Sigmatropic Rearrangements:Types, Mechanisms, Stereochemistry

Sigmatropic Rearrangements

Sigmatropic is a fused word of Sigma meaning Single Carbon-Carbon bonds, and Tropos, a Greek word that means turn. So, we can say that sigmatropic means migration or movement of single bond. Sigmatropic rearrangements are unimolecular processes involve the movement of a σ bond with the simultaneous rearrangement of the π-system. In this rearrangement reaction, a σ bond is broken in the reactant, a new σ bond is formed in the product and the π bonds rearrange. However, the number of π bonds does not change; that is, the reactant and the product possess the same number of π bonds (see example [3, 3]shift). Sigmatropic reactions are usually uncatalysed, although Lewis acid catalysts are sometimes used. This rearrangement can occur through hydrogen shift, alkyl shift (Cope rearrangement) or Claisen rearrangement.

Numbering for Sigmatropic Rearrangement

In sigmatropic rearrangement, substarte can be divided into two parts: alkenyl (or polyakenyl) group and migrating group. All substrates have at least one allylic carbon in alkenyl chain for sigmatropic rearrangement.

Numbering of alkenyl chain is always started from the allylic carbon and this carbon is numbered 1. Atom (Carbon, Hydrogen or heteroatom) of migrating group bonded with allylic carbon by sigma bond is always given number 1.

Classification of Sigmatropic Rearrangement

Sigmatropic rearrangements are indicated by two members within a square bracket as [i, j] or [n, m]. The first number shows the original position of the migrating sigma bond (of atom or group) and the Second member is for a new position of migrating sigma bond (of an atom or group). In this rearrangement, the group in pink color migrates from the initial position 1 to new position 3. So, the sigmatropic rearrangement is [1, 3] Shift. In this rearrangement, the total number of σ and π bonds remains unchanged. Similarly, for [1,5], [1.7] shift

Sigmatropic rearrangements can be divided into two categories:

1. Migrating atom or group is bonded through the same atom in both reactant and product. In this example, migrating group is bonded to carbon in reactant as well as in product.

2. Migrating atom or group is bonded through different atoms in the reactant and in the product.
In [3,3] sigmatropic rearrangement, allyl group bonded to oxygen in reactant and carbon in the product. In [2,3] sigmatropic rearrangement, allyl group bonded to nitrogen in reactant and oxygen in the product.

Mechanism of Sigmatropic Rearrangements

Frontier Orbital Approach

Mechanism of sigmatropic rearrangement can be explained by Frontier molecular orbital (FMO) theory. Let's take an example of [1,3] hydrogen shift in propene.
H-CH₂-CH=CH₂ → CH₂=CH-CH₂-H
Let's consider the sigma bond connecting to the migrating group to its original position undergoes homolytic cleavage to two free radicals. This is not how the reaction takes place because sigmatropic rearrangement is concerted. It helps to understand the mechanism.

The product of hypothetical cleavage are a hydrogen radical and an allyl free radical which contains three p-orbitals. The π-molecular orbital of allyl free radical is shown below-

The hydrogen shift can take place in two directions but in actual, takes place in only one direction. In the first case, migrating group remains on the side of the π-orbital system. Such migration is called suprafacial process. In 1,3 thermal sigmatropic rearrangement, suprafacial migration is geometrically feasible but symmetry forbidden.

In the second case, migrating group must shift by antarafacial process (migrates to the opposite face). This migration is symmetry allowed but geometrically forbidden.

Geometrically forbidden is due to the smallest size of 1s orbital of hydrogen atom and the distance between two lobes of interacting p orbitals of carbon is maximum hence, 1s orbital cannot interact effectively with p-orbitals at the same time in the formation of transition state.

[1,3] sigmatropic shift takes place in the presence of UV light. In the presence of UV light, the lowest unoccupied molecular orbital (LUMO) of ground state becomes highest occupied molecular orbital (HOMO) of excited state.

[1,5] Sigmatropic Rearrangements

[1,5] sigmatropic shift is thermally allowed suprafacial with retention of configuration but photochemically forbidden. This can also be explain by FMO.

[1,7] Sigmatropic Rearrangements

[1,7] sigmatropic rearrangement involves the migration of a sigma (σ) bond from the first to the seventh position in a carbon chain across a π-system. [1,7] sigmatropic shift is photochemically allowed suprafacial shift mostly occurs in bicyclic compounds while thermal [1,7] sigmatropic shifts proceed in an antarafacial manner.

[3,3] Sigmatropic Rearrangements

Claisen and cope rearrangement is a very important example of [3,3] sigmatropic shift because of the ability to generate stereogenic centers from the sp² hybridized carbons.

Selection Rules and Stereochemistry of Slgmatroplc Rearrangement

Stereochemistry of the products under thermal or photochemical condition can be explained using Woodward-Hoffmann Rule:

Selection Rule for Sigmatropic Rearrangement
Shift	Order [i,j]	Suprafacial		Antarafacial
		Thermal	Photochemical	Thermal	Photochemical
H-Shift	4n [1,3] Shift	Not Allowed	Allowed	Allowed	Not Allowed
	4n+2 [1,5] Shift	Allowed	Not Allowed	Not Allowed	Allowed
Alkyl-Shift	4n [1,3] Shift	Allowed Inversion	Allowed Retension
	4n+2 [1,5] Shift	Allowed Retension	Allowed Inversion