# Organophosphine

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{{Short description|Class of chemical compounds}}
'''Organophosphines''' are [organophosphorus compound](/source/organophosphorus_compound)s with the formula PR<sub>''n''</sub>H<sub>3−''n''</sub>, where R is an organic substituent.  These compounds can be classified according to the value of ''n'': primary phosphines (''n''&nbsp;=&nbsp;1), secondary phosphines (''n''&nbsp;=&nbsp;2), tertiary phosphines (''n''&nbsp;=&nbsp;3). All adopt pyramidal structures.<ref>{{cite book|editor=Paul C. J. Kamer, Piet W. N. M. van Leeuwen|title=Phosphorus(III)Ligands in Homogeneous Catalysis: Design and Synthesis|publisher=Wiley|location=New York|year=2012|isbn=978-0-470-66627-2}}</ref> Organophosphines are generally colorless, lipophilic liquids or solids.<ref>{{cite book|title=Organic Phosphorus Compounds, Volume 1|author1=G.M. Kosolapoff |author2=L. Maier |publisher=John Wiley|location=New York, N. Y.|year=1972}}</ref>  The parent of the organophosphines is [phosphine](/source/phosphine) (PH<sub>3</sub>).
<ref name=Schmidbaur>Annette Schier and Hubert Schmidbaur"P-Donor Ligands" in Encyclopedia of Inorganic Chemistry 2006, Wiley-VCH, Weinheim.  {{doi|10.1002/0470862106.ia177}}</ref>

== 1° vs 2° vs 3° phosphines==
Organophophines are classified according to the number of organic substituents.

===Primary phosphines===
Primary (1°) phosphines, with the formula RPH<sub>2</sub>, in principle are derived by alkylation of phosphine.  Some simple alkyl derivatives such as [methylphosphine](/source/methylphosphine) (CH<sub>3</sub>PH<sub>2</sub>) can be prepared by alkylation of phosphine in the presence of base:<ref>{{cite book|author=W. L. Jolly|title=Methylphosphine|series=Inorganic Syntheses|year=1968| volume=11|page=124|doi=10.1002/9780470132425.ch25}}</ref>
:{{chem2|MPH2  +  RX  ->  RPH2 + MX}} (M = Li, Na, K)
A more common synthetic route involves reduction of chlorophosphines with hydride reagents.  For example, reduction of [dichlorophenylphosphine](/source/dichlorophenylphosphine) with [lithium aluminium hydride](/source/lithium_aluminium_hydride) affords [phenylphosphine](/source/phenylphosphine) according to the following idealized equation:<ref>{{cite journal|doi=10.1002/anie.200602143|title=Taming a Functional Group: Creating Air-Stable, Chiral Primary Phosphanes|year=2006|journal=Angewandte Chemie International Edition|volume=45|issue=43|pages=7248–7251|last1=Hiney|first1=Rachel M.|last2=Higham|first2=Lee J.|last3=Müller-Bunz|first3=Helge|last4=Gilheany|first4=Declan G.|pmid=17022105}}</ref>
:{{chem2|2 RPCl2  +  LiAlH4  ->  2 RPH2 + LiCl  +  AlCl3}}

Most primary phosphines are [pyrophoric](/source/pyrophoric) in air.<ref>{{cite journal|doi=10.1021/om200070a|journal=Organometallics|title=Predicting the air stability of phosphines|first1=Beverly|last1=Stewart|first2=Anthony|last2=Harriman|first3=Lee&nbsp;J.|last3=Higham|orig-date=January 25, 2011|date=September 21, 2011|publisher=American Chemical Society|page=5338|volume=30}}</ref>

===Secondary phosphines===
Secondary (2°) phosphines, with the formula R<sub>2</sub>PH, are prepared analogously to the primary phosphines.  They are also obtained by alkali-metal reductive cleavage of triarylphosphines followed by hydrolysis of the resulting phosphide salt.  The latter route is employed to prepare [diphenylphosphine](/source/diphenylphosphine) (Ph<sub>2</sub>PH).  Diorganophosphinic acids, R<sub>2</sub>P(O)OH, can also be reduced with [diisobutylaluminium hydride](/source/diisobutylaluminium_hydride).  Secondary phosphines are mildly protic in character.

