Cykladditioner

Cykladditioner, or cycloadditions, are a class of chemical reactions in which two or more unsaturated molecules combine to form a single cyclic product, with the new sigma bonds formed by joining a set of atoms from the reacting partners.

Most cycloadditions proceed via a concerted, pericyclic mechanism in which electrons move through a cyclic transition state. The reaction is often stereospecific and governed by orbital symmetry (Woodward–Hoffmann rules); thermal cycloadditions follow rules depending on the number of pi electrons involved.

Common types include [4+2] cycloaddition (Diels–Alder reaction) forming six-membered rings; [2+2] cycloaddition forming four-membered rings, typically photochemically allowed; and [3+2] cycloadditions such as 1,3-dipolar cycloadditions that give five-membered rings. The 1,3-dipolar cycloaddition is exemplified by Huisgen cycloaddition, which yields triazoles.

Applications of cycloadditions span the construction of complex ring systems in natural product synthesis, medicinal chemistry, polymer science, and materials science. The Diels–Alder reaction is widely used for stereocontrolled synthesis of cyclohexenes, while 1,3-dipolar cycloadditions underpin many click-chemistry approaches.

Historical note: cycloadditions were first described by Otto Diels and Kurt Alder in 1928, and the broader pericyclic framework was developed later by Woodward and Hoffmann. Variants include intramolecular cycloadditions and catalyzed versions that improve rate and selectivity.

Terminology: in Swedish, cykladditioner denotes the plural of cycloaddition and covers a range of reactions sharing the cyclic-forming characteristic.