methylenationtype

Methylenation type refers to a class of chemical transformations that introduce a methylene group (CH2) into a molecule. The term typically describes methods that convert carbonyl-containing substrates into products featuring a methylene unit, often by converting C=O into C=CH2 or by inserting a methylene as a bridge or substituent.

Two broad families dominate methylenation chemistry: carbonyl methylenation and methylenation by methylene transfer reagents. Carbonyl methylenation converts aldehydes and ketones into alkenes bearing a terminal CH2 group. Classic Wittig-type methylenation uses methylenetriphenylphosphorane to generate R2C=CH2 products after oxidative workup; variants such as Horner–Wadsworth–Emmons provide mainly E-alkenes. Alternative methylenation methods employ metal-organic carbenoids to transfer a CH2 unit directly to carbonyls, including titanium methylidene (Tebbe reagent) and related systems.

Applications of methylenation include the synthesis of alkenes from carbonyl precursors, installation of exocyclic methylene groups in natural products and pharmaceuticals, and broader use as a planning tool in organic synthesis. Methylenation is valued for its ability to install a CH2 unit with relatively straightforward operation, broad substrate scope, and compatibility with diverse functional groups when conditions are carefully chosen.

Limitations and considerations include functional group tolerance, stereochemical outcomes in olefination, and selectivity in complex molecules. Some methylenation conditions can be sensitive to air or moisture, and different reagent families require different protective-group strategies and workup protocols.

See also: Wittig reaction, Tebbe reagent, methylenation reagents.