The document defines and provides examples of various sigmatropic reactions, including: 1. The Claisen rearrangement, which involves the [3,3] rearrangement of an allyl vinyl ether. 2. The Cope rearrangement, which involves the [3,3] sigmatropic rearrangement of 1,5-dienes. 3. The Oxy-Cope rearrangement, which has a hydroxyl substituent and proceeds faster when deprotonated. 4. Other reactions discussed include the Fischer indole synthesis, aromatic Claisen rearrangement, [2,3]-Wittig rearrangement, Carroll rearrangement, and walk rearrangements. Mechanisms

1. Assignment On ‘Sigmatropic Reaction’
Submitted By Submitted To
Md. Azamu Shahiullah Prottoy Rezwana Nasrin Chowdhury
Syed Hasan Mahmud Shoaib Lecturer, BRAC university.
Nusrat Akbar
Najib Hasnat
Shafiul Mujnabin

2. Index
1. Definition Of Sigmatropic Reaction
2. Claisen rearrangement
3. Cope –Rearrangement
4. (Anionic) Oxy-Cope Rearrangement
5. [1,5] Hydrogen shift
6. Fischer indole synthesis
7. Aromatic Claisen Rearrangement
8. [2,3]-Wittig Rearrangement
9. Carrol Rearrangement
10. Walk rearrangements

3. 1. Definition
A sigmatropic reaction in organic chemistry is a pericyclic reaction (a pericyclic reaction is
a type of organic reaction wherein the transition state of the molecule has a cyclic geometry,
and the reaction progresses in a concerted fashion. Pericyclic reactions are usually
rearrangement reactions.) 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 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 rearrangement, Claisen
rearrangement, Carroll rearrangement and the Fischer indole synthesis.
2. Claisen rearrangement:
Mechanism:
The Claisen rearrangement is an exothermic (about 84 kJ mol−1
),
concerted pericyclic reaction which according to theWoodward–Hoffmann rules shows a

4. suprafacial reaction pathway. Crossover experiments eliminate the possibility of the
rearrangement occurring via an intermolecular reaction mechanism and are consistent with
an intramolecular process, now understood as a [3,3]-electrocyclic reaction.
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.
Common Example:
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 irreversible.
3. Cope –Rearrangement:

5. 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.
The Cope rearrangement causes the fluxional states of the molecules in
the bullvalene family.
Mechanism:
Although the Cope rearrangement is concerted and pericyclic, it can also be considered to go
via a transition state that is energetically and structurally equivalent to a diradical. This is an
alternative explanation which remains faithful to the uncharged nature of the Cope transition
state, while preserving the principles of orbital symmetry. This also explains the high energy
requirement to perform a Cope rearrangement. Although illustrated in the chair
conformation, the Cope can also occur with cyclohexadienes in the "boat" conformation.

6. The above description of the transition state is not quite correct. It is currently generally
accepted that the Cope rearrangement follows an allowed concerted route through a
homoaromatic transition state and not a diradical. That is unless the potential energy surface
is perturbed to favor the diradical.
.
Examples:
The rearrangement is widely used in organic synthesis. It is symmetry-allowed when it
is suprafacial on all components. The transition state of the molecule passes through a boat
or chair like transition state. An example of the Cope rearrangement is the expansion of
a cyclobutane ring to a 1,5-cyclooctadiene ring:
In this case, the reaction must pass through the boat transition state to produce the
two cis doubl bonds. A trans double bond in the ring would be too strained. The reaction
occurs under thermal conditions. The driving force of the reaction is the loss of strain from
the cyclobutane ring.
4. (Anionic) Oxy-Cope Rearrangement :

7. Mechanism:
The Cope Rearrangement is the thermal isomerization of a 1,5-diene leading to a
regioisomeric 1,5-diene. The main product is the thermodynamically more stable
regioisomer. The Oxy-Cope has a hydroxyl substituent on an sp3
-hybridized carbon of the
starting isomer.
The driving force for the neutral or anionic Oxy-Cope Rearrangement is that the product is
an enol or enolate (resp.), which can tautomerize to the corresponding carbonyl compound.
This product will not equilibrate back to the other regioisomer.
The Oxy-Cope Rearrangement proceeds at a much faster rate when the starting alcohol is
deprotonated, e.g. with KH. The reaction is then up to 1017
times faster, and may be
conducted at room temperature. Aqueous work up then gives the carbonyl compound.

