hydrophosphination

Although most of this article concerns addition of P-H bonds to alkenes, an important variant is the hydrophosphination of formaldehyde. Tetrakis(hydroxymethyl)phosphonium chloride (THPC) is prepared as follows from phosphine:

:PH₃ + 4 H₂C=O + HCl → [P(CH₂OH)₄]Cl

It is a white water-soluble salt with applications as a precursor to fire-retardant materials{{Ullmann|doi=10.1002/14356007.a19_545.pub2|title=Phosphorus Compounds, Organic|year=2006|last1=Svara|first1=Jürgen|last2=Weferling|first2=Norbert|last3=Hofmann|first3=Thomas|isbn=3527306730}} and as a microbiocide in commercial and industrial water systems. The hydroxymethyl groups on THPC undergo replacement reactions when THPC is treated with α,β-unsaturated nitrile, acid, amide, and epoxides. For example, base induces condensation between THPC and acrylamide with displacement of the hydroxymethyl groups. (Z = CONH₂)

:[P(CH₂OH)₄]Cl + NaOH + 3 CH₂=CHZ → P(CH₂CH₂Z)₃ + 4 CH₂O + H₂O + NaCl

Similar reactions occur when THPC is treated with acrylic acid; only one hydroxymethyl group is displaced, however.{{cite journal | author = Vullo, W. J. | title = Hydroxymethyl Replacement Reactions of Tetrakis(hydroxymethyl)phosphonium Chloride |journal=Ind. Eng. Chem. Prod. Res. Dev.| year = 1966 | volume = 58 | issue = 4 | pages = 346–349 | doi = 10.1021/i360020a011}}

THPC converts to tris(hydroxymethyl)phosphine upon treatment with aqueous sodium hydroxide:M. Caporali, L. Gonsalvi, F. Zanobini, M. Peruzzini "Synthesis of the Water-Soluble Bidentate (P,N) Ligand PTN(Me)" Inorg. Syntheses, 2011, Vol. 35, p. 92–108. {{doi|10.1002/9780470651568.ch5}}

:[P(CH₂OH)₄]Cl + NaOH → P(CH₂OH)₃ + H₂O + H₂C=O + NaCl

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:{{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 }}

: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:{{cite journal|doi=10.1021/ar50053a003|title=Poly(tertiary Phosphines) and Their Metal Complexes|journal=Accounts of Chemical Research|volume=5|pages=177–185|year=1972|last1=King|first1=R. Bruce|issue=5}}

:HPR₂ + CH₂=CHPR'₂ → R₂PCH₂CH₂PR'₂

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.

File:Radical hydrophosphination.png

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.Quin, L. D. A Guide to Organophosphorus Chemistry; John Wiley & Sons: New York, 2000; pp 28-29. Some highly efficient hydrophosphinations appear not to proceed via radicals, but alternative explanations are lacking.{{cite journal|doi=10.1039/c2gc35898k|title=Solvent- and catalyst-free regioselective hydrophosphanation of alkenes|journal=Green Chemistry|volume=14|pages=2699|year=2012|last1=Alonso|first1=Francisco|last2=Moglie|first2=Yanina|last3=Radivoy|first3=Gabriel|last4=Yus|first4=Miguel|issue=10|hdl=11336/95357|hdl-access=free}}

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.{{Cite journal | doi = 10.1039/B612306F| pmid = 18648674| title = Stoichiometric and catalytic activation of P–H and P–P bonds| journal = Chemical Society Reviews| volume = 37| issue = 8| pages = 1482–1489| year = 2008| last1 = Greenberg | first1 = S. | last2 = Stephan | first2 = D. W. }}{{Cite book | doi = 10.1007/978-3-642-12073-2_4| chapter = Recent Advances in Metal-Catalyzed C–P Bond Formation| title = C-X Bond Formation| volume = 31| pages = 65–100| series = Topics in Organometallic Chemistry| year = 2010| last1 = Glueck| first1 = David S.| isbn = 978-3-642-12072-5}}Rosenberg, L. R. ACS Catal. 2013, 3, 2845.{{Cite journal | doi = 10.1021/cs400685c| title = Mechanisms of Metal-Catalyzed Hydrophosphination of Alkenes and Alkynes| journal = ACS Catalysis| volume = 3| issue = 12| pages = 2845–2855| year = 2013| last1 = Rosenberg | first1 = L. }}

