Question

The following Diene does not undergo Diels–Alder reactions. Explain.

Short answer

Due to the steric hindrance of two methyl groups in s-cis conformation, diene doses do not undergo a Diels-Alder reaction.

Step-by-step solution

s-cis Diene 

Dienerotates along with carbon-2 and carbon-3 through a sigma bond, resulting in s-cis and s-trans Diene equilibrium.

The s-trans Diene is more stable; therefore, it cannot undergoes a Diel-Alder reaction, so its single bond is rotated to form the s-cis Diene, which undergoes Diels-Alder cycloaddition reaction.

Diene not undergoing Diels-Alder Reaction

The given Diene 1-methyl-2-(2-methylprop-1-enyl) cyclohex-1-ene is s-trans, but to undergo the Diels-Alder reaction, the s-trans has to rotate to the s-cis arrangement. When the s-trans Diene is rotated to s-cis conformation, the two methyl groups ( shown by a circle) experience steric by being too close to each other, thus preventing the Diene from adopting the s-cis conformation.

Therefore when the Diene 1-methyl-2-(2-methylprop-1-enyl) cyclohex-1-ene is reacted with a dienophile, no Diels-Alder reaction occurs.

Rotation of s-trans Diene to s-cis