Sigmatropic reaction

A **sigmatropic reaction** in organic chemistry is a pericyclic reaction. A sigmatropic reaction does not use a catalyst and involves a single molecule (an uncatalyzed intramolecular process). It changes one σ-bond into a different σ-bond. The name *sigmatropic* is the result of a compounding of the long-established "sigma" name for single carbon-carbon bonds and the Greek word *tropos*, meaning turn.  This is a rearrangement reaction which means that the bonds in a molecule  between atoms without any atoms leaving or new atoms added to the molecule. In a sigmatropic reaction, a substituent moves from one part of a π-bonded system to another part in an intramolecular reaction with  rearrangement of the π system. True sigmatropic reactions usually do not need a catalyst. Some sigmatropic reactions are catalyzed by a Lewis acid.  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 rearrangement, Claisen rearrangement, Carroll rearrangement and the Fischer indole synthesis.

## Overview of Sigmatropic Shifts
### Woodward-Hoffman Sigmatropic Shift Nomenclature
A special notation is used to describe sigmatropic shifts.  Each of the carbon atoms on the backbone of the molecule are assigned a position number.  Sigmatropic rearrangements are described by an **order** term **[i,j]**. This means the  of a σ-bond adjacent to one or more π systems to a new position (i-1) and (j-1) atoms removed from the original location of the σ-bond. Transformations that occur within small- or medium-sized rings cannot make antarafacial shifts.

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## Classes of Sigmatropic Rearrangements
### [1,3] Shifts
#### Thermal Hydride Shifts
In a thermal [1,3] hydride shift, a hydride moves three atoms.  The Woodward-Hoffmann rules dictate that it would proceed in an antarafacial shift.  Although such a shift is symmetry allowed, the Mobius topology required in the transition state prohibits such a shift. It is geometrically impossible. This is why enols do not isomerize without an acid or base catalyst.

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#### Thermal Alkyl Shifts
Thermal alkyl [1,3] shifts, similar to [1,3] hydride shifts, must proceed antarafacially.  The geometry of the transition state is prohibitive. But an alkyl group, due to the nature of its orbitals, can invert its geometry and form a new bond with the back lobe of its sp3 orbital. This reaction will result a suprafacial shift. These reactions are still not common in open chain systems because of the highly ordered nature of the transition state. So, the reactions work better in cyclic molecules.

:[[Image:1,5hantarafacialfixed.png|center|600px|Antarafacial [1,5] Hydride Shift]]
 
In contrast to hydrogen [1,5] shifts, there have never been any observed [1,5] alky shifts in an open-chain system.

Alkyl groups undergo [1,5] shifts very poorly and usually require high temperatures. However, for cyclohexadienes, the temperature for alkyl shifts is not much higher than that for carbonyls, the best migratory group. A study showed that this is because alkyl shifts on cyclohexadienes proceed through a different mechanism.  First the ring opens, followed by a [1,7] shift, and then the ring reforms electrocyclically:

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This same mechanistic process is seen below, without the final electrocyclic ring-closing reaction, in the interconversion of lumisterol to vitamin D2.

### [1,7] Shifts
[1,7] sigmatropic shifts are predicted by the Woodward-Hoffmann rules to proceed in an antarafacial fashion, by a Mobius topology transition state. An antarafacial **[1,7]** shift is observed in the conversion of lumisterol to vitamin D2, where following an electrocyclic ring opening to previtamin D2, a methyl hydrogen shifts.

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Bicyclic nonatrienes also undergo [1,7] shifts in a so called walk rearrangement, which is the shift of divalent group, as part of a three-membered ring, in a bicyclic molecule.

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### [3,3] Shifts
[3,3] sigmatropic shifts are well studied sigmatropic rearrangements. The Woodward-Hoffman rules predict that these six electron reactions would proceed suprafacially, using a Huckel topology transition state.

#### Claisen Rearrangement
Discovered in 1912 by Rainer Ludwig Claisen, the Claisen rearrangement is the first recorded example of a [3,3]-sigmatropic rearrangement. 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 .

##### 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.

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.

#### Cope Rearrangement
The **Cope rearrangement** is an extensively studied organic reaction involving the [3,3] sigmatropic rearrangement of 1,5-dienes. It was developed by Arthur C. Cope. For example 3,4-dimethyl-1,5-hexadiene heated to 300&nbsp;°C yields 2,6-octadiene.

##### Oxy-Cope rearrangement
The **Oxy-Cope rearrangement** a hydroxyl group is added at C3 forming an enal or enone after Keto-enol tautomerism of the intermediate enol:

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#### Carroll Rearrangement
The **Carroll rearrangement** is a rearrangement reaction in organic chemistry and involves the transformation of a β-keto allyl ester into a α-allyl-β-ketocarboxylic acid. This organic reaction can be followed by decarboxylation and the final product is a γ,δ-allylketone. The Carroll rearrangement is an adaptation of the Claisen rearrangement and effectively a **decarboxylative allylation**.

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#### Fischer Indole Synthesis
The **Fischer indole synthesis** is a chemical reaction that produces the aromatic heterocycle indole from a (substituted) phenylhydrazine and an aldehyde or ketone under acidic conditions. The reaction was discovered in 1883 by Hermann Emil Fischer.

The choice of acid catalyst is very important.  Successful acid catalysts include: Bronsted acids such as HCl, H2SO4, polyphosphoric acid and p-toluenesulfonic acid. Lewis acids such as boron trifluoride, zinc chloride, iron chloride, and aluminium chloride are also useful catalysts.

Several reviews have been published.

### [5,5] Shifts
Similar to [3,3] shifts, the Woodward-Hoffman rules predict that [5,5] sigmatropic shifts would proceed suprafacially, Huckel topology transition state.  These reactions are rarer than [3,3] sigmatropic shifts, but this is mainly a function of the fact that molecules that can undergo [5,5] shifts are rarer than molecules that can undergo [3,3] shifts. An example of such a rearrangement is the shift of substituents on tropilidenes (1,3,5-cycloheptatrienes). When heated, the pi-system goes through an electrocyclic ring closing to form bicycle[4,1,0]heptadiene (norcaradiene). Thereafter follows a [1,5] alkyl shift and an electrocyclic ring opening.

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Proceeding through a [1,5] shift, the walk rearrangement of norcaradienes is expected to proceed suprafacially with a retention of stereochemistry. Experimental observations, however, show that the 1,5-shifts of norcaradienes proceed antarafacially. Theoretical calculations found the [1,5] shift to be a diradical process, but without involving any diradical minima on the potential energy surface.

## Related pages
*2,3-sigmatropic rearrangement
*NIH shift
*Frontier Molecular Orbital Theory
*Woodward–Hoffmann rules

## References