Other classes of pericyclic reactions

Many diverse reactions of synthetic interest could be classified as pericyclic reactions. 

A reaction in which formation of two a-bonds occurs at a single atom (monocentric) is called a cheletropic reaction. The addition of SO2 to diene is a well-known example of cheletropic reaction. 

The reverse of this process is known as cheletropic extrusion (or cheletropic elimination). 

The cheletropic elimination of nitrogen from diazenes 8.1 and 8.2 is a stereospecific reaction (for definition of a stereospecific reaction, see section 1.5). 

A co-arctic reaction involves the formation or cleavage of four a-bonds at a single atomic centre. 

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Analogously to the example mentioned above, the other classes of pericyclic reactions with even numbers of chemical centers can be related to isomorphic topologies of edges. The structure does not change if we increase the number of atoms taking part during these reactions. The topologies of pericyclic reactions are equal up to isomorphism. 

So far, cycloadditions have been our only examples of pericyclic reactions. There are several other classes of pericyclic reactions, of which the most notable are cheletropic reactions, sigmatropic rearrangements and electrocyclic reactions. In essence, frontier orbital theory treats each of them as a cycloaddition reaction. 

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