Triphenylphosphine
Triphenylphosphine is a common organophosphorus compound with the formula P3 and often abbreviated to PPh3 or Ph3P. It is versatile compound that is widely used as a reagent in organic synthesis and as a ligand for transition metal complexes, including ones that serve as catalysts in organometallic chemistry. PPh3 exists as relatively air stable, colorless crystals at room temperature. It dissolves in non-polar organic solvents such as benzene and diethyl ether.
Preparation and structure
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:Triphenylphosphine crystallizes in triclinic and monoclinic modification. In both cases, the molecule adopts a pyramidal structure with propeller-like arrangement of the three phenyl groups.
Principal reactions with chalcogens, halogens, and acids
Oxidation
Triphenylphosphine undergoes slow oxidation by air to give triphenylphosphine oxide, Ph3PO:This impurity can be removed by recrystallisation of PPh3 from either hot ethanol or isopropanol. This method capitalizes on the fact that OPPh3 is more polar and hence more soluble in polar solvents than PPh3.
One electron oxidation generates the triphenylphosphonium radical cation, which dimerizes to the hexaphenyl-1,2-diphosphonium dication 2+. Irradiation by light cleaves the P–P bond and regenerates transient triphenylphosphonium radical cations.
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 triphenylphosphine sulfide, Ph3PS. This reaction can be employed to assay the "labile" S0 content of a sample, say vulcanized rubber. Triphenylphosphine selenide, Ph3PSe, may be easily prepared via treatment of PPh3 with red Se. Salts of selenocyanate, SeCN−, are used as the Se0 source. PPh3 can also form an adduct with Te, although this adduct primarily exists as 2Te rather than PPh3Te.
Aryl azides react with PPh3 to give phosphanimines, analogues of OPPh3, via the Staudinger reaction. Illustrative is the preparation of triphenylphosphine phenylimide:
The phosphanimine can be hydrolyzed to the amine. Typically the intermediate phosphanimine is not isolated.
Chlorination
Cl2 adds to PPh3 to give triphenylphosphine dichloride, which exists as the moisture-sensitive phosphonium halide. This reagent is used to convert alcohols to alkyl chlorides in organic synthesis. Bis(triphenylphosphine)iminium chloride 2N]Cl is prepared from triphenylphosphine dichloride:Protonation
PPh3 is a weak base, although it is a considerably stronger base than NPh3. It forms isolable triphenylphosphonium salts with strong acids such as HBr:Organic reactions
PPh3 is widely used in organic synthesis. The properties that guide its usage are its nucleophilicity and its reducing character. The nucleophilicity of PPh3 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
PPh3 combines with alkyl halides to give phosphonium salts. This quaternization reaction is particularly fast for benzylic and allylic halides:These salts, which can often be isolated as crystalline solids, react with strong bases to form ylides, which are reagents in the Wittig reactions.
Aryl halides will quaternize PPh3 to give tetraphenylphosphonium salts:
The reaction however requires elevated temperatures and metal catalysts.
Mitsunobu reaction
In the Mitsunobu reaction, a mixture of triphenylphosphine and diisopropyl azodicarboxylate converts an alcohol and a carboxylic acid to an ester. DIAD is reduced as it serves as the hydrogen acceptor, and the PPh3 is oxidized to OPPh3.Appel reaction
In the Appel reaction, a mixture of PPh3 and CX4 is used to convert alcohols to alkyl halides. Triphenylphosphine oxide is a byproduct.This reaction commences with nucleophilic attack of PPh3 on CBr4, an extension of the quaternization reaction listed above.
Deoxygenation
The easy oxygenation of PPh3 is exploited in its use to deoxygenate organic peroxides, which generally occurs with retention of configuration:It is also used for the decomposition of organic ozonides to ketones and aldehydes, although dimethyl sulfide is more popular for the reaction as the side product, dimethyl sulfoxide is more readily separated from the reaction mixture than triphenylphosphine oxide. Aromatic N-oxides are reduced to the corresponding amine in high yield at room temperature with irradiation:
Sulfonation
of PPh3 gives trisphosphine, P3, usually isolated as the trisodium salt. In contrast to PPh3, TPPTS is water-soluble, as are its metal derivatives. Rhodium complexes of TPPTS are used in certain industrial hydroformylation reactions.[Image:TPPTS.png|thumb|left|3,3,3-Phosphanetriyltris(benzenesulfonic acid) trisodium salt is a water-soluble derivative of triphenylphosphine.]