Methylenetriphenylphosphorane

Methylenetriphenylphosphorane is an organophosphorus compound with the formula Ph₃PCH₂. It is the parent member of the phosphorus ylides, popularly known as Wittig reagents. It is a highly polar, highly basic species.

Preparation and use

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_a 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.

Structure

Crystallographic characterization of the colourless ylide reveals that the phosphorus atom is approximately tetrahedral. The PCH₂ centre is planar and the P=CH₂ distance is 1.661 Å, which is much shorter than the P-Ph distances. The compound is usually described as a combination of two resonance structures:

Uses

Methylenetriphenylphosphorane has become a standard tool for synthetic organic chemists.

Related reagents