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