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.

Methylenetriphenylphosphorane

Names
Preferred IUPAC name Methylidenetri(phenyl)phosphane
Identifiers
CAS Number	3487-44-3 Y
3D model (JSmol)	Interactive image
ChemSpider	121606
PubChem CID	137960
UNII	8GN8K95F62 Y
CompTox Dashboard (EPA)	DTXSID20188410
InChI InChI=1S/C19H17P/c1-20(17-11-5-2-6-12-17,18-13-7-3-8-14-18)19-15-9-4-10-16-19/h2-16H,1H2 Key: XYDYWTJEGDZLTH-UHFFFAOYSA-N InChI=1/C19H17P/c1-20(17-11-5-2-6-12-17,18-13-7-3-8-14-18)19-15-9-4-10-16-19/h2-16H,1H2 Key: XYDYWTJEGDZLTH-UHFFFAOYAU
SMILES C=P(C1=CC=CC=C1)(C2=CC=CC=C2)C3=CC=CC=C3
Properties
Chemical formula	C19H17P
Appearance	yellow solid
Density	1.19 g/cm3
Solubility in water	decompose
Solubility	THF
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Preparation and use

Methylenetriphenylphosphorane is prepared from methyltriphenylphosphonium bromide by its deprotonation using a strong base like butyllithium:^[1]

Ph₃PCH₃Br + BuLi → Ph₃PCH₂ + LiBr + BuH

The phosphorane is generally not isolated, instead it is used in situ. The estimated pK_a of this carbon acid is near 15.^[2] Potassium tert-butoxide has been used in place of butyl lithium.^[3] Sodium amide has also been used a base.^[4]

Methylenetriphenylphosphorane is used to replace oxygen centres in aldehydes and ketones with a methylene group, i.e., a methylenation:

R₂CO + Ph₃PCH₂ → R₂C=CH₂ + Ph₃PO

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 (1.823 Å).^[5] The compound is usually described as a combination of two resonance structures:

Ph₃P⁺CH₂⁻ ↔ Ph₃P=CH₂

Uses

Two examples of the Wittig reaction using methylenetriphenylphosphorane

Methylenetriphenylphosphorane has become a standard tool for synthetic organic chemists.^[6]

Related reagents

• (Chloromethylene)triphenylphosphorane
• Methoxymethylenetriphenylphosphine
• Carbomethoxymethylenetriphenylphosphorane