Chemistry:Methylenation

Short description: Chemical reaction which adds a –CH2– group to a molecule

In organic chemistry, methylenation is a chemical reaction that inserts a methylene (–CH
2–) group into a chemical compound:

[math]\displaystyle{ \ce{A-B} + {\color{red}\ce{CH2}} \longrightarrow \ce{A}{\color{red}\ce{-CH2 -}}\ce{B} }[/math]

Typically, the reaction is used to prepare terminal alkenes from aldehydes and, less frequently, ketones.

Methods

Methylene-for-oxo reactions

A common method for methylenation involves the Wittig reaction using methylenetriphenylphosphorane with an aldehyde (Ph = phenyl, C
6H
5):

[math]\ce{ RCHO + Ph3P=CH2 -> RCH=CH2 + Ph3PO }[/math]

A related reaction can be accomplished with Tebbe's reagent, which is sufficiently versatile to allow methylenation of esters:

[math]\ce{ RCO2R' + Cp2Ti(Cl)CH2AlMe2 -> RC(OR')=CH2 + Cp2TiOAlMe2Cl }[/math]

Other less well-defined titanium reagents, e.g., Lombardo's reagent, effect similar transformations.

Carbanions derived from methylsulfones have also been employed, equivalently to the Wittig reaction.

Other approaches

Ethenolysis is a method for methylenation of internal alkene as illustrated by the following example:

[math]\displaystyle{ \ce{RCH=CHR} + {\color{red}\ce{CH2=CH2}} \longrightarrow \ce{2 RCH=}{\color{red}\ce{CH2}} }[/math]

In principle, the addition of CH
2 across a C=C double bond could be classified as a methylenation, but such transformations are commonly described as cyclopropanations.