Methylenetriphenylphosphorane is an organophosphorus compound with the formula Ph<sub>3</sub>PCH<sub>2</sub>. It is the parent member of the phosphorus ylides, popularly known as Wittig reagents. It is a highly polar, highly basic species.
Methylenetriphenylphosphorane is prepared from methyltriphenylphosphonium bromide by its deprotonation using a strong base like butyllithium:
The phosphorane is generally not isolated, instead it is used in situ. The estimated pK<sub>a</sub> of this carbon acid is near 15. Potassium tert-butoxide has been used in place of butyl lithium. Sodium amide has also been used a base.
Methylenetriphenylphosphorane is used to replace oxygen centres in aldehydes and ketones with a methylene group, i.e., a methylenation:
The phosphorus-containing product is triphenylphosphine oxide.
Crystallographic characterization of the colourless ylide reveals that the phosphorus atom is approximately tetrahedral. The PCH<sub>2</sub> centre is planar and the P=CH<sub>2</sub> distance is 1.661 ÃÂ , which is much shorter than the P-Ph distances (1.823 ÃÂ ). The compound is usually described as a combination of two resonance structures:
Methylenetriphenylphosphorane has become a standard tool for synthetic organic chemists.