Methylenetriphenylphosphorane

Methylenetriphenylphosphorane
Names
	IUPAC name Methylene(triphenyl)phosphorane
Identifiers
CAS Number	3487-44-3 ;
3D model (JSmol)	Interactive image;
ChemSpider	121606;
PubChem CID	137960;
UNII	8GN8K95F62 ;
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

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 + 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]

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 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).[3] The compound is usually described as a combination of two resonance structures:

Ph₃P⁺CH₂⁻ ↔ Ph₃P=CH₂

Hexaphenylcarbodiphosphorane is one of several unusual phosphorus ylides inspired by Wittig reagents.[4]

Related compounds

Electron-withdrawing groups (EWGs) enhance the ease of deprotonation of phosphonium salts. This behavior is illustrated by triphenylcarbethoxymethylphosphonium derived from chloroacetic acid esters and triphenylphosphine. It deprotonation requires only sodium hydroxide. The resulting triphenylcarbethoxymethylenephosphorane is somewhat air-stable. It is however less reactive than ylides lacking EWGs. For example they usually fail to react with ketones, necessitating the use of the Horner–Wadsworth–Emmons reaction as an alternative. Such stabilized ylides usually give rise to an E-alkene product when they react, rather than the more usual Z-alkene.

Although these ylides are "electron-rich", they are susceptible to deprotonation. Treatment of Me₃PCH₂ with butyl lithium affords Me₂P(CH₂)₂Li.[5]

Having carbanion-like properties, lithiated ylides function as ligands. Thus Me₂P(CH₂)₂Li is a bidentate ligand.[6][5]

Related reagents

(Chloromethylene)triphenylphosphorane

References

Wittig, Georg; Schoellkopf, U. (1960). "Methylenecyclohexane". 40: 66. doi:10.15227/orgsyn.040.0066. Cite journal requires |journal= (help)
Ling-Chung, Sim; Sales, Keith D.; Utley, James H. P. (1990). "Measurement of pK_a 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.
Bart, J. C. J. "Structure of the non-stabilized phosphonium ylid methylenetriphenylphosphorane". Journal of the Chemical Society B. 1969: 350–365. doi:10.1039/J29690000350.
Tonner, Ralf; Oexler, Florian; Neumueller, Bernhard; Petz, Wolfgang; Frenking, Gernot (2006). "Carbodiphosphoranes: The Chemistry of Divalent Carbon(0)". Angewandte Chemie International Edition. 45 (47): 8038–8042. doi:10.1002/anie.200602552. PMID 17075933.CS1 maint: uses authors parameter (link)
Fackler, J. P.; Basil, J. D. (1982). "Oxidative Addition of Methyl Iodide to a Dinuclear gold(I) Complex. The X-Ray Crystal Structure of Bis[mu-(Dimethyldimethylenephosphoranyl-C,C)]-iodomethyldigold(II)(Au-Au), Au₂[(CH₂)₂P(CH₃)₂]₂(CH3)I". Organometallics. 1 (6): 871–873. doi:10.1021/om00066a021.CS1 maint: uses authors parameter (link)
Schmidbaur, H. (1983). "Phosphorus Ylides in the Coordination Sphere of Transition Metals: An Inventory". Angewandte Chemie International Edition in English. 22 (12): 907–927. doi:10.1002/anie.198309071.

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[1] Wittig, Georg; Schoellkopf, U. (1960). "Methylenecyclohexane". 40: 66. doi:10.15227/orgsyn.040.0066. Cite journal requires |journal= (help)

[2] Ling-Chung, Sim; Sales, Keith D.; Utley, James H. P. (1990). "Measurement of pK_a 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.

[3] Bart, J. C. J. "Structure of the non-stabilized phosphonium ylid methylenetriphenylphosphorane". Journal of the Chemical Society B. 1969: 350–365. doi:10.1039/J29690000350.

[4] Tonner, Ralf; Oexler, Florian; Neumueller, Bernhard; Petz, Wolfgang; Frenking, Gernot (2006). "Carbodiphosphoranes: The Chemistry of Divalent Carbon(0)". Angewandte Chemie International Edition. 45 (47): 8038–8042. doi:10.1002/anie.200602552. PMID 17075933.CS1 maint: uses authors parameter (link)

[Fackler-5] Fackler, J. P.; Basil, J. D. (1982). "Oxidative Addition of Methyl Iodide to a Dinuclear gold(I) Complex. The X-Ray Crystal Structure of Bis[mu-(Dimethyldimethylenephosphoranyl-C,C)]-iodomethyldigold(II)(Au-Au), Au₂[(CH₂)₂P(CH₃)₂]₂(CH3)I". Organometallics. 1 (6): 871–873. doi:10.1021/om00066a021.CS1 maint: uses authors parameter (link)

[6] Schmidbaur, H. (1983). "Phosphorus Ylides in the Coordination Sphere of Transition Metals: An Inventory". Angewandte Chemie International Edition in English. 22 (12): 907–927. doi:10.1002/anie.198309071.