Methylenetriphenylphosphorane
Identifiers
CAS number	3487-44-3
Properties
Molecular formula	C19H17P
Appearance	yellow solid
Density	1.19 g/cm3
Solubility in water	decompose
Solubility	THF
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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 triphenylphosphine and methyl bromide followed by deprotonation of the resulting phosphonium salt using a strong base like butyllithium:^[1]

Ph₃PCH₃Br + BuLi → Ph₃PCH₂ + LiBr + BuH

The compound is generally not isolated, instead it is used in situ.

Methylenetriphenylphosphorane is used to replace oxygen centres in aldehydes and ketones with a methylene group:

R₂CO + Ph₃PCH₂ → R₂C=CH₂ + Ph₃PO

The phosphorus-containing product is triphenylphosphine oxide.

Structure

Crystallographic characterization of the colourless ylide reveals that the phosphorus 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 (1.823 Å).^[2] The compound is usually described as a combination of two resonance structures:

Ph₃P⁺CH₂^- ↔ Ph₃P=CH₂

References

1. ^ Georg Wittig U. Schoellkopf (1973), "Methylenecyclohexane", Org. Synth.; Coll. Vol. 5: 751
2. ^ J. C. J. Bart "Structure of the non-stabilized phosphonium ylid methylenetriphenylphosphorane" Journal of the Chemical Society B: Physical Organic J. Chem. Soc. B, 1969, 350–365. doi:10.1039/J29690000350