Cycloaddition

The cycloaddition is a chemical reaction in organic chemistry . Generally two (or more) unsaturated systems react with one another by ring closure. The cycloadditions are usually referred to with an abbreviation in front of the name, which indicates how many electrons are involved in the reaction. [4 + 2] cycloaddition means that 4 electrons from one molecule and 2 electrons from another molecule are involved. The atom economy of cycloadditions is always 100%, since the concerted course of the reaction does not result in any by-products. ${\displaystyle \pi }$ ${\displaystyle \pi }$ ${\displaystyle \pi }$

The cycloaddition is a special case of the pericyclic reactions . The best known and most frequent special case of the cycloaddition is the [4 + 2] cycloaddition, including the Diels-Alder reaction . There is also the Huisgen 1,3-dipolar cycloaddition , which is useful for the preparation of heterocycles . The [2 + 2] cycloaddition is an example of a photochemical reaction. In the case of a Paternò-Büchi reaction, an alkene and a ketone react with one another and form an oxetane .

These reactions can take place both photochemically and thermally, this generally depends on the number of electrons. All suprafacial-suprafacial cycloadditions with 4n electrons (ie [2 + 2], [4 + 4]) always run photochemically, all with 4n + 2 electrons [4 + 2] always run thermally. In contrast, suprafacial-antarafacial cycloadditions with 4n electrons (ie [2 + 2], [4 + 4]) run thermally and those with 4n + 2 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 accordingly means that the bond is formed on different sides of the molecular orbital. An example may illustrate this case: ${\displaystyle \pi }$ ${\displaystyle \pi }$ ${\displaystyle \pi }$ ${\displaystyle \pi }$ ${\displaystyle \pi }$

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

notation

A cycloaddition reaction can be named in two different ways , according to the IUPAC recommendations.

( i + j +…) cycloaddition

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

[ m + n +…] cycloaddition

In this preferred notation, m and n and ... electrons are involved in the formation of the ring system in the reacting species . Both the above Diels-Alder reaction and the first step of ozonolysis are thus a [4 + 2] cycloaddition.

Cheletropic reaction

In cheletropic reactions is a special case of cycloaddition. With them, the newly made bonds originate from the same atom. An example is the addition of a carbene to the double bond of an alkene . These reactions can also be traced using the frontier orbital method. Two σ-bonds starting from the same atom are opened or closed simultaneously.

Individual evidence

1. ↑ ^{a b} Entry on cycloaddition . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.C01496 Version: 2.1.5.
2. ↑ Ivan Ernest: Binding, Structure and Reaction Mechanisms in Organic Chemistry , Springer-Verlag, 1972, p. 349, ISBN 3-211-81060-9 .