Cycloaddition

A pericyclic reaction where the net result is two pi electrons being converted to two sigma electrons.

Examples of cycloaddition reactions are: the Diels-Alder reaction. alkyne-azide reaction.

Cycloadditions are usually described by the backbone size of the participants. This would make the Diels-Alder a (4 + 2) cycloaddition, and the alkyne-azide reaction a (3 + 2) cycloaddition.

Thermal cycloadditions usually have 4n + 2 pi electrons participating in the starting material. These occur in a suprafacial-suprafacial or antarafacial-antarfacial manner (rare). There are thermal cycloadditions whcih have 4n pi electrons; (for example the (2 + 2) cycloaddition), these proceed in a suprafacial-antarafacial sense.