Ylides

Ylides are a class of dipolar compounds in which a positively charged atom, typically phosphorus or sulfur, is adjacent to a negatively charged carbon atom. The molecule is overall neutral, but it can be represented by resonance forms showing charge separation. The most common ylides are phosphorane (phosphorus) ylides and sulfonium (sulfur) ylides. They are usually generated by deprotonation of phosphonium or sulfonium salts with strong bases, and their reactivity is influenced by stabilization of the adjacent carbanion by electron-withdrawing groups or other substituents.

Phosphorus ylides, often written as R3P+-CR'2–, are central to a wide range of carbon–carbon bond-forming reactions. Stabilized ylides, where the negative charge on carbon is delocalized onto nearby carbonyl or aryl groups, tend to deliver different selectivity than non-stabilized ylides. Sulfur ylides, produced from sulfonium salts, are used in other complementary transformations and can participate in reactions that form epoxides and related structures under basic conditions.

A key application is the Wittig reaction, in which a phosphorane ylide reacts with an aldehyde or ketone to give an alkene and a phosphine oxide byproduct. The process proceeds via a cycloaddition to form a betaine or oxaphosphetane intermediate, and the resulting alkene geometry can be influenced by the nature of the ylide and its substituents. Sulfur ylides are employed in the Corey-Chaykovsky reactions to convert carbonyl compounds into epoxides, expanding the range of accessible oxygen-containing products.

Ylide chemistry provides versatile routes for constructing alkenes, epoxides, and related motifs, making it a foundational area in organic synthesis.