Heterocyclic diene employed in cyclo addition reactions is Furan.

Explanation:
Cycloaddition reactions involve the formation of a cyclic compound from two or more reactants. In this reaction, a diene and a dienophile undergo a cycloaddition reaction to form a cyclic compound. The diene used in this reaction is usually a heterocyclic diene, which contains at least one heteroatom (non-carbon atom) in the ring structure.

Furan is a heterocyclic diene that contains an oxygen atom in its five-membered ring structure. Furan is an excellent diene for cycloaddition reactions because it readily undergoes a Diels-Alder reaction with a dienophile to form a cyclic compound. Furan is also a useful diene because it is readily available and can be easily functionalized to give a wide range of derivatives.

Pyrrole, 2,5-dimethylpyrrole, and thiophene are also heterocyclic dienes that are used in cycloaddition reactions, but they are less commonly used than furan. Pyrrole and 2,5-dimethylpyrrole are less reactive than furan because they contain a nitrogen atom in the ring structure, which decreases the electron density of the diene. Thiophene is less reactive than furan because it contains a sulfur atom in the ring structure, which decreases the electron density of the diene and makes it more prone to oxidation.

In summary, furan is the preferred heterocyclic diene for cycloaddition reactions because it is readily available, easily functionalized, and highly reactive.