Question

Bicyclo-2,5-heptadiene can be prepared in two steps from cyclopentadiene and vinyl chloride. Provide a mechanism for each step.

Short answer

Question: Describe the mechanism of how bicyclo[2.2.1]heptadiene can be prepared from cyclopentadiene and vinyl chloride in two steps.

Step-by-step solution

Formation of the Diene and Dienophile

Cyclopentadiene is the diene, and vinyl chloride is the dienophile. The diene needs to be in the s-cis conformation to be reactive in the Diels-Alder reaction. To convert cyclopentadiene to its s-cis conformation, the double bond allows the rotation of the ring to obtain the required conformation. Vinyl chloride remains unchanged as the dienophile.

Diels-Alder Reaction Mechanism

In this step, the diene and dienophile will react with each other through a concerted process, meaning all bond-making and bond-breaking events will happen at the same time. The carbonyl atom of the vinyl chloride forms a bond with one of the carbon atoms of the diene, and the other carbon atom of the diene forms a bond with the alkyl group of vinyl chloride. Additionally, one double bond in the diene will be converted to a single bond. Step 2: Aromatization

Formation of the Aromatic Compound

The product formed in the previous step is not yet the desired bicyclo[2.2.1]heptadiene. We need to aromatize the compound to form the desired product. This can be achieved by a heat-induced elimination reaction, where the chlorine atom and a hydrogen atom from the adjacent carbon will be removed.

Aromatization Mechanism

The elimination reaction involves the breaking of the C-Cl bond and the C-H bond on the adjacent carbon. The electrons from both bonds will form a new double bond, creating an aromatic system in the bicyclic compound. The elimination product will be hydrogen chloride (HCl). The overall two-step mechanism can be summarized as follows: 1. Diels-Alder reaction between cyclopentadiene (in s-cis conformation) and vinyl chloride. 2. Aromatization through heat-induced elimination of HCl.