User:Rperreau

thumb|# To do: #Change electron arrows to red color? #Fill out all sections until sufficient coverage, including applications section #Remove storage section from bottom #ortho vs para claisen for disubstituted (or even monosubstituted ortho positions #Remove [5,5] Claisen? #fluxional tautomerism of bullvalene #Text description for Murray series #Mukai antara-antara [3,3] shift Tet Lett 1967, 3433 in HCH 126 #[1,4] rearrangement Swatton-Hart rearrangement delocalized positive charge and "migrating" OH in D.W. Swatton and H. Hart, J. Am. Chem. Soc. 89, 5-75 (1967); occurs with inversion #[3,4] in competition with [1,2] and [3,3] from H.J. Hansen, B. Sutter, and H. Schmid, Helv. Chim. Acta 51, 828 (1968) # Berson and Willcott with [1,5] sequential shifts in norcaradienes, separate section from J.A. Berson and M.R. Willcott III, J. Am. Chem. Soc., 87, 2751, 2752 1965 88, 2494 (1966) # Woodward Hoffmann Concise Rules Chart and more FO examples etc. #Photochemical [1,5] shifts? in WH 124

A 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 intramolecular process. 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 "a 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.

Woodward-Hoffman Sigmatropic Shift Nomenclature
Sigmatropic rearrangements are concisely described by an order term [i,j], which is defined as the migration 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. Hydrogens are omitted in the third example for clarity.


 * [[Image:Woodward hoffmann order nomenclature detailed 2.png|center|600px|]]

A convenient means of determining the order of a given sigmatropic rearrangement is to number the atoms of the bond being broken as atom 1, and then count the atoms in each direction from the broken bond to the atoms that form the new σ-bond in the product, numbering consecutively. The numbers that correspond to the atoms forming the new bond are then separated by a comma and placed within brackets to create the sigmatropic reaction order descriptor.


 * [[Image:Woodward hoffmann order nomenclature bond break.png|center|600px|]]

In the case of hydrogen atom migrations, a similar technique may be applied. When determining the order of a sigmatropic shift involving a hydrogen atom migration it is critical in to count across all atoms involved in the reaction rather than only across the closest atoms. For example, the following hydrogen atom migration is of order [1,5], attained by counting counterclockwise, rather than the [1,3] order designation that would mistakenly result if counted clockwise.


 * [[Image:Woodward hoffmann order nomenclature hydrogen -1,5-.png|center|300px|]]

Suprafacial and Antarafacial Shifts
In principle, all sigmatropic shifts can occur with either a retention or inversion of the geometry of the migrating group, depending upon whether whether the original bonding lobe of the migrating atom or its other lobe is used to form the new bond.


 * [[Image:Stereochemistry rentention inversion.png|center|700px|]]

In cases of stereochemical retention, the migrating group translates without rotation into the bonding position, while in the case of stereochemical inversion the migrating group both rotates and translates to reach its bonded conformation.

However, another stereochemical transition effect equally capable of producing inversion or retention products is whether the migrating group remains on the original face of the π system after rebonding or instead transfers to the opposite face of the π system. If the migrating group remains on the same face of the π system, the shift is known as suprafacial, while if the migrating group transfers to the opposite face is called an antarafacial shift.


 * [[Image:Suprafacial antarafacial -1,5-.png|center|600px|]]

Suprafacial antarafacial -1,5-.png

Woodward-Hoffman
Main Article: Woodward-Hoffmann rules

Frontier Molecular Orbital Theory
Main Articles: HOMO

Considerations from Sterics
Antarafacial shifts are impossible for transformations which occur within small- or medium-sized rings.

Thermal Hydride Shifts
In a thermal [1,3] hydride shift, a hydride moves three atoms. The Woodward-Hoffman rules dictate that it would proceed in an antarafacial shift. Although such a shift is symmetry allowed allowed, the Mobius topology required in the transition state prohibits such a shift because it is geometrically impossible, which accounts for the fact that enols do not isomerizes without an acid or base catalyst.


 * [[Image:Hydride Sigmatropic.png|center|400px|Impossible Shift]]

Pasto and Brophy, however, show that alkyl substituted allenes, due to the geometry of the pi orbitals of the central sp-hybridized carbon, do undergo [1,3] hydride shifts. :


 * [[Image:1,3exception.png|center|675px|allene exception to [1,3] hydride impossiblity]]

Thermal Alkyl Shifts
Thermal alkyl [1,3] shifts, similar to [1,3] hydride shifts, must proceed antarafacially. Here the geometry of the transition state is prohibitive, but an alkyl group (due to the nature of its orbitals) can invert (form a new bond with the back lobe of its sp3 orbital picture) its geometry and therefore proceed via a suprafacial shift. These reactions are still not common in open chain systems because of the highly ordered nature of the transition state, which is more readily achieved in cyclic molecules.


