Everything about Triphenylphosphine totally explained
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Triphenylphosphine (in Europe: triphenylphosphane) is a common
organophosphorus compound with the formula P(C
6H
5)
3 - often abbreviated to
PPh3 or Ph
3P. It is widely used in the synthesis of
organic and
organometallic compounds. PPh
3 exists as relatively air stable, colorless crystals at room temperature. It dissolves in non-polar organic solvents such as
benzene and
diethyl ether.
Preparation, structure, handling
Although it's inexpensive, triphenylphosphine can be prepared in the laboratory by treatment of
phosphorus trichloride with
phenylmagnesium bromide or
phenyllithium. The industrial synthesis involves the reaction between
phosphorus trichloride,
chlorobenzene, and sodium. PPh
3 is pyramidal with a chiral propeller-like arrangement of the three phenyl rings. The rigidity of PPh
3 contributes to the ease with which its derivatives crystallize.
Principal reactions with chalcogens, halogens, and acids
Triphenylphosphine undergoes slow oxidation by air to give
triphenylphosphine oxide, Ph
3PO:
» 2 PPh
3 + O
2 → 2 OPPh
3
This impurity can be removed by
recrystallisation of PPh
3 from either hot
ethanol or hot
isopropanol. This method capitalizes on the fact that OPPh
3 is more polar and hence more soluble in hydroxylic solvents than PPh
3.
The easy oxygenation of PPh
3 is exploited in its use to deoxygenate organic peroxides, which generally occurs with retention of configuration:
» PPh
3 + RO
2H → OPPh
3 + ROH (R = alkyl)
Triphenylphosphine abstracts
sulfur from
polysulfide compounds, episulfides, and elemental
sulfur. Simple organosulfur compounds such as
thiols and
thioethers are unreactive, however. The phosphorus-containing product is Ph
3PS. This reaction can be employed to assay the "labile" S
0 content of a sample, say vulcanized rubber. Triphenylphosphine selenide, Ph
3PSe, doesn't readily form upon treatment of PPh
3 with either
allotrope of
Se. Salts of
selenocyanate, SeCN
-, are used as the Se
0 source. Ph
3PTe is unknown and apparently unstable.
Aryl
azides react with PPh
3 to give imido analogue of OPPh
3 via the
Staudinger reaction:
» PPh
3 + PhN
3 → PhNPPh
3 + N
2
The product imides can be hydrolyzed to the amine. Typically the intermediate imidophosphorane isn't isolated.
» PPh
3 + RN
3 + H
2O → OPPh
3 + N
2 + RNH
2
Cl
2 adds to PPh
3 to give [PPh
3Cl]Cl, which exists as the moisture-sensitive phosphonium salt, This reagent is used to convert
alcohols to
alkyl chlorides in
organic synthesis.
PPh
3 is a weak base, but does form stable salts with strong acids such as HBr. The product contains the phosphonium cation [HPPh
3]
+.
Principal organic reactions
PPh
3 is widely used in
organic synthesis. The properties that guide its usage are its nucleophilicity and its reducing character. The nucleophilicity of PPh
3 is indicated by its reactivity toward electrophilic alkenes, such as Michael-acceptors, and alkyl halides. It is also used in the synthesis of biaryl compounds, such as the
Suzuki reaction.
Quaternization
PPh
3 combines with most
alkyl halides to give
phosphonium salts. The facility of the reaction follows the usual pattern whereby alkyl iodides and benzylic and allylic halides are particularly efficient reactants:
» PPh
3 +
CH3I → [CH
3PPh
3]
+I
-
These salts, which are readily isolated as crystalline solids, react with strong bases to form
ylides:
» CH
3PPh
3+ + base → [CH
2PPh
3] + baseH
+
Such ylides are key reagents in the
Wittig reactions, used to convert
aldehydes and
ketones into
alkenes. Nickel salts are required to react PPh
3 with PhBr to give [PPh
4]Br. The tetraphenylphosphonium cation is widely used to prepare crystallizable lipophilic salts.
