16.24:
Thermal Sigmatropic Reactions: Overview
Sigmatropic rearrangements involve the migration of a σ bond from one location to another within a π system.
The order of sigmatropic rearrangements is denoted by two numbers in brackets, indicating the number of atoms across which the σ bond migrates.
For example, in 1,5-hexadiene, both ends of the σ bond migrate by three atoms. So, it is called [3,3] sigmatropic rearrangement.
Next, consider 1,3-pentadiene. Here, only one end of the sigma bond moves by five atoms, and the other remains fixed. This is a [1,5] sigmatropic rearrangement.
Thermal sigmatropic shifts proceed via the ground state HOMO of the π system.
If the migrating σ bond moves across the same face of the π system, the rearrangement is suprafacial, whereas, if it moves to the opposite face, the rearrangement is antarafacial.
In summary, thermal sigmatropic rearrangements follow a suprafacial pathway if the number of interacting electron pairs is odd, and an antarafacial pathway if it is even.
16.24:
Thermal Sigmatropic Reactions: Overview
Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in 1,5-hexadiene, referred to as the Cope rearrangement, and a [1,7] hydride shift in 1,3,5-heptatriene.
Thermally induced sigmatropic shifts proceed suprafacially if the sum of π and σ electron pairs participating in the rearrangement is odd. For example, the [3,3] shift in 1,5-hexadiene involves two π bonds and one σ sigma bond, or three electron pairs, and therefore follows a suprafacial pathway.
In contrast, the antarafacial pathway is symmetry-allowed if the sum is even, like in 1,3,5-heptatriene, which involves three π bonds and one σ sigma bond, or a total of four electron pairs.