Claisen rearrangement

The Claisen rearrangement is a reaction from the field of organic chemistry and after the German chemist Ludwig Claisen (1851 - 1930) named. It provides a [3,3 ] sigmatropic rearrangement of allyl vinyl ethers to γ -, δ -unsaturated carbonyl compounds represents a different name for the reaction is oxa -Cope rearrangement. In various textbooks, various statements find whether first phenyl or Vinylallylether were investigated.

Importance

The reaction has very interesting properties for the synthetic chemist. Obtained together with simultaneous extension of the carbon chain of the interesting for further syntheses carbonyl compounds. In addition, products are obtained, starting from enantiomerically pure starting materials with high enantiomeric excess.

Mechanism

The Claisen rearrangement is in principle reversible, but the equilibrium is due to the greater thermodynamic stability of the carbonyl on the product page. The allyl vinyl ether represented by mercury or acid-catalyzed reaction of ethyl vinyl ether with allyl alcohol. Since the allyl vinyl already rearranged during the synthesis part, they are usually not isolated.

When heated to 200 ° C allyl phenyl ether, according to standing down mechanism starts to form the expected ketone, which then rearranges under rearomatization and proton transfer analogous to a keto -enol tautomerism in the corresponding ortho- allyl phenol.

Studies that used the kinetic isotope effect, show that the O- Cα bond already from 50 to 60% dissolved, while - C? Cortho bond is first formed from 10 to 20%. This means that the Claisen rearrangement is indeed a concerted but asynchronous rearrangement.

If the two ortho positions occupied by substituents ( a Cope rearrangement in this case) is done by a second [3,3 ] - sigmatropic rearrangement, the formation of the corresponding para- product.

Variants

Because of these advantages, numerous variants have been developed. The following five -known variants of the Claisen rearrangement is presented.

Johnson variant

In the orthoester or Johnson - variant, a carboxylic acid ortho ester is reacted under acid catalysis with the allyl alcohol. It successively two molecules of methanol are eliminated. Following the rearrangement to the unsaturated esters.

Eschenmoser variant

In the Eschenmoser variant of a carboxamide dimethyl acetal used. This reacts with the allyl alcohol with elimination of a molecule of methanol and formation of a mixed acetal. The acetal eliminated another molecule of methanol. Then the rearrangement to an unsaturated amide.

Carroll version

When Carroll variant of allyl alcohol is first reacted with diketene. It primarily produces a β -keto acid, which is decarboxylated to give the unsaturated ketone.

Arnold variant

When Arnold- variant allyl esters catalyzed by strong bases (B ) is deprotonated. The resulting ester enolate then superimposed to the salt of the unsaturated carboxylic acid.

Ireland- variant

In Ireland variant is first deprotonated as well as at the Arnold variant, the allyl esters. But then the trimethylsilyl ethers of the ester enolate is generated. This rearranges under particularly mild conditions. The silyl ester can be cleaved very gently.