Thermal Sigmatropic Reactions: Overview

A sigmatropic rearrangement is a pericyclic reaction in which a σ bond migrates from one part of a π system to another. These intramolecular rearrangements maintain the total number of σ and π bonds. Classification uses an order term [i,j], where i and j indicate the number of atoms across which each end of the σ bond migrates, such as [3,3] or [1,5] shifts.

The order notation [i,j] counts the atoms across which each end of the σ bond migrates. In 1,5-hexadiene, both ends migrate by three atoms, giving [3,3]. In 1,3-pentadiene, one end moves five atoms while the other remains fixed, yielding [1,5]. Each number represents the atomic distance traveled by one end of the migrating bond.

In suprafacial rearrangements, the migrating σ bond moves across the same face of the π system. In antarafacial rearrangements, it moves to the opposite face. Thermal sigmatropic shifts proceed suprafacially when the number of interacting electron pairs is odd, and antarafacially when it is even.

Thermal sigmatropic shifts proceed via the ground state HOMO of the π system, which has specific symmetry properties. When the sum of π and σ electron pairs is odd, suprafacial migration is symmetry-allowed. When even, antarafacial migration is symmetry-allowed. This follows Woodward-Hoffmann selection rules governing pericyclic reactions.

A [3,3] shift in 1,5-hexadiene involves two π bonds and one σ bond, totaling three electron pairs. Since three is odd, this rearrangement follows a suprafacial pathway. This is the basis for the 3,3 sigmatropic rearrangement of 1,5-dienes Cope rearrangement, a common thermal sigmatropic transformation.

A [1,7] hydride shift in 1,3,5-heptatriene involves three π bonds and one σ bond, totaling four electron pairs. Since four is even, this rearrangement follows an antarafacial pathway. The even electron pair count makes antarafacial migration the symmetry-allowed thermal process.

Sigmatropic rearrangements are intramolecular reactions where the σ bond migrates within the same molecule's π system. The total number of σ and π bonds remains unchanged during the rearrangement. This distinguishes them from intermolecular cycloaddition reactions involving separate reactants and different mechanisms.