A form of @C01496@ across the terminal atoms of a fully @C01267@ with formation of two new σ-bonds to a single atom of the ('monocentric') @R05190@. There is formal loss of one π-bond in the substrate and an increase in @C01331@ of the relevant atom of the @R05190@. An example is the @A00132@ of sulfur dioxide to butadiene:
The reverse of this type of reaction is designated 'cheletropic @E02038@'.
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1094 [Terms] [Paper]