Question

Question: Use curved arrows to show how E is converted to F by a two-step reaction sequence consisting of a [1,5] sigmatropic rearrangement followed by a [4 + 2] cycloaddition.

Short answer

Answer

Step-by-step solution

[1,5] sigmatropic rearrangement

It is the shift of a group, mainly hydride, alkyl, or aryl, from ${\mathbf{C}}_{\mathbf{1}}$to ${\mathbf{C}}_{\mathbf{5}}$ carbon in a $\mathbf{\pi }$-system. This shift can take place in a cyclic as well as in an open-chain structure, hence making this rearrangement more useful.

[4 + 2] cycloaddition

It is a pericyclic reaction in which one of the reactants contributes 4 $\mathrm{\pi }$-electrons to the bonding. At the same time, another reactant contributes 2$\mathrm{\pi }$-electrons to the bonding giving a six-membered ring.

Mechanism of the given reaction

The mechanism of the given reaction is shown hereunder.