Buta-1,3-diene is , a four-carbon diene with a conjugated system. Adjacent p orbitals allow electron delocalization across the two double bonds and the intervening single bond.
A suitable cisoid conformation can react with a dienophile in a Diels-Alder reaction. The need for the reactive conformation connects internal rotation with cycloaddition reactivity.
[ref]
Organic Chemistry by John McMurry. Chapter14, Conjugated Compounds: conjugated dienes and the Diels-Alder reaction
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