Question

a. Propose a mechanism for the following reaction. (Hint:An electrocyclic reaction is followed by a Diels–Alder reaction.)

b. What would be the product if trans-2-butene were used instead of ethene?

Short answer

a. The mechanism of the reaction is shown below

b. The product obtained when trans-2-butene is used as the reactant is given below.

Step-by-step solution

Electrocyclic reaction

Electrocyclic reactions fall under the category of intramolecular reactions. In an electrocyclic reaction, a new sigma bond is created at the end of a conjugated pi system. This kind of reaction is a reversible reaction.

Proposing a mechanism for the reaction

a. In the reaction, the reactant reacts with an ethylene molecule under thermal conditions. The product is formed by an electrocyclic reaction that is followed by a Diels-Alder reaction. The mechanism of the reaction is shown below.

b. If trans-2-butene is used instead of ethane, two products are obtained. The products are (2R,3R)-2,3-dimethyl-1,2,3,4-tetrahydronaphthalene and (2S,3S)-2,3-dimethyl-1,2,3,4-tetrahydronaphthalene. The reaction is shown below.

The reaction when trans-2-butene is used as the reactant