sigma designation from single carbon–carbon bonds and the Greek word tropos, meaning turn. In this type of rearrangement reaction, a substituent moves from one part of a π-bonded system to another part in an intramolecular reaction with simultaneous rearrangement of the π system. True sigmatropic reactions are usually uncatalyzed, although Lewis acid catalysis is possible. Sigmatropic reactions often have transition-metal catalysts that form intermediates in analogous reactions. The most well-known of the sigmatropic rearrangements are the [3,3] Cope rearrangementClaisen rearrangement,Carroll rearrangement and the Fischer indole synthesis.









 3,3-sigmatropic reaarangment after that decarboxylation

Woodward hoffmann order nomenclature bond break.png


The mecahnism involves the abstraction of proton from the active methylene group of the keto-ester followed by intramolecular Michael addition and decarboxylation to yield the unsaturated ketone compound

Firstly the ketoester tautomerize to give a enol ester and then a [3,3]-sigmatropic rearrangement occur (or simply called Claisen Rearrangement). After that, the substrate become a keto-acid and it undergoes decarboxylation under heat.


Claisen rearrangement

Main article: Claisen rearrangement
Discovered in 1912 by Rainer Ludwig Claisen, the Claisen rearrangement is the first recorded example of a [3,3]-sigmatropic rearrangement.[10][11][12] This rearrangement is a useful carbon-carbon bond-forming reaction. An example of Claisen rearrangement is the [3,3] rearrangement of an allyl vinyl ether, which upon heating yields a γ,δ-unsaturated carbonyl. The formation of a carbonyl group makes this reaction, unlike other sigmatropic rearrangements, inherently irreversible.
The Claisen rearrangement

Aromatic Claisen rearrangement
The ortho-Claisen rearrangement involves the [3,3] shift of an allyl phenyl ether to an intermediate which quickly tautomerizes to an ortho-substituted phenol.

Aromatic Claisen rearrangement
When both the ortho positions on the benzene ring are blocked, a second ortho-Claisen rearrangement will occur. This para-Claisen rearrangement ends with the tautomerization to a tri-substituted phenol.
Para-Claisen rearrangement

Cope rearrangement

Main article: Cope rearrangement
The Cope rearrangement is an extensively studied organic reaction involving the [3,3] sigmatropic rearrangement of 1,5-dienes.[13][14][15] It was developed by Arthur C. Cope. For example 3,4-dimethyl-1,5-hexadiene heated to 300 °C yields 2,6-octadiene.
The Cope rearrangement of 3,4-dimethyl-1,5-hexadiene

Oxy-Cope rearrangement
In the Oxy-Cope rearrangement, a hydroxyl group is added at C3 forming an enal or enone after Keto-enol tautomerism of the intermediate enol:[16]


Oxy-Cope rearrangement













Stereochemistry rentention inversion.png





References


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