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.