methylenation

Methylenation is a chemical transformation that introduces a methylene group (-CH2-) into a substrate. In organic synthesis it is commonly used to convert carbonyl compounds into alkenes bearing a one-carbon methylene unit, or to form methylene bridges that connect molecular fragments.

Several approaches are used. The Wittig methylenation employs methylenetriphenylphosphorane (Ph3P=CH2) to convert aldehydes and ketones into terminal alkenes, R-CH=CH2, after removal of the oxide byproduct. The reaction generally provides high chemoselectivity and can be tuned to influence alkene geometry under suitable conditions. Tebbe-type methylenation uses titanium-based carbenoid reagents to transform carbonyl groups into methylenes under relatively mild conditions, often accommodating sensitive substrates.

Other modern methods include related olefination strategies that install a methylene unit at carbonyl sites, yielding vinylidene or exocyclic methylene products. The scope typically covers aldehydes and ketones, with some methods extending to esters and other carbonyl derivatives, though functional-group compatibility varies by reagent and reaction conditions. Mechanistic differences among these approaches account for variations in selectivity, substrate tolerance, and operational simplicity.

Applications of methylenation appear in natural product synthesis, medicinal chemistry, and materials science, where the introduction of a methylene unit can alter stereochemistry, polarity, or reactivity and enable the formation of vinyl or exocyclic methylene motifs. The choice of method depends on substrate sensitivity, desired alkene geometry, and compatibility with other functional groups. See also Wittig reaction and Tebbe olefination for related carbonyl-to-alkene transformations.