16.24 : 熱シグマトロピック反応: 概要

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.