A sigmatropic rearrangement is a type of pericyclic reaction, characterized by the migration of a sigma bond from one atom to another in a conjugated system which leads to new sigma-bond formation and intramolecular molecular rearrangement. This is a concerted process, where the steps involved in the breaking of bonds as well as in the formation of bonds happen simultaneously. This category of sigmatropic rearrangements is, in turn, divided by the number of atoms in migration as well as movement nature that occurs.
There are many types of Sigmatropic Rearrangement For instance [1,2]-shift, [2,3]-shift, and [3,3]-shift. In a [1,2]-sigmatropic-shift, one hydrogen atom shifts between two adjacent carbon atoms. In a [2,3]-shift, two atoms or groups shift over three carbon atoms. In a [3,3]-shift, three atoms or groups shift place. These reactions are in accordance with the Woodward-Hoffmann rules which rationalize the conditions of symmetry and orbitals necessary for the rearrangement to take place.
Now let's take a step-by-step look at a more common example of a sigmatropic rearrangement: the Cope rearrangement. In the Cope rearrangement, a 1,5-hexadiene rearranges to give a new product through the migration of a sigma bond.
1. Starting Material: The starting material of Cope rearrangement is a 1,5-hexadiene molecule with a conjugated system of double bonds. The conjugation in the molecule allows the migration of sigma bonds to easily take place.
2. Formation of Transition State Upon the initiation of reaction, this 1,5-hexadiene molecule forms a concerted process where pi electrons in the central bond shift to produce a new transition state. This transition state is highly energetic and very transitory in which there can be smooth migration of sigma bonds.
3. Sigma Bond Migration: In the transition state, there is a migration of sigma bond between two neighboring carbon atoms, forming a new sigma bond between two other carbon atoms. This migration of the sigma bond is done in a cyclic fashion by moving the electrons in concert to encourage the rearrangement of the molecular structure.
New product After the migration of the sigma bond, new product is formed with the changed structure. For instance of the Cope rearrangement, in that case, the molecule of 1,5-hexadiene changes into a new diene molecule where the arrangement of the double bonds has also changed.
4. Product Stabilization: The new bonding interactions within the molecule stabilize the product of the sigmatropic rearrangement. New bonding interactions may also result in stabilizing conjugation effects, resonance structures, or other such effects depending upon the type of rearrangement reaction.
However, one should notice that sigmatropic rearrangements are processes that occur via a concerted mechanism; that is to say, the sigma bonds migration involves one step without the generation of any intermediate species. It is just this concerted nature of the rearrangement that would ensure the overall stability of the reaction and that the correct rearrangement of the bond in the molecule takes place.
In summary, sigmatropic rearrangements such as the Cope rearrangement involve the migration of sigma bonds within a conjugated system, leading to the formation of new products with rearranged molecular structures. These rearrangements proceed via a concerted mechanism where all bond movements occur simultaneously, resulting in efficient and predictable transformations of organic molecules.
Since chemists know step by step the process of sigmatropic rearrangements, for example, the Cope rearrangement, they can predict and control the outcome that such a reaction could take in order to synthesize organic complex molecules, mainly in specific structural arrangements. For more details about sigmatropic Rearrangement and it's shifts Click Here
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