Cycloaddition

A reaction in which unsaturated molecules combine to form a ring by creating new bonds between their atoms. Cycloadditions are classified by the numbers of atoms contributed by each reactant.

Diels–Alder reaction: A thermal reaction of a conjugated diene with an alkene that forms a six-membered ring. This widely used six-atom cycloaddition illustrates how conjugation enables ring formation.
Alkene: A hydrocarbon containing a carbon–carbon double bond. Alkenes commonly supply a two-atom unit in cycloaddition reactions.
Click chemistry: A design approach favoring modular reactions that join molecular components efficiently and selectively. Cycloadditions, especially azide–alkyne reactions, became central click-chemistry tools.
Cycloreversion: A reaction that breaks a ring and forms multiple bonds by reversing a cycloaddition. It reverses the bond-making pattern that defines cycloaddition.
Huisgen cycloaddition: A 1,3-dipolar cycloaddition between an azide and an alkyne that forms a triazole. This named example shows how a three-atom dipole and a two-atom partner build a ring.
Conjugated diene: A molecule containing two double bonds separated by one single bond. Its four connected π electrons make it a common four-atom cycloaddition partner.
Strain-promoted azide–alkyne cycloaddition: An azide–alkyne cycloaddition accelerated by alkyne ring strain, requiring no copper catalyst. It enables bioorthogonal labeling under mild conditions.
Michael addition: Conjugate addition of a nucleophile to an activated alkene or alkyne. It consumes unsaturation through sequential bond formation rather than closing a ring between unsaturated partners.
Woodward–Hoffmann rules: Rules predicting whether pericyclic reactions are allowed from orbital symmetry and reaction conditions. They explain why thermal and photochemical cycloadditions can have different outcomes.
1,3-Dipole: A three-atom system with delocalized charge that can undergo 1,3-dipolar cycloaddition. It contributes three contiguous atoms to a five-membered ring.

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