Fosfoniylidyns

Fosfoniylidyns are a class of chemical compounds characterized by a phosphorus atom double-bonded to a carbon atom, where the carbon atom also bears a negative charge and is bonded to two other substituents. This structure can be represented as R2C=P, with the carbon atom having a formal negative charge and the phosphorus atom a formal positive charge, often referred to as a phosphorane ylide or simply a ylide. These compounds are highly reactive intermediates in organic synthesis, playing a crucial role in various named reactions.

The most well-known application of fosfoniylidyns is in the Wittig reaction, where they react with aldehydes or ketones to form alkenes. In this reaction, the nucleophilic carbon of the fosfoniylidyn attacks the electrophilic carbonyl carbon, leading to the formation of a betaine intermediate, which then eliminates triphenylphosphine oxide to yield the desired alkene. Fosfoniylidyns can be generated in situ by the deprotonation of phosphonium salts, typically with a strong base such as an organolithium reagent or an alkali metal hydride. The stability and reactivity of fosfoniylidyns can be modulated by the nature of the substituents on the carbon and phosphorus atoms. Electron-withdrawing groups on the carbon atom generally increase its acidity, facilitating deprotonation, while electron-donating groups can stabilize the ylide.