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:1
Ph3PCH3Br + BuLi → Ph3PCH2 + LiBr + BuHThe phosphorane is generally not isolated, instead it is used in situ. The estimated pKa 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 + 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).5 The compound is usually described as a combination of two resonance structures:
Ph3P+CH2− ↔ Ph3P=CH2Uses
Methylenetriphenylphosphorane has become a standard tool for synthetic organic chemists.6
Related reagents
- (Chloromethylene)triphenylphosphorane
- Methoxymethylenetriphenylphosphine
- Carbomethoxymethylenetriphenylphosphorane
References
Wittig, Georg; Schoellkopf, U. (1960). "Methylenecyclohexane". Organic Syntheses. 40: 66. doi:10.15227/orgsyn.040.0066. /wiki/Doi_(identifier) ↩
Ling-Chung, Sim; Sales, Keith D.; Utley, James H. P. (1990). "Measurement of pKa Values for Phosphonium Salts via the Kinetics of Proton Transfer to an Electrogenerated Base". Journal of the Chemical Society, Chemical Communications (9): 662. doi:10.1039/C39900000662. /wiki/Doi_(identifier) ↩
Fitjer, L.; Quabeck, U. Synthetic Communications 1985, 15(10), 855–864. ↩
F. A. Bottino, G. Di Pasquale, A. Pollicino, A. Recca and D. T. Clark (1990). "Synthesis of 2-(2-hydroxyphenyl)-2H-benzotriazole monomers and studies of the surface photostabilization of the related copolymers". Macromolecules. 23 (10): 2662–2666. Bibcode:1990MaMol..23.2662B. doi:10.1021/ma00212a011.{{cite journal}}: CS1 maint: multiple names: authors list (link) /wiki/Macromolecules_(journal) ↩
Bart, J. C. J. (1969). "Structure of the non-stabilized phosphonium ylid methylenetriphenylphosphorane". Journal of the Chemical Society B. 1969: 350–365. doi:10.1039/J29690000350. /wiki/Doi_(identifier) ↩
B. E. Maryanoff & A. B. Reitz (1989). "The Wittig olefination reaction and modifications involving phosphoryl-stabilized carbanions. Stereochemistry, mechanism, and selected synthetic aspects". Chem. Rev. 89 (4): 863–927. doi:10.1021/cr00094a007. /wiki/Chem._Rev. ↩