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

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, C6H5):[1]

A related reaction can be accomplished with Tebbe's reagent, which is sufficiently versatile to allow methylenation of esters:[2]

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:

In principle, the addition of CH2 across a C=C double bond could be classified as a methylenation, but such transformations are commonly described as cyclopropanations.

References

  1. ↑ Eric J. Leopold (1986). "Selective Hydroboration of a 1,3,7-Triene: Homogeraniol". Organic Syntheses. 64: 164. doi:10.15227/orgsyn.064.0164.
  2. ↑ 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.
  3. ↑ Luciano Lombardo (1987). "Methylenation of Carbonyl Compounds: (+)-3-Methylene-cis-p-menthane". Organic Syntheses. 65: 81. doi:10.15227/orgsyn.065.0081.
  4. ↑ 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. PMC 1399453. PMID 16542018.
  5. ↑ 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.
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