Density 1.19 g/cm³ | Appearance yellow solid | |
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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.
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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:
Ph3PCH3Br + BuLi → Ph3PCH2 + LiBr + BuHThe 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 + Ph3POThe 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). The compound is usually described as a combination of two resonance structures:
Ph3P+CH2− ↔ Ph3P=CH2