Secondary phosphines occur in cyclic forms. Three-membered rings are [phosphirane](/source/phosphirane)s (unsaturated: [phosphirene](/source/phosphirene)s), five-membered rings are [phospholane](/source/phospholane)s (unsaturated: [phosphole](/source/phosphole)), and six-membered rings are [phosphinane](/source/phosphinane)s.

===Tertiary phosphines===
Tertiary (3°) phosphines, with the formula R<sub>3</sub>P,  are traditionally prepared by alkylation of [phosphorus trichloride](/source/phosphorus_trichloride) using [Grignard reagent](/source/Grignard_reagent)s or related [organolithium](/source/organolithium) compounds:
:3{{nbsp}}RMgX  +  PCl<sub>3</sub>    →   PR<sub>3</sub>   +  3{{nbsp}}MgX<sub>2</sub>
In the case of [trimethylphosphine](/source/trimethylphosphine), [triphenyl phosphite](/source/triphenyl_phosphite) is used in place of the highly electrophilic PCl<sub>3</sub>:<ref>{{cite book |author1=Leutkens, M. L. Jr. |author2=Sattelberger, A. P. |author3=Murray, H. H. |author4=Basil, J. D. |author5-link=John P. Fackler Jr. |author5=Fackler, J. P. Jr. |title=Inorganic Syntheses | year = 1990 | chapter = Trimethylphosphine | volume = 28 | pages = 305–310 | doi = 10.1002/9780470132593.ch76|isbn=9780470132593 }}</ref>
: 3&nbsp;CH<sub>3</sub>MgBr + P(OC<sub>6</sub>H<sub>5</sub>)<sub>3</sub> → P(CH<sub>3</sub>)<sub>3</sub> + 3&nbsp;C<sub>6</sub>H<sub>5</sub>OMgBr

Slightly more elaborate methods are employed for the preparation of unsymmetrical tertiary phosphines, with the formula R<sub>2</sub>R'P.  The use of organophosphorus-based [nucleophile](/source/nucleophile)s is typical.  For example, [lithium diphenylphosphide](/source/lithium_diphenylphosphide) is readily methylated with [methyl iodide](/source/methyl_iodide) to give [methyldiphenylphosphine](/source/methyldiphenylphosphine):
:LiiP(C<sub>6</sub>H<sub>5</sub>)<sub>2</sub> + CH<sub>3</sub>I → CH<sub>3</sub>P(C<sub>6</sub>H<sub>5</sub>)<sub>2</sub> + LiI

[Phosphine](/source/Phosphine) is a precursor to some tertiary phosphines by [hydrophosphination](/source/hydrophosphination) of alkenes. For example, in the presence of basic catalysts PH<sub>3</sub> adds of [Michael acceptor](/source/Michael_acceptor)s such as [acrylonitrile](/source/acrylonitrile):<ref name=RussRev>{{cite journal|title=Phosphine in the Synthesis of Organophosphorus Compounds|first1=Boris A. |last1=Trofimov|first2=Svetlana N. |last2=Arbuzova|first3=Nina K. |last3=Gusarova|year=1999|journal=Russian Chemical Reviews|volume=68|issue=3 |pages=215–227 |doi=10.1070/RC1999v068n03ABEH000464|bibcode=1999RuCRv..68..215T|s2cid=250775640 }}</ref>
:PH<sub>3</sub> + 3&nbsp;CH<sub>2</sub>=CHZ → P(CH<sub>2</sub>CH<sub>2</sub>Z)<sub>3</sub> (Z = NO<sub>2</sub>, CN, C(O)NH<sub>2</sub>)

Tertiary phosphines of the type PRR′R″ are "[''P''-chiral](/source/P-Chiral_phosphine)" and optically stable.

