Cycloaddition

The cycloaddition reaction is a chemical reaction of organic chemistry. It generally react two (or more) unsaturated systems by ring closure with each other. It refers to the cycloadditions usually with a suffix attached to the name indicates how many electrons are involved in the reaction. [ 4 2 ] cycloaddition is to say, there are four electrons of one molecule and two electrons of another molecule involved. The atom economy of the cycloaddition is consistently excellent.

The cycloaddition reaction is a special case of the pericyclic reactions. The best known and most common special case of the cycloaddition is the [4 2 ] cycloaddition, including falls, the Diels- Alder reaction. In addition, still exist the Huisgen 1,3 -dipolar cycloaddition, which is useful for the preparation of heterocycles. The [2 2 ] cycloaddition is an example of a photochemically ongoing reaction. These react in case of a Paterno - Büchi reaction, an alkene and a ketone with each other and form an oxetane.

The sequence of these reactions can be effected both thermally and photochemically, this is generally dependent on the number of electrons. All suprafacially - suprafacial cycloadditions with 4n electrons ( ie [2 2 ], [4 4 ] ) always run photochemically, all run with 4n 2 electrons [ 4 2 ] always thermally. In contrast, suprafacially - antarafacial cycloadditions run with 4n electrons ( ie [2 2 ], [4 4 ] ) thermally and 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 means accordingly, the bond is formed on different sides of the molecular orbital. An example may illustrate this case:

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

Notation

The designation of a cycloaddition reaction can be carried out according to IUPAC recommendations in two different ways.

( i j ... ) cycloaddition

[m n ... ] cycloaddition

Cheletropic reaction

In cheletropic reactions this is a special case of the cycloaddition. For them, the newly formed bonds of the same atom go out. One example is the addition of carbenes to double bonds. These reactions are understandable on the frontier orbital method. There are two σ - bonds originating from the same atom, simultaneously opened or closed.

Cycloaddition | Electrophilic Addition | Electrophilic Substitution | Electrophilic Aromatic Substitution | elimination | Nucleophilic Addition | Nucleophilic substitution | oxidation | Radical Addition | Radical Substitution | Reduction | rearrangement