Hydrophosphination

Hydrophosphination is the insertion of a carbon-carbon multiple bond into a phosphorus-hydrogen bond forming a new phosphorus-carbon bond. Like other hydrofunctionalizations, the rate and regiochemistry of the insertion reaction is influenced by the catalyst. Catalysts take many forms, but most prevalent are bases and free-radical initiators.

Acid-base routes

The usual application of hydrophosphination involves reactions of phosphine (PH₃). Typically base-catalysis allows addition of Michael acceptors such as acrylonitrile to give tris(cyanoethyl)phosphine:
:PH₃ + 3 CH₂=CHZ → P(CH₂CH₂Z)₃ (Z = NO₂, CN, C(O)NH₂)

Acid catalysis is applicable to hydrophosphination with alkenes that form stable carbocations. These alkenes include isobutylene and many analogues:
:PH₃ + R₂C=CH₂ → R₂(CH₃)CPH₂ (R = Me, alkyl, etc)
Bases catalyze the addition of secondary phosphines to vinyldiphenylphosphine:
:HPR₂ + CH₂=CHPR'₂ → R₂PCH₂CH₂PR'₂

Free-radical methods

Many hydrophosphination reactions are initiated by free-radicals. AIBN and peroxides are typical initiators, as well as Ultraviolet irradiation. In this way, the commercially important tributylphosphine and trioctylphosphine are prepared in good yields from 1-butene and 1-octene, respectively.

The reactions proceed by abstraction of an H atom the phosphine precursor, producing the phosphino radical, a seven electron species. This radical then adds to the alkene, and subsequent H-atom transfer completes the cycle. Some highly efficient hydrophosphinations appear not to proceed via radicals, but alternative explanations are lacking.

Metal-catalyzed reactions

Metal-catalyzed hydrophosphinations are not widely used, although they have been extensively researched. Studies mainly focus on secondary and primary organophosphines (R₂PH and RPH₂, respectively). These substrates bind to metals, and the resulting adducts insert alkenes and alkynes into the P-H bonds via diverse mechanisms.

Early transition metal and lanthanide catalysts

Metal complexes of ''d''⁰ configurations are effective catalysts for hydrophosphinations of simple alkenes and alkynes. Intramolecular reactions are facile, e.g. starting with ''α,ω''-pentenylphosphine. The primary phosphine undergoes a ''σ''-bond metathesis with the bis(trimethylsilyl)methylene ligand forming the lanthanide-phosphido complex. Subsequently the pendant terminal alkene or alkyne inserts into the Ln-P bond. Finally, protonolysis of the Ln-C bond with the starting primary phosphine releases the new phosphine and regenerates the catalyst. Given that the metal is electron-poor, the M-C bond is sufficiently enough to be protonolyzed by the substrate primary phosphine.

Most metal catalyzed hydrophosphinations proceed via metal phosphido intermediates. Some however proceed by metal-phosphinidene intermediates, i.e. species with M=PR double bonds. One such example is the Ti-catalyzed hydrophosphination of diphenylacetylene with phenylphosphine. This system involves a cationic catalyst precursor that is stabilized by the bulky 2,4,6-tri(isopropyl)phenyl- substituent on the phosphinidene and the close ionic association of methyltris(pentafluorophenyl)borate. This precursor undergoes exchange with phenylphosphine to give the titanium-phenylphosphinidene complex, which is the catalyst. The Ti=PPh species undergoes a +2cycloaddition with diphenylacetylene to make the corresponding metallacyclobutene. The substrate, phenylphosphine, protonolyzes the Ti-C bond and after a proton shift regenerates the catalyst and releases the new phosphine.

Titanium-catalyzed 1,4-hydrophosphination of 1,3-dienes with diphenylphosphine has been demonstrated. It is a rare example of a ''d²'' catalyst. In the first step, the Ti(II) precursor inserted in the P-H bond of diphenylphosphine (Ph₂PH).

Late transition metal catalysts

Late transition metal hydrophosphination catalysts, i.e. those reliant on the nickel-triad and neighboring elements, generally require alkenes and alkynes with electron withdrawing substituents. A strong base is required as a cocatalyst.

Some late metal hydrophosphination catalysts proceed via oxidative addition of a P-H bond. For example, a Pt(0) catalyst undergoes oxidative addition of a secondary phosphine to form the corresponding hydrido Pt(II) phosphido complex. These systems catalyze hydrophosphination of acrylonitrile, although this reaction can be achieved without metal catalysts. The key P-C bond-forming step occurs through an outer-sphere, Michael-type addition.

The usual mechanism for hydrophosphination for late metal catalysts involves insertion of the alkene into the metal-phosphorus bond. Insertion into the metal-hydrogen bond is also possible. The product phosphine is produced through reductive elimination of a P-C bond rather than a P-H bond in Glueck's system. The Ni(0) catalyst involves oxidation addition of a P-H bond to the metal, followed by insertion of the alkene into the M-H bond.

Hydrophosphorylation and related reactions

Utilizing phosphorus(V) precursors hydrophosphorylation entails the insertion of alkenes and alkynes into the P-H bonds of secondary phosphine oxides:
:R₂P(O)H + CH₂=CHR → R₂P(O)CH₂CH₂R
The reaction can be effected both using metal catalysts or free-radical initiators.

Further reading

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References

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Addition reactions
Green chemistry
Organophosphanes
Stoichiometry