Q37E - Organic Chemistry (Mcmurry)

Question

Propose a mechanism to account for the following transformation. What two kinds of reactions are occurring?

Step-by-Step Solution

Answer

The two kinds of reactions occurring in the formation of the given compound are Claisen rearrangement and Diels-Alder reaction.

Step-by-Step Solution Step 1: Claisen rearrangement

Claisen rearrangement is a carbon-carbon bond-forming chemical reaction shown by aliphatic ethers and aromatic ethers. The Claisen rearrangement is specific to allyl aryl ethers $({\mathrm{H}}_2\mathrm{C}={\mathrm{CHCH}}_2-\mathrm{O}-\mathrm{Ar})$ and allyl vinyl ethers $({\mathrm{H}}_2\mathrm{C}={\mathrm{CHCH}}_2-\mathrm{O}-\mathrm{CH}={\mathrm{CH}}_2)$ . Heating the allyl vinyl ether to 200 – 250 °C causes Claisen rearrangement, leading to a pent-4-enal. 

Claisen rearrangement

Step 2: Diels-Alder reaction

The Diels-Alder reaction is an important organic chemical reaction discovered by German chemists Otto Diels and Kurt Alder in 1928. 

It is a type of reaction in which the reactant includes a substituted alkene and conjugated diene. This substituted alkene is commonly referred to as dienophile.

The Diels-Alder reaction proceeds via a concerted mechanism in which bond breakage and bond formation occur in a single step.

            Diene     dienophile

Step 3: Proposed mechanism of the given transformation

A Claisen rearrangement is followed by a Diels-Alder reaction.

Tautomerization of the Claisen product to a phenol doesn’t occur because no hydrogen is available for donation to oxygen.

                                     Proposed mechanism of the given reaction