16.15:
Cycloaddition Reactions: Overview
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.
16.15:
Cycloaddition Reactions: Overview
Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.
The feasibility of cycloaddition reactions under thermal and photochemical conditions can be predicted using a set of selection rules. If the total number of π electrons involved in the rearrangement is a multiple of 4n, the reaction is photochemically allowed. The reaction is thermally allowed if the total number of π electrons involved is 4n + 2.