Coarctate reaction


In the classification of organic reactions by transition state topology, a coarctate reaction is a third, comparatively uncommon topology, after linear topology and pericyclic topology.

Transition state topologies

Reactions of linear topology are the most common, and consist of all transformations whose transition states are acyclic, including addition, elimination, substitution, and fragmentation reactions. In contrast, in pericyclic reactions, the atoms under chemical change form a closed cycle, and include reactions like the Diels-Alder reaction and Cope rearrangement, among many others.
In contrast to these types of reactions, a coarctate reaction is characterized by a doubly cyclic transition state, in which at least one atom undergoes the simultaneous making and breaking of two bonds. Thus, the topology of the transition state of a coarctate reaction is a constricted cycle that meets with itself while the topology of pericyclic and linear reactions are a circle and line segment, respectively. The concept was first proposed by Herges.

Examples

The most well-known example of a coarctate transition state is that of the epoxidation of an olefin by dimethyldioxirane. In this transition state, the oxygen atom transferred to the olefin forms a cycle with the acetone leaving group and a cycle with the olefin undergoing epoxidation. Another well-studied reaction is the fragmentation of spirocyclic ozonides into formaldehyde, CO2, and an olefin.
Selection rules, resembling the Woodward-Hoffmann rules, have been proposed to explain patterns in reaction activation energy related to transition state topology or orbital symmetry.