Question

The following rearrangement was devised and carried out to prove the stereochemistry of [1,5] sigmatropic hydrogen shifts. Explain how the observed result confirms the predictions of orbital symmetry.

Short answer

A [1,5] sigmatropic rearrangement involves three electron pairs (two π bonds and one σ bond), the orbital-symmetry rules predict a suprafacial reaction and the product formed is also giving confirmation of a suprafacial reaction.

Step-by-step solution

Orbital-symmetry rules

A [1,5] sigmatropic rearrangement involves three electron pairs (two π bonds and one σ bond), and the orbital-symmetry rules predict a suprafacial reaction. In fact, the [1,5] suprafacial shift of a hydrogen atom across two double bonds of a π system is one of the most commonly observed of all sigmatropic rearrangements.

According to the rules

Electron pairs (double bonds)	Thermal reaction	Photochemical reaction
Even number	Antarafacial	Suprafacial
Odd number	Suprafacial	Antarafacial

Mechanism

The mechanism for [1,5] sigmatropic rearrangement is shown as: