cycloadditions

Cycloadditions are a class of pericyclic reactions in which two or more unsaturated molecules or molecular fragments combine to form a cyclic product in a single, concerted step, with several new sigma bonds created by the redistribution of pi electrons. The overall process is typically stereospecific and preserves carbon skeletons. The name of a cycloaddition is given by the number of atoms contributed by each partner, for example a [4+2] cycloaddition involves four atoms from a diene and two from a dienophile.

The best known example is the Diels–Alder reaction, a [4+2] cycloaddition that forms cyclohexene rings and is widely used in synthesis. Other common types include [2+2] cycloadditions, which form cyclobutanes; these often require photochemical activation or catalysts because thermal [2+2] reactions are symmetry-forbidden in many cases. [3+2] cycloadditions, such as 1,3-dipolar cycloadditions, join a 1,3-dipole to an alkene or alkyne to give five-membered heterocycles; the azide–alkyne cycloaddition (CuAAC) is a prominent example that yields triazoles.

Mechanistically, cycloadditions are governed by orbital symmetry rules (Woodward–Hoffmann). Most proceed in a single concerted step with suprafacial addition on both partners; product stereochemistry is often predictable, as in the endo selectivity observed in many Diels–Alder reactions.

Cycloadditions are versatile tools in organic synthesis and polymer chemistry, enabling rapid construction of ring systems and complex architectures.