From the commercial perspective, the most important phosphine is [triphenylphosphine](/source/triphenylphosphine), several million kilograms being produced annually. It is prepared from the reaction of [chlorobenzene](/source/chlorobenzene), [PCl<sub>3</sub>](/source/phosphorus_trichloride), and sodium.<ref name=Ullmann>{{cite book|author1=Svara, Jürgen |author2=Weferling, Norbert |author3=Hofmann, Thomas  |name-list-style=amp |chapter=Phosphorus Compounds, Organic|title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley-VCH|place= Weinheim|year= 2006|doi=10.1002/14356007.a19_545.pub2|isbn=978-3527306732}}</ref>  Phosphines of a more specialized nature are usually prepared by other routes.<ref>{{cite journal | last1 = Downing | first1 = J.H. | last2 = Smith | first2 = M.B. | year = 2003| title = Phosphorus Ligands | journal = Comprehensive Coordination Chemistry II | volume = 2003 | pages = 253–296 | doi = 10.1016/B0-08-043748-6/01049-5 | isbn = 9780080437484}}</ref>

===Di- and triphosphines===
[[File:Generic-diphoshine-ligand-2D-skeletal.png|thumb|right|class=skin-invert-image|[Skeletal formula](/source/Skeletal_formula) of a generic diphosphine ligand. R represents a [side chain](/source/side_chain); the phosphine donors are connected by a backbone linker.]]
[Diphosphines](/source/Diphosphines) are also available in primary, secondary, and tertiary phosphorus substituents.  Triphosphines etc. are similar.

==Structure and bonding==
Organophosphines, like phosphine itself, are [pyramidal molecule](/source/pyramidal_molecule)s with approximate ''C''<sub>3''v''</sub> [symmetry](/source/point_group). The C–P–C bond angles are approximately 98.6°.<ref name=Schmidbaur/> The C–P–C bond angles are consistent with the notion that phosphorus predominantly uses the 3p orbitals for forming bonds and that there is little sp hybridization of the phosphorus atom. The latter is a common feature of the chemistry of phosphorus. As a result, the lone pair of trimethylphosphine has predominantly s-character as is the case for phosphine, PH<sub>3</sub>.<ref>E. Fluck, The Chemistry of Phosphine, Topics in Current Chemistry Vol. 35, 64 pp, 1973.</ref>

Tertiary phosphines are pyramidal. When the organic substituents all differ, the phosphine is [chiral](/source/chirality_(chemistry)) and configurationally stable (in contrast to NRR'R").  Complexes derived from the chiral phosphines can catalyse reactions to give [chiral](/source/Chirality_(chemistry)), [enantioenriched](/source/Enantiomer) products.

===Comparison of phosphines and amines===
The phosphorus atom in phosphines has a formal oxidation state −3 (σ<sup>3</sup>λ<sup>3</sup>) and are the phosphorus analogues of [amine](/source/amine)s.  Like amines, phosphines have a [trigonal pyramidal molecular geometry](/source/trigonal_pyramidal_molecular_geometry) although often with smaller C-E-C angles (E = N, P), at least in the absence of steric effects.  The C-P-C [bond angle](/source/bond_angle) is 98.6° for trimethylphosphine increasing to 109.7° when the methyl groups are replaced by [''tert''-butyl](/source/tert-butyl) groups.  When used as ligands, the steric bulk of tertiary phosphines is evaluated by their [cone angle](/source/ligand_cone_angle).  The barrier to [pyramidal inversion](/source/pyramidal_inversion) is also much higher than [nitrogen inversion](/source/nitrogen_inversion) to occur, and therefore phosphines with three different [substituent](/source/substituent)s can be resolved into thermally stable [optical isomer](/source/optical_isomer)s.  Phosphines are often less basic than corresponding amines, for instance the phosphonium ion itself has a [p''K''<sub>a</sub>](/source/pKa) of −14 compared to 9.21 for the ammonium ion; [trimethylphosphonium](/source/Trimethylphosphine) has a p''K''<sub>a</sub> of 8.65 compared to 9.76 for [trimethylammonium](/source/trimethylammonium). However, triphenylphosphine (p''K''<sub>a</sub> 2.73) is more basic than [triphenylamine](/source/triphenylamine)  (p''K''<sub>a</sub> −5), mainly because the lone pair of the nitrogen in NPh<sub>3</sub> is partially delocalized into the three phenyl rings.  Whereas the lone pair on nitrogen is [delocalized](/source/delocalization) in [pyrrole](/source/pyrrole), the lone pair on phosphorus atom in the phosphorus equivalent of [pyrrole](/source/pyrrole) ([phosphole](/source/phosphole)) is not. The reactivity of phosphines matches that of amines with regard to [nucleophilicity](/source/nucleophilicity) in the formation of [phosphonium salt](/source/phosphonium_salt)s with the general structure PR<sub>4</sub><sup>+</sup>X<sup>−</sup>. This property is used in the [Appel reaction](/source/Appel_reaction) for converting [alcohol](/source/Alcohol_(chemistry))s to [alkyl halide](/source/alkyl_halide)s. Phosphines are easily [oxidized](/source/oxidation) to the corresponding [phosphine oxide](/source/phosphine_oxide)s, whereas amine oxides are less readily generated.  In part for this reason, phosphines are very rarely encountered in nature.

