Cheletropic reaction

A Cheletropic reaction is a pericyclic reaction where the net result is the conversion of a pi bond and a lone pair into a pair of sigma bonds; with both new sigma bonds adding into the same atom.

An example of such a reaction is the [1+4] reaction of sulfur dioxide with butadiene to form an adduct. The reverse of this process is often called Cheletropic extrusion or Cheletropic elimination, and is often driven by the entropic benefit of gaseous evolution.