Question

Question: What type of sigmatropic rearrangement is illustrated in each reaction?

Short answer

Answer

a. $\left[1,5\right]$sigmatropic shift

b.$\left[2,3\right]$sigmatropic shift

Step-by-step solution

Sigmatropic shift

• A sigmatropic rearrangement is an intramolecular pericyclic reaction.

• The reactant’s $\sigma$ bond is broken, and the$\mathrm{\pi }$bonds rearrange to form a new bond in the product.

• In a sigmatropic rearrangement, the number of $\mathrm{\pi }$ bonds is constant in the reactant and product

 [1,5] and [2,3] sigmatropic shifts

The $\left[1,5\right]$sigmatropic shift is associated with the rearrangement of the group at the first and fifth positions.

The $\left[2,3\right]$sigmatropic shift is associated with the rearrangement of the group at the second and third positions.

Explanation

a. In the given conversion, the groups at the first position rearrange themselves at the fifth position. Therefore, it is a $\left[1,5\right]$sigmatropic shift.

b. In the given conversion, the groups at the second position rearrange themselves at the third position. Therefore, it is a $\left[2,3\right]$ sigmatropic shift.