In this reaction, a mixture of PPh
3 and
diisopropyl azodicarboxylate (“DIAD”, or its diethyl analogue, DEAD) converts an alcohol and a carboxylic acid to an ester. The DIAD is reduced as it serves as the hydrogen acceptor, and the PPh
3 is oxidized to
OPPh3.
PPh
3 is oxidized again to
OPPh3 in this application, which covert alcohols to alkyl halides using CX
4 (X = Cl, Br):
» PPh
3 + CBr
4 + RCH
2OH → OPPh
3 + RCH
2Br + HCBr
3
This reaction commences with nucleophilic attack of PPh
3 on CBr
4, an extension of the quaternization reaction listed above.
Principal transition metal derivatives
Triphenylphosphine binds well to most
transition metals, especially those in the middle and late transition metals of groups 7-10. In terms of steric bulk, PPh
3 has a
cone angle of 145°, which is intermediate between those of P(C
6H
11)
3 (170°) and P(CH
3)
3 (115°). In an early application in
homogeneous catalysis, NiBr
2(PPh
3)
2 was used by
Walter Reppe for the synthesis of acrylate esters from
alkynes,
carbon monoxide, and
alcohols.
Wilkinson's further popularized the use of PPh
3, including the then revolutionary
hydroformylation catalyst RhH(PPh
3)
2(CO)
2.
It is telling that the corresponding
triphenylamine shows little tendency to bind to metals. The difference between the coordinating power of PPh
3 and NPh
3 reflects the greater steric crowding around the nitrogen atom, which is smaller and favors a more tetrahedral geometry. Far more similar to PPh
3 in terms of its coordinating properties is
triphenylarsine, AsPh
3.
An important technique for the characterization of metal-PPh
3 compounds is
31P
NMR spectroscopy. Substantial shifts occur upon complexation and
31P-
31P spin-spin coupling can provide insight into the structure of complexes containing multiple phosphine ligands.
Illustrative PPh
3 complexes:
Organophosphorus chemistry
Conversion to PPh2 derivatives
Triphenylphosphine is commonly employed as a precursor to other organophosphines.
Lithium in
THF and Na or K in NH
3 react with PPh
3 to give Ph
2PM (M = Li, Na, K). These reactions generate equal amounts of phenyllithium (or sodium, potassium analogs thereof) C
6H
5M, which can be selectively converted to
benzene by selective protonation. Treatment of the resulting alkali metal diphenylphosphide with an
alkylating agent RX gives PRPh
2. This method can be used to prepare such
ligands as PMePh
2, methyldiphenylphosphine. The corresponding reaction with dihaloalkanes gives bis(diphenylphosphino)alkanes. For example,
1,2-dibromoethane and Ph
2PM react to give Ph
2PCH
2CH
2PPh
2, known as
1,2-bis(diphenylphosphino)ethane or dppe. Weak acids such
ammonium chloride, converts Ph
2PM (M = Li, Na, K) into Ph
2PH, known as diphenylphosphine.
Sulfonation - access to water-soluble phosphine ligands
Sulfonation of PPh
3 gives tris(3-sulfophenyl)phosphine, P(C
6H
4-3-SO
3-)
3. This anionic phosphine is usually isolated as the trisodium salt and is known as
TPPTS. In contrast to PPh
3, TPPTS is a water soluble as are its metal derivatives. Rhodium complexes of TPPTS are used in certain industrial
hydroformylation reactions because the water-soluble catalyst is readily separated from the organic products.
Polymer-anchored PPh3 derivatives
Polymeric analogues of PPh
3 are known whereby polystyrene is modified with PPh
2 groups at the para position. Such polymers can be employed in many of the applications used for PPh
3 with the advantage that the polymer, being insoluble, can be separated from products by simple filtration of reaction slurries. Such polymers are prepared via treatment of 4-lithiophenyl-substituted polystyrene with PPh
2Cl.
Safety
PPh
3 should be handled in a well ventillated area, preferably a fume hood.
Further Information
Get more info on 'Triphenylphosphine'.
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