==Reactions==
===Coordination chemistry===
{{see also|Metal phosphine complex}}
Tertiary phosphines are often used as [ligands](/source/Phosphine_ligand) in coordination chemistry. The binding of phosphines bind to metals, which serve as [Lewis acid](/source/Lewis_acid)s. For example, [silver chloride](/source/silver_chloride) reacts with triphenylphosphine to 1;1 and 1:2 complexes:
:PPh<sub>3</sub> + AgCl → ClAgPPh<sub>3</sub>
:PPh<sub>3</sub> + ClAgPPh<sub>3</sub> → ClAg(PPh<sub>3</sub>)<sub>2</sub>
The adducts formed from phosphines and borane are useful reagents.  These [phosphine-borane](/source/phosphine-borane)s are air-stable, but the borane [protecting group](/source/protecting_group) can be removed by treatment with amines.<ref>{{cite book |doi=10.1007/128_2014_565|chapter=Recent Advances in Synthesis of P-BH<sub>3</sub> Compounds|title=Phosphorus Chemistry II|series=Topics in Current Chemistry|year=2014|last1=Alayrac|first1=Carole|last2=Lakhdar|first2=Sami|last3=Abdellah|first3=Ibrahim|last4=Gaumont|first4=Annie-Claude|volume=361|pages=1–82|pmid=25504072|isbn=978-3-319-15511-1}}</ref><ref>{{cite journal |doi=10.1016/S0010-8545(98)00072-1|title=Phosphane–Boranes: Synthesis, Characterization and Synthetic Applications|year=1998|last1=Brunel|first1=Jean Michel|last2=Faure|first2=Bruno|last3=Maffei|first3=Michel|journal=Coordination Chemistry Reviews|volume=178-180|pages=665–698}}</ref>

===Quaternization===
Akin to complexation, phosphines are readily alkylated.  For example, [methyl bromide](/source/methyl_bromide) converts triphenylphosphine to the [methyltriphenylphosphonium bromide](/source/methyltriphenylphosphonium_bromide), a "quat salt":
:PPh<sub>3</sub>  +  CH<sub>3</sub>Br  →  [CH<sub>3</sub>PPh<sub>3</sub><sup>+</sup>]Br<sup>−</sup>

Phosphines are [nucleophilic catalysts](/source/organocatalysis) in [organic synthesis](/source/organic_synthesis), e.g. the [Rauhut–Currier reaction](/source/Rauhut%E2%80%93Currier_reaction) and [Baylis-Hillman reaction](/source/Baylis-Hillman_reaction).

===Protonation and deprotonation===
Like phosphine itself, but easier, organophosphines undergo protonation.  The reaction is reversible.  Whereas organophosphines are oxygen-sensitive, the protonated derivatives are not.

Primary and secondary derivatives, they can be deprotonated by strong bases to give organo[phosphide](/source/phosphide) derivatives.  Thus [diphenylphosphine](/source/diphenylphosphine) reacts with [organolithium reagent](/source/organolithium_reagent) to give [lithium diphenylphosphide](/source/lithium_diphenylphosphide):
:HPPh<sub>2</sub> + RLi → LiPPh<sub>2</sub> + RH

===Oxidation and sulfiding===
Tertiary phosphines characteristically oxidize to give [phosphine oxide](/source/phosphine_oxide)s with the formula R<sub>3</sub>PO. The reaction with oxygen is spin-forbidden but still proceeds at sufficient rate that samples of tertiary phosphines are characteristically contaminated with phosphine oxides. Qualitatively, the rates of oxidation are higher for trialkyl vs triarylphosphines. Faster still are oxidations using [hydrogen peroxide](/source/hydrogen_peroxide). Primary and secondary phosphines also oxidize, but the product(s) are subject to tautomerization and further oxidation.

Tertiary phosphines characteristically oxidize to give [phosphine sulfide](/source/phosphine_sulfide)s.