 * [[Image:1,3alkyl.png|center|600px|[1,3] Alkyl Shifts]]

Photochemical [1,3] Shifts
Photochemical [1,3] shifts should show proceed through suprafacial shifts, however, most show non-concertedness because they proceed through a triplet state, i.e.: have a diradical mechanism, and therefore the Woodward-Hoffmann rules do not apply.

Hydride Shifts
A [1,5] shift involves the shift of 1 substituent (-H, -R or -Ar) down 5 atoms of a pi system. Hydrogen has been shown to shift in both cyclic and open chain systems at temperatures at or above 200˚C. These reactions are predicted to proceed suprafacially, via a Huckel-topology transition state.


 * [[Image:1,5hydridecyclic.png|center|600px|[1,5] Hydride shift in a cyclic system]]

Photoirradiation would require an antarafacial shift of hydrogen. Although rare, there are examples where antarafacial shifts are favored :


 * [[Image:1,5hantarafacial.png|center|800px|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. Several studies have been done and determined preferences for [1,5] shifts  :  carbonyl and carboxyl> hydride> phenyl and vinyl>> alkyl.

Alkyl groups undergo [1,5] shifts very poorly, usually requiring high temperatures, however, on cyclohexadienes, the temperature for alkyl shifts isn’t 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:


 * [[Image:1,5alkyl.png|center|600px|alkyl shift on cyclohexadiene]]

[1,7] Shifts
[1,7] sigmatropic shifts are predicted by the Woodward-Hoffmann rules to proceed in an antarafacial fashion, via a Mobius topology transition state. Antarafacial [1,7] shifts are observed in the conversion of lumisterol to vitamin D and in walk (link to below) reactions of bicyclic nonatrienes.


 * [[Image: Vitamin D -1,7- shift.png|center|800px|conversion of lumisterol to vitamin D]]


 * [[Image: 1,7 shift in nonatriene.png|center|400px|walk rearrangement of bicycle nonatriene]]

[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, Hückel topology transition state.

Claisen Rearrangement
Main article: 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 [3,3]-sigmatropic rearrangement to give a γ,δ-unsaturated carbonyl.



Discovered in 1912, the Claisen rearrangement is the first recorded example of a [3,3]-sigmatropic rearrangement.

An allowed [5,5] Claisen rearrangement has also been characterized.



Cope Rearrangment
Main article: 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-methyl-1,5-hexadiene heated to 300°C yields 1,5-heptadiene.



Carroll Rearrangement
Main article: 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 is accompanied by decarboxylation and the final product is a γ,δ-allylketone. The Carrol rearrangement is an adaptation of the Claisen rearrangement and effectively a decarboxylative Allylation.

Fischer Indole Synthesis
Main article: 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. Today antimigraine drugs of the triptan class are often synthesized by this method.



The choice of acid catalyst is very important. Bronsted acids such as HCl, H2SO4, polyphosphoric acid and p-toluenesulfonic acid have been used successfully. 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, Hückel 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 rare than molecules that can undergo [3,3] shifts.


 * [[Image:5,5shift.png|center|800px|[5,5] shift of phenyl pentadienyl ether]]

Walk Rearrangements
The migration of a divalent group, such as O, S, NR or CR2, which is part of a three-membered ring in a bicyclic molecule, is a commonly referred to as a walk rearrangement. This can be formally characterized according to the Woodward-Hofmann rules as being a (1, n) sigmatropic shift. 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.


 * [[Image:Walkrearrangement.png|center|700px|norcaradiene rearrangment]]

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, “there is no significant bonding in the transition state between C7 and C2 or C6 and that the 1,5-shifts of norcaradienes are diradical processes, but do not involve any diradical minima on the potential energy scale. .

Murray and Kaplan [1,5] and [1,7] Sigamatropic Hydrogen Shift Series
Insert descriptive text for R.W. Murray and M.L. Kaplan, J. Am. Chem. Soc. 88 3527 1966.


 * [[Image:Murray kaplan unlinked.png|center|600px|]]


 * [[Image:Murray kaplan linked.png|center|600px|]]

STORAGE
Sigmatropic rearrangements are classified by the substituent that moves and the order of the rearrangement, which is given in brackets [i,j] with i and j the number of atoms that each sigma terminus has moved. For example, in a [1,5]hydride shift, a proton moves over 5 carbon positions.

Photolysis of 7-dehydrocholesterolin 6-electron conrotatory electrocyclic reaction to generate pre-vitamin D3. Antarafacial hydride [1,7] Sigmatropic shift spontaneousl isomerization to form Vitamin D3 in a antarafacial hydride [1,7] sigmatropic shift.


 * [[Image:SigmatropicHydrideShifts.png|center|400px|Sigmatropic Hydride Shifts]]