A form of cycloaddition across the terminal atoms of a fully conjugated system with formation of two new σ-bonds to a single atom of the ('monocentric') reagent. There is formal loss of one π-bond in the substrate and an increase in coordination number of the relevant atom of the reagent. An example is the addition of sulfur dioxide to butadiene:
The reverse of this type of reaction is designated 'cheletropic elimination'.
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1094 (https://doi.org/10.1351/pac199466051077)