Ring expansion and contraction

Loss of N2 from the diazo cations results in secondary carbocations, which tend to rearrange and then undergo hydrolysis.

[1] These expansions can be further broken down by what type of atom they incorporate (a carbon or a heteroatom) into the expanded ring.

The group to which the endocyclic bond migrates can also be selectively added to the ring based on the functionality already present, for example 1,2 addition into a cyclic ketone.

A common method for expanding a ring involves opening cyclopropane-containing bicyclic intermediate.

The Buchner ring expansion is encouraged to open to the desired product by placing electron withdrawing groups on the carbon added.

[11] While it proceeds through an intermediate bicycle the final cyclization and ring opening take place within the same radical reaction.

[13] These introductions are primarily ring expansions because they often take place through migration/insertion pathways similar to those mentioned above for carbon.

[18] It proceeds through a carbanion that attacks an endocyclic carbon and expels a leaving group (a halide) forming a bicyclic molecule with rings smaller than the original.

[17] It should also be noted that the so-called "quasi-Favorskii rearrangement" proceeds without an additional nucleophile to form the final contracted product.

The cationic rearrangement contraction proceeds through the loss of a leaving group and the migration of an endocyclic bond to the carbocation.