Diarylphosphoranes

Diarylphosphoranes are a class of phosphorus ylides in which the phosphorus atom bears two aryl substituents. In their prominent role, they function as Wittig reagents, enabling the transformation of carbonyl compounds into alkenes in a broadly used method for carbon–carbon bond formation.

Structurally, diarylphosphoranes are best described as zwitterionic species with a positively charged phosphorus and a carbanionic carbon (R2P+–CR'2−). In many representations, a P=C bond character is used to emphasize the ylide nature. The aryl groups contribute stabilization through conjugation, and substituents on the rings influence reactivity and selectivity.

Preparation typically involves the deprotonation of the corresponding diarylphosphonium salts (Ar2P+–CHR2) with strong bases such as n-butyllithium or similar reagents under strictly anhydrous conditions. The choice of base, solvent, and substituents on the aryl rings affect the stability and reactivity of the resulting diarylphosphorane.

Reactions of diarylphosphorane ylides with carbonyl compounds proceed via formation of a betaine/oxaphosphetane intermediate, which collapses to yield an alkene and the corresponding phosphine oxide. The stereochemical outcome is influenced by the stabilization of the ylide and the substituents on the carbonyl compound; stabilized and unstabilized diarylphosphoranes can give different selectivities, and mixtures are common in some systems. These reagents are widely used in organic synthesis to construct alkenes with defined substituents and in the development of stereoselective methodologies.

See also: Wittig reaction, phosphorus ylide, phosphorane, triphenylphosphine oxide.