8. 5. [1,5] Hydrogen shift:
Mechanism:
The most common category of hydrogen shift involves a so called [1,5] hydrogen
sigmatropic shift (n=1 in the above diagram). A practical example of this reaction involves
the preparation of Chiral ethanoic (acetic) acid (CHDTCO2H, where D= 2
H and T=3
H) which
has long been an invaluable tool for elucidating biochemical mechanisms, but whose
synthesis has been long, difficult and in low yield. Recently a new and particularly efficient
route to this molecule has been developed ased on the reaction shown below, which
involves 6 electrons, and hence falls into the 4n+2 thermal onverted to chiral ethanoic acid
(along with the three other products shown) by the sequence of three reagents shown. With
R=t
Bu only a single product is formed, whereas with smaller R groups two compounds are

9. formed.
6. Fischer indole synthesis
Mechanism:
The Fischer indole synthesis is a chemical reaction that produces
the aromatic heterocyclic indole from phenylhydrazine,aldehyde or ketone under
conditions. The reaction was discovered in 1883 by Hermann Emil Fischer.

10. 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.
7. Aromatic Claisen rearrangement
Mechanism:
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.

11. 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.
8. [2,3]-Wittig Rearrangement
The [2,3]-Wittig Rearrangement allows the synthesis of homoallylic alcohols by the base-
induced rearrangement of allyl ethers at low temperatures.

12. Mechanism of the [2,3]-Wittig Rearrangement
The [2,3]-Wittig Rearrangement is a [2,3]-sigmatropic reaction, a thermal isomerization that
proceeds through a six-electron, five-membered cyclic transition state. A general scheme for
[2,3]-sigmatropic reactions is given here:
[2,3]-Sigmatropic reactions encompass a vast number of synthetically useful variants in
terms of both the atom pair involved (X, Y) and the electronic state (Y: anions, non-bonding
electron pairs, ylides).
The transformation of deprotonated allyl ethers into homoallylic alcohols is the
[2,3]-sigmatropic version of the [1,2]-Wittig Rearrangement, and is therefore termed [2,3]-
Wittig Rearrangement:

13. These [2,3]-rearrangements feature regioselective carbon-carbon bond formation with allylic
transposition of the oxygen, generation of specific olefin geometries and transfer of chirality.
9. Walk rearrangements
The migration of a divalent group, such as O, S, N R or C R2, which is part of a three-
membered ring in a bicyclic molecule, is 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.
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 tha

14. t the 1,5-shifts of norcaradienes proceed antarafaciallyTheoretical calculations found the
[1,5] shift to be a diradical process, but without involving any diradical minima on the
potential energy surface
10. 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. The Carroll rearrangement is an adaptation of
the Claisen rearrangement and effectively a decarboxylative Allylation.
Mechanism:
The Carroll rearrangementin the presence of base and with high reaction temperature takes
place through an intermediate enol which then rearranges in an electrocyclic Claisen
rearrangement. The follow-up is a decarboxylation. With palladium as a catalyst, the
reaction is much milder (path B) with an intermediate allyl cation /carboxylic acid anion
organometallic complex.

16. 11. Reference:
• http://www.name-reaction.com/cope-rearrangement
• http://organicreactions.org/index.php/2,3-Wittig_rearrangement
• http://www.organic-chemistry.org/namedreactions/cope-
rearrangement.shtm
• http://pubs.acs.org/doi/abs/10.1021/ja00793a023
• link.springer.com/chapter/10.1007%2F3-540-30031-7_53#page-1
• http://www.cedricbrule.com/page.asp?rec=118&dossier=10
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rearrangement.shtm
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v
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Q&sig2=C-mJGvOGlai8BQ9pwC7d3w&bvm=bv.64764171,d.bmk