Claisen rearrangement
The Claisen rearrangement is a powerful carbon-carbon bond-forming chemical reaction discovered by Rainer Ludwig Claisen.  The heating of an allyl vinyl ether will initiate a -sigmatropic rearrangement to give a ,-unsaturated carbonyl.
Discovered in 1912, the Claisen rearrangement is the first recorded example of a -sigmatropic rearrangement.
Many reviews have been written.

# Mechanism
The Claisen rearrangement (and its variants) are exothermic (about 84 kJ/mol), concerted pericyclic reactions which according to the Woodward-Hoffmann rules show a suprafacial reaction pathway.
There are substantial solvent effects in the Claisen reactions.  More polar solvents tend to accelerate the reaction to a greater extent.  Hydrogen-bonding solvents gave the highest rate constants.  For example, ethanol/water solvent mixtures give rate constants 10-fold higher than sulfolane.
Trivalent organoaluminium reagents, such as trimethylaluminium, have been shown to accelerate this reaction.

# Variations

## Aromatic Claisen rearrangement
The aromatic variation of the Claisen rearrangement is the -sigmatropic rearrangement of an allyl phenyl ether to an intermediate which quickly tautomerizes to an ortho-substituted phenol.

## Bellus-Claisen rearrangement
The Bellus-Claisen rearrangement is the reaction of allylic ethers, amines, and thioethers with ketenes to give ,-unsaturated esters, amides, and thioesters.

## Eschenmoser-Claisen rearrangement
The Eschenmoser-Claisen rearrangement proceeds from an allylic alcohol to a ,-unsaturated amide, and was developed by Albert Eschenmoser in 1964.

## Ireland-Claisen rearrangement
The Ireland-Claisen rearrangement is the reaction of an allylic acetate with strong base (such as Lithium diisopropylamide) to give a ,-unsaturated carboxylic acid.

## Johnson-Claisen rearrangement
The Johnson-Claisen rearrangement is the reaction of an allylic alcohol with trimethyl orthoacetate to give a ,-unsaturated ester.

# Hetero-Claisens

## Aza-Claisen
An iminium can serve as one of the pi-bonded moieties in the rearrangement.

## Chromium Oxidation
Chromium can oxidize allylic alcohols to alpha-beta unsaturated ketones on the opposite side of the unsaturated bond from the alcohol.  This is via a concerted hetero-claisen reaction, although there are mechanistic differences since the chromium atom has access to d- shell orbitals which allow the reaction under a less constrained set of geometries.

## Chen-Mapp Reaction
The Chen-Mapp reaction also known as the -Phosphorimidate Rearrangement or Staudinger-Claisen Reaction installs a phosphite in the place of an alcohol and takes advantage of the Staudinger Ligation to convert this to an imine.  The subsequent claisen is driven by the fact that a P=O double bond is more energetically favorable than a P=N double bond.

## Overman rearrangement
The Overman rearrangement (named after Larry Overman) is a Claisen rearrangement of allylic trichloroacetimidates to allylic trichloroacetamides.

## Zwitterionic Claisen rearrangement
Unlike typical Claisen rearrangements which require heating, zwitterionic Claisen rearrangements take place at or below room temperature.  The acyl ammonium ions are highly selective for Z-enolates under mild conditions.

# Claisen rearrangement in nature
The enzyme Chorismate mutase (EC 5.4.99.5) catalyzes the Claisen rearrangement of chorismate ion to prephenate ion, a key intermediate in the shikimic acid pathway (the biosynthetic pathway towards the synthesis of phenylalanine and tyrosine).