Cycloadditions are a class of pericyclic reactions in which two unsaturated compounds combine to form a cyclic product.

They are designated by the notation [m + n], where "m" and "n"  indicate the number of π electrons in the interacting components.

For example, the reaction between a four π and a two π electron system is designated as a [4 + 2] cycloaddition, also known as the Diels–Alder reaction.

Similarly, the interaction between two systems, each with two π electrons, is denoted as a [2 + 2] cycloaddition.

These reactions can occur under thermal or photochemical conditions.

If the total number of π electrons is a multiple of 4n, where n is an integer, the cycloaddition is photochemically allowed and thermally forbidden.

However, if the total number of π electrons is 4n + 2, the reaction is thermally allowed and photochemically forbidden.

So, a [4 + 2] cycloaddition is thermally favorable, whereas a [2 + 2] cycloaddition is photochemically favorable.