Cycloaddition

Cycloaddition is a chemical reaction in organic chemistry. In general, two (or more) unsaturated systems react with each other by ring closure. Cycloadditions are usually designated by an abbreviation in front of the name, which indicates how many π electrons are involved in the reaction. Thus, [4+2]-cycloaddition means there are 4 π one molecule and 2 π -electrons of another molecule involved. The atomic economy of cycloadditions is always 100%, since no coupling products are formed due to the concerted reaction process.

Cycloaddition is a special case of pericyclic reactions. The best known and most common special case of cycloaddition is the [4+2]-cycloaddition, which also includes the Diels-Alder reaction. In addition, Huisgen 1,3-dipolar cycloaddition still exists, which is useful for the preparation of heterocycles. The [2+2]-cycloaddition is an example of a photochemical reaction. Here, in the case of a Paternò-Büchi reaction, an alkene and a ketone react to form an oxetane.

The course of these reactions can be both photochemical and thermal, this is generally dependent on the number of π -electrons. All suprafacial-suprafacial cycloadditions with 4n π (i.e. [2+2], [4+4]) always run photochemically, all with 4n+2 π -electrons [4+2] always run thermally. In contrast, suprafacial-antarafacial cycloadditions with 4n π (i.e., [2+2], [4+4]) run thermally and those with 4n+2 π -electrons [4+2] always run photochemically. This difference is due to the geometry of the reactants. Suprafacial means that both bonds are formed on the same side of the molecular orbital. Antarafacial means accordingly, the bond is formed on different sides of the molecular orbital. An example may illustrate this case:

In the illustrated case of the Diels-Alder thermal reaction, a suprafacial attack occurs; both bonds are formed on the same side of the molecule.

Notation

The naming of a cycloaddition reaction can be done in two different ways according to the IUPAC recommendations.

(i + j + ...)-cycloaddition

In this notation, i and j and ... contiguous atoms of the reacting species are linked to form a ring system with a total of i + j + ... atoms. Accordingly, the above Diels-Alder reaction is a (4+2)-cycloaddition. The formation of the primary product of ozonolysis is called (3+2)-cycloaddition.

[m + n + ...]-cycloaddition

In this preferred notation, m and n and ... electrons are involved in the formation of the ring system on the reacting species, respectively. Thus, both the above Diels-Alder reaction and the first step of ozonolysis are [4+2]-cycloadditions.

Cheletrope reaction

Cheletropic reactions are a special case of cycloaddition. In these reactions, the newly formed bonds start from the same atom. An example is the addition of a carbene to the double bond of an alkene. These reactions can also be reproduced using the frontier orbital method. Two σ-bonds starting from the same atom are opened or closed simultaneously.