The [reducing properties](/source/reducing_agent) of organophosphiines is also illustrated in the [Staudinger reduction](/source/Staudinger_reduction) for the conversion of organic azides to amines and in the [Mitsunobu reaction](/source/Mitsunobu_reaction) for converting alcohols into esters. In these processes, the phosphine is oxidized to phosphorus(V). Phosphines have also been found to reduce activated carbonyl groups, for instance the reduction of an α-keto ester to an α-hydroxy ester in ''scheme 2''.<ref>{{cite journal | last1 = Zhang | first1 = W. | last2 = Shi | first2 = M. | year = 2006| title = Reduction of activated carbonyl groups by alkyl phosphines: formation of α-hydroxy esters and ketones | journal = [ChemComm](/source/ChemComm) | volume = 2006 | issue = 11| pages = 1218–1220 | doi = 10.1039/b516467b | pmid = 16518496 }}</ref> In the proposed [reaction mechanism](/source/reaction_mechanism), the first proton is on loan from the methyl group in trimethylphosphine (triphenylphosphine does not react).
:350px|thumb|class=skin-invert-image|center|Reduction of activated carbonyl groups by alkyl phosphines

===Hydrophosphination===
Primary (RPH<sub>2</sub>) and secondary phosphines (RRPH and R<sub>2</sub>PH) add to [alkene](/source/alkene)s in presence of a strong base (e.g., [KOH](/source/potassium_hydroxide) in [DMSO](/source/Dimethyl_sulfoxide)). [Markovnikov's rule](/source/Markovnikov's_rule)s apply. Similar reactions occur involving [alkyne](/source/alkyne)s.<ref>{{cite journal|author1=Arbuzova, S. N. |author2=Gusarova, N. K. |author3=Trofimov, B. A. |title=Nucleophilic and free-radical additions of phosphines and phosphine chalcogenides to alkenes and alkynes|journal=[Arkivoc](/source/Arkivoc)|year=2006|volume=v|pages= 12–36|doi=10.3998/ark.5550190.0007.503|issue=5|doi-access=free|hdl=2027/spo.5550190.0007.503|hdl-access=free}}</ref> Base is not required for electron-deficient alkenes (e.g., derivatives of [acrylonitrile](/source/acrylonitrile)) and alkynes.
:class=skin-invert-image|center|240px|Scheme 1. Addition of phosphine and phosphines to alkenes and alkynes

Primary and secondary phosphines do not normally add to ketones and aldehydes unless the addition closes a ring:<ref>{{cite journal |doi=10.1039/b306153a |title=Highly stereoselective, thermodynamically controlled and reversible formation of a new P-chiral phosphine |date=2003 |last1=Komarov |first1=Igor V. |last2=Spannenberg |first2=Anke |last3=Holz |first3=Jens |last4=Börner |first4=Armin |journal=Chem. Commun. |issue=17 |pages=2240–2241 |pmid=13678220 }}</ref>
:{{chem2|R2PH  +  R'2C\dO  ->  R2PC(OH)HR'2}}

==See also==
*[Diphosphines](/source/Diphosphines), R<sub>2</sub>PPR<sub>2</sub>, R<sub>2</sub>P(CH<sub>2</sub>)<sub>''n''</sub>PR<sub>2</sub>
*[Phosphine oxide](/source/Phosphine_oxide), R<sub>3</sub>P=O
*[Phosphorane](/source/Phosphorane), PR<sub>5</sub>, R<sub>3</sub>P=CR<sub>2</sub>
*[Phosphinite](/source/Phosphinite), P(OR)R<sub>2</sub>
*[Phosphonite](/source/Phosphonite), P(OR)<sub>2</sub>R
*[Phosphite](/source/Phosphite), P(OR)<sub>3</sub>
*[Phosphinate](/source/Phosphinate), R<sub>2</sub>P(RO)O
*[Phosphonate](/source/Phosphonate), RP(RO)<sub>2</sub>O

==References==
{{Reflist}}

{{Organophosphorus}}

Category:Functional groups
Category:Phosphorus(−III) compounds
Category:Organophosphanes

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Adapted from the Wikipedia article [Organophosphine](https://en.wikipedia.org/wiki/Organophosphine) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Organophosphine?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.