Organophosphine

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Organophosphines are

organophosphorus compounds with the formula PRnH3−n, where R is an organic substituent. These compounds can be classified according to the value of n: primary phosphines (n = 1), secondary phosphines (n = 2), tertiary phosphines (n = 3). All adopt pyramidal structures.[1] Organophosphines are generally colorless, lipophilic liquids or solids.[2] The parent of the organophosphines is phosphine
(PH3). [3]

1° vs 2° vs 3° phosphines

Organophophines are classified according to the number of organic substituents.

Primary phosphines

Primary (1°) phosphines, with the formula RPH2, are typically prepared by alkylation of phosphine. Simple alkyl derivatives such as methylphosphine (CH3PH2) are prepared by alkylation of alkali metal derivatives MPH2 (M is Li, Na, or K). Another synthetic route involves treatment of the corresponding chlorophosphines with hydride reagents. For example, reduction of dichlorophenylphosphine with lithium aluminium hydride affords phenylphosphine (PhPH2).[4]

Example of reduction of phosphonate ester to a primary phosphine.

Primary (RPH2) and secondary phosphines (RRPH and R2PH) add to alkenes in presence of a strong base (e.g., KOH in DMSO). Markovnikov's rules apply. Similar reactions occur involving alkynes.[5] Base is not required for electron-deficient alkenes (e.g., derivatives of acrylonitrile) and alkynes.

Scheme 1. Addition of phosphine and phosphines to alkenes and alkynes
Scheme 1. Addition of phosphine and phosphines to alkenes and alkynes

Secondary phosphines

Secondary (2°) phosphines, with the formula R2PH, 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 (Ph2PH). Diorganophosphinic acids, R2P(O)OH, can also be reduced with diisobutylaluminium hydride. Secondary phosphines are typically protic in character. But when modified with suitable substituents, as in certain (rare) diazaphospholenes (scheme 3), the polarity of the P-H bond can be inverted (see: umpolung) and the resulting phosphine hydride can reduce a carbonyl group as in the example of benzophenone in yet another way.[6] Secondary phosphines occur in cyclic forms. Three-membered rings are phosphiranes (unsaturated: phosphirenes), five-membered rings are phospholanes (unsaturated: phosphole), and six-membered rings are phosphinanes.

Scheme 3. diazaphospholene phosphine hydride
Scheme 3. diazaphospholene phosphine hydride

Tertiary phosphines

Tertiary (3°) phosphines, with the formula R3P, are traditionally prepared by alkylation of

organolithium
compounds:

3 RMgX + PCl3 → PR3 + 3 MgX2

In the case of trimethylphosphine, triphenyl phosphite is used in place of the highly electrophilic PCl3:[7]

3 CH3MgBr + P(OC6H5)3 → P(CH3)3 + 3 C6H5OMgBr

Slightly more elaborate methods are employed for the preparation of unsymmetrical tertiary phosphines, with the formula R2R'P. The use of organophosphorus-based

methyl iodide to give methyldiphenylphosphine
:

LiiP(C6H5)2 + CH3I → CH3P(C6H5)2 + LiI

Michael acceptors such as acrylonitrile:[8]

PH3 + 3 CH2=CHZ → P(CH2CH2Z)3 (Z = NO2, CN, C(O)NH2)

Tertiary phosphines of the type PRR′R″ are "P-chiral" and optically stable.

From the commercial perspective, the most important phosphine is triphenylphosphine, several million kilograms being produced annually. It is prepared from the reaction of chlorobenzene, PCl3, and sodium.[9] Phosphines of a more specialized nature are usually prepared by other routes.[10]

Di- and triphosphines

Skeletal formula of a generic diphosphine ligand. R represents a side chain; the phosphine donors are connected by a backbone linker.

Diphosphines
are also available in primary, secondary, and tertiary phosphorus substituents. Triphosphines etc. are similar.

Structure and bonding

Organophosphines, like phosphine itself, are

pyramidal molecules with approximate C3v symmetry. The C–P–C bond angles are approximately 98.6°.[3] 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, PH3.[11]

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

Comparison of phosphines and amines

The phosphorus atom in phosphines has a formal oxidation state −3 (σ3λ3) and are the phosphorus analogues of

oxidized to the corresponding phosphine oxides
, 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

Lewis acids. For example, silver chloride
reacts with triphenylphosphine to 1;1 and 1:2 complexes:

PPh3 + AgCl → ClAgPPh3
PPh3 + ClAgPPh3 → ClAg(PPh3)2

The adducts formed from phosphines and borane are useful reagents. These phosphine-boranes are air-stable, but the borane protecting group can be removed by treatment with amines.[12][13]

Quaternization

Akin to complexation, phosphines are readily alkylated. For example,

methyl bromide converts triphenylphosphine to the methyltriphenylphosphonium bromide
, a "quat salt":

PPh3 + CH3Br → [CH3PPh3+]Br−

Phosphines are

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 organophosphide derivatives. Thus diphenylphosphine reacts with organolithium reagent to give lithium diphenylphosphide:

HPPh2 + RLi → LiPPh2 + RH

Oxidation and sulfiding

Tertiary phosphines characteristically oxidize to give phosphine oxides with the formula R3PO. 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. Primary and secondary phosphines also oxidize, but the product(s) are subject to tautomerization and further oxidation.

Tertiary phosphines characteristically oxidize to give phosphine sulfides.

The

Staudinger reduction for the conversion of organic azides to amines and in the 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.[14] In the proposed reaction mechanism
, the first proton is on loan from the methyl group in trimethylphosphine (triphenylphosphine does not react).

Reduction of activated carbonyl groups by alkyl phosphines

See also

References

  1. ISBN 978-0-470-66627-2
    .
  2. ^ G.M. Kosolapoff; L. Maier (1972). Organic Phosphorus Compounds, Volume 1. New York, N. Y.: John Wiley.
  3. ^
    doi:10.1002/0470862106.ia177
  4. PMID 17022105
    .
  5. hdl:2027/spo.5550190.0007.503
    .
  6. PMID 16551102
    .
  7. ISBN 9780470132593
    .
  8. S2CID 250775640
    .
  9. ISBN 978-3527306732
    .
  10. ISBN 9780080437484
    .
  11. ^ E. Fluck, The Chemistry of Phosphine, Topics in Current Chemistry Vol. 35, 64 pp, 1973.
  12. PMID 25504072
    .
  13. doi:10.1016/S0010-8545(98)00072-1
    .
  14. PMID 16518496
    .
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