Methylenation

In organic chemistry, methylenation is a chemical reaction that inserts a methylene (−CH₂−) group into a chemical compound:

${\displaystyle {\ce {A-B}}\longrightarrow {\ce {A}}{\color {red}{\ce {-CH2 -}}}{\ce {B}}}$

In a related sense, it also describes a process in which a divalent group of a starting material is removed and replaced with a terminal CH₂ group:

${\displaystyle {\ce {A=E}}\longrightarrow {\ce {A}}{\color {red}{\ce {=CH2}}}}$

Methylenation in this context is also known as methenylation. Most commonly, E is an oxygen atom, so that the reaction results in terminal alkenes from aldehydes and ketones, or more rarely, enol ethers from esters or enamines from amides.