What is a suprafacial cycloaddition reaction

Cycloaddition

The Cycloaddition is a chemical reaction in organic chemistry. Generally two (or more) unsaturated systems react with one another by ring closure.[1] The cycloadditions are usually referred to with an abbreviation in front of the name, which indicates how many $ \ pi $ electrons are involved in the reaction. [4 + 2] cycloaddition means that 4 $ \ pi $ -electrons of one molecule and 2 $ \ pi $ -electrons of another molecule are involved. The atom economy of the cycloadditions is consistently excellent.

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. In addition, those still exist Huisgen 1,3-dipolar cycloadditionuseful for making heterocycles. The [2 + 2] cycloaddition is an example of a photochemical reaction. In the case of a Paterno-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 $ \ pi $ electrons. All suprafacial-suprafacial cycloadditions with 4n $ \ pi $ electrons (ie [2 + 2], [4 + 4]) always run photochemically, all with 4n + 2 $ \ pi $ electrons [4 + 2] always run thermally . In contrast, suprafacial-antarafacial cycloadditions with 4n $ \ pi $ electrons (ie [2 + 2], [4 + 4]) run thermally and those with 4n + 2 $ \ pi $ electrons [4 + 2] always run photochemical. 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:

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

The naming of a cycloaddition reaction can be done in two different ways according to the IUPAC recommendations.[1]

(i + j +…) Cycloaddition

In this notation, i and j and ... atoms of the reacting species connected to one another to form a ring system with a total of i + j +… Atoms linked. 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, the reacting species are in each case m and n and ... electrons are involved in the formation of the ring system. Both the above Diels-Alder reaction and the first step of ozonolysis are thus a [4 + 2] cycloaddition.

Cheletropic reaction

At cheletropic reactions it is a special case of cycloaddition. With them, the newly made bonds come from the same atom. The addition of carbenes to double bonds is an example. 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. 1,01,1 Entry: cycloaddition. In: IUPAC Compendium of Chemical Terminology (the “Gold Book”). doi: 10.1351 / goldbook.C01496 (Version: 2.1.5).