Chemistry:Methylenation
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):[1]
- [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:[2]
- [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.[3][4]
Carbanions derived from methylsulfones have also been employed, equivalently to the Wittig reaction.[5]
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.
References
- ↑ Eric J. Leopold (1986). "Selective Hydroboration of a 1,3,7-Triene: Homogeraniol". Organic Syntheses 64: 164. doi:10.15227/orgsyn.064.0164.
- ↑ Straus, Daniel A.; Morshed, M. Monzur; Dudley, Matthew E.; Hossain, M. Mahmun (2006). "μ-Chlorobis(cyclopentadienyl)(dimethylaluminium)-μ-methylenetitanium". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rc073.pub2. ISBN 0471936235.
- ↑ Luciano Lombardo (1987). "Methylenation of Carbonyl Compounds: (+)-3-Methylene-cis-p-menthane". Organic Syntheses 65: 81. doi:10.15227/orgsyn.065.0081.
- ↑ Marsden, Stephen P; Ducept, Pascal C (2005). "Synthesis of highly substituted allenylsilanes by alkylidenation of silylketenes". Beilstein Journal of Organic Chemistry 1 (1): 5. doi:10.1186/1860-5397-1-5. PMID 16542018.
- ↑ Ando, Kaori; Oguchi, Mai; Kobayashi, Takahisa; Asano, Haruka; Uchida, Nariaki (2020). "Methylenation for Aldehydes and Ketones Using 1-Methylbenzimidazol-2-yl Methyl Sulfone". The Journal of Organic Chemistry 85 (15): 9936–9943. doi:10.1021/acs.joc.0c01227. PMID 32608238.
![]() | Original source: https://en.wikipedia.org/wiki/Methylenation.
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