Wittig reagents are used in organic synthesis. They are derived from phosphonium salts. A strong base such as butyllithium or sodium amide is required for the deprotonation:
:[PhPCHR]X + CHLi → PhP=CHR + LiX + CH
One of the simplest ylides is methylenetriphenylphosphorane (PhP=CH).
The compounds PhPX (X = Cl, Br) are used in the Kirsanov reaction.
The Kinnear–Perren reaction is used to prepare alkylphosphonyl dichlorides (RP(O)Cl) and esters (RP(O)(OR′)). A key intermediate are alkyltrichlorophosphonium salts, obtained by the alkylation of phosphorus trichloride:
:RCl + PCl + AlCl → [RPCl]