Sigmatropic Reactions

Introduction

In a sigmatropic rearrangement, a σ-bond is broken in the reactant, a new σ- bond is formed in the product, and the bonds π- rearrange.

The number of π- bonds does not change (the reactant and the product have the same number of π- bonds).

The σ- bond that is broken can be in the middle π- of the system or at the end of the π- system.

The system π- consists of the double-bonded carbons and the carbons immediately adjacent to them.

There are some other type of pericyclic reactions that are known as sigmatropic reactions.

1) Ene Eeaction (1,5 Sigmatropic):

[1,5] Sigmatropic migrations of hydrogen are well known.

They involve three pairs of electrons, so they take place by a suprafacial pathway under thermal conditions

The reactivity of the starting material increases by addition of the lewis acid then the reaction of an aldehyde and an alkene in the presence of a Lewis acid provides a convenient route to some homoallylic alcohols.

Thus, using dimethylaluminium chloride as the Lewis acid, limonene reacts selectively with acetaldehyde to give the following alcohol.

The thermal or Lewis acid-catalysed imino-ene reaction gives rise to homoallylic amines.

For example, the ene reaction of N-sulfonyl imines, such as, with alkenes gives adducts that are readily converted into γ,δ-unsaturated α-amino-acids.

1.2) Assymetric Ene Reaction:

By using some chiral reagent can perform asymmetric reaction to give exclusive singe isomer .

The intermolecular metallo-ene reaction is often unselective and product yields are commonly poor, except with certain (e.g. metal-substituted) alkene or alkyne enophiles.

Intramolecular metallo-ene reactions have received more interest for the synthesis of natural products.

2) Cope Rearrangement & Claisen Rearrangement (3,3 Sigmatropic):

A Cope rearrangement is a [3,3] sigmatropic rearrangement of a 1,5-diene.

A Claisen rearrangement is a [3,3] sigmatropic rearrangement of an allyl vinyl ether.

Both rearrangements form six-membered-ring transition states. The reactions, therefore, must be able to take place by a suprafacial pathway.

2.1) Claisen Rearrangement:

The [3,3]-sigmatropic rearrangement of 1,5-hexadienes is a reversible process and the position of equilibrium depends on the substitution pattern and on the relative strain of the two 1,5-dienes.

The thermodynamic stability of an alkene increases with increasing substitution or with increasing conjugation and the equilibrium therefore normally lies in favour of the more-substituted, more conjugated product.

Heating the 1,5-diene gave the new 1,5-diene, in which two carbonyl groups come into conjugation with one of the new alkene π-bonds on rearrangement.

The synthesis of δ,ε-unsaturated carbonyl compounds, 1,6-dicarbonyl compounds and for substrates in which the equilibrium would otherwise lie on the side of the starting 1,5-diene.

For example, the 1,5-diene rearranges on heating to give the ten-membered ring, whereas the equilibrium for the Cope rearrangement lies in favour of the six-membered ring.

The E,E- and E,Z-isomers of the starting 1,5-diene In addition, as a consequence of the ordered transition state, chirality transfer across the allylic system is possible.

2.2) With Aromatic Substituents :

2.3) Indole Synthesis:

The condensation of the ketone and phenylhydrazine in the presence of polyphosphoric acid to give the indole.

3) 1,3 Sigmatropic :

1,3-Hydrogen shifts can take place if the reaction is carried out under photochemical conditions because the HOMO is symmetric under photochemical conditions, which means that hydrogen can migrate by a suprafacial pathway.

Via Mechanism:

4) 1,7- Sigmatropic:

[1,7] Sigmatropic hydrogen migrations involve four pairs of electrons.

They can take place under thermal conditions because the eight-membered-ring transition state allows the required antarafacial rearrangement.

5) 2,3 - Sigmatropic :

[2,3] sigmatropic rearrangement, two atoms connect the old and new σ-bonds in one fragment and three atoms (C, C, C) connect the old and new σ- bonds in the other fragment.

The [2,3]-sigmatropic rearrangement is the allylic variant of the [1,2]-sigmatropic rearrangement of sulfonium or ammonium ylides or σ-metalated ethers.

The new σ-bond forms at the end of the allylic system by a concerted process, with simultaneous cleavage of the allylic–heteroatom bond.

The [2,3]-sigmatropic rearrangement is often highly stereoselective, with a marked preference for the formation of the E-alkene product.

5.1) Sulfoxide :

A useful [2,3]-Sigmatropic rearrangement in which a new carbon–heteroatom bond is formed is the sulfoxide–sulfenate rearrangement.

On warming, allyl sulfoxides (normally prepared by oxidation of allylic sulfides) are partly converted in a reversible reaction into rearranged allyl sulfenates.

The equilibrium is usually much in favour of the sulfoxide, but if the mixture is treated with a thiophile (such as trimethyl phosphite) then the oxygen–sulfur bond of the sulfenate is cleaved to give an allylic alcohol.

Even if the sulfenate is present in low equilibrium concentration, its removal by reaction with the thiophile results in conversion of the sulfoxide to the rearranged allylic alcohol in high yield.

The rearrangement step occurs through a five-membered cyclic transition state and is stereoselective, leading, in the acyclic series, to predominantly the E-allylic alcohol.

5.2) Reaction Mechanism :

Step 1:

Step 2:

Step 3:

Step 4: