Coarctate reaction

In organic chemistry, a coarctate reaction is a concerted reaction whose transition state involves two rings, 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 (resembling a figure eight) while the topology of pericyclic and linear reactions are a circle (or Möbius strip) and line segment, respectively.

[1][2] The most well-known example of a coarctate transition state is that of the epoxidation of an olefin by dimethyldioxirane.

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.