File:Ln cycle.png

Metal complexes of d⁰ configurations are effective catalysts for hydrophosphinations of simple alkenes and alkynes.{{cite journal|doi=10.1002/chem.201602749|pmid=27405918|title=Challenges in Catalytic Hydrophosphination|journal=Chemistry - A European Journal|volume=22|issue=36|pages=12598–12605|year=2016|last1=Bange|first1=Christine A.|last2=Waterman|first2=Rory}}

{{cite journal|doi=10.1039/C6DT03913H|pmid=27891536|title=Use of organolanthanides in the catalytic intermolecular hydrophosphination and hydroamination of multiple C–C bonds|journal=Dalton Transactions|volume=45|issue=48|pages=19172–19193|year=2016|last1=Trifonov|first1=A. A.|last2=Basalov|first2=I. V.|last3=Kissel|first3=A. A.}} 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.{{Cite journal | doi = 10.1021/ja064853o| pmid = 17031972| title = Neutral and Zwitterionic Low-Coordinate Titanium Complexes Bearing the Terminal Phosphinidene Functionality. Structural, Spectroscopic, Theoretical, and Catalytic Studies Addressing the Ti−P Multiple Bond| journal = Journal of the American Chemical Society| volume = 128| issue = 41| pages = 13575–85| year = 2006| last1 = Zhao | first1 = G. | last2 = Basuli | first2 = F. | last3 = Kilgore | first3 = U. J. | last4 = Fan | first4 = H. | last5 = Aneetha | first5 = H. | last6 = Huffman | first6 = J. C. | last7 = Wu | first7 = G. | last8 = Mindiola | first8 = D. J. }} 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 [2+2] cycloaddition 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.{{Cite journal | doi = 10.1002/chem.200901863| pmid = 19918817| title = First Titanium-Catalyzed 1,4-Hydrophosphination of 1,3-Dienes| journal = Chemistry – A European Journal| volume = 16| issue = 1| pages = 64–67| year = 2010| last1 = Perrier | first1 = A. | last2 = Comte | first2 = V. | last3 = Moïse | first3 = C. | last4 = Le Gendre | first4 = P. }} 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 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.

File:Pt cycle.png

File:Nickel catalyst.png

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.{{Cite journal | doi = 10.1021/ol017238s| pmid = 11869121| title = Catalytic Hydrophosphination of Styrenes| journal = Organic Letters| volume = 4| issue = 5| pages = 761–3| year = 2002| last1 = Shulyupin | first1 = M. O. | last2 = Kazankova | first2 = M. A. | last3 = Beletskaya | first3 = I. P. }}{{Cite journal | doi = 10.1023/A:1022552404812| year = 2002| last1 = Kazankova | first1 = M. A.| journal = Russian Journal of Organic Chemistry| volume = 38| issue = 10| pages = 1479| last2 = Shulyupin | first2 = M. O.| last3 = Borisenko | first3 = A. A.| last4 = Beletskaya | first4 = I. P.|title=Synthesis of Alkyl(diphenyl)phosphines by Hydrophosphination of Vinylarenes Catalyzed by Transition Metal Complexes| s2cid = 94929172}} 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.

Calcium-catalysed intermolecular hydrophosphination is known, using a β-diketiminato complex.{{cite journal |last1=Crimmin |first1=Mark |last2=Barrett |first2=Anthony |last3=Hill |first3=Michael |last4=Hitchcock |first4=Peter |last5=Procopiou |first5=Panayiotis |title=Calcium-Catalyzed Intermolecular Hydrophosphination |journal=Organometallics |date=2007 |volume=26 |issue=12 |pages=2953–2956 |doi=10.1021/om070200k |url=https://pubs.acs.org/doi/10.1021/om070200k |access-date=3 March 2025|url-access=subscription }}

Utilizing phosphorus(V) precursors hydrophosphorylation entails the insertion of alkenes and alkynes into the P-H bonds of secondary phosphine oxides:{{cite journal|title=Highly Selective Markovnikov Addition of Hypervalent H-Spirophosphoranes to Alkynes Mediated by Palladium Acetate: Generality and Mechanism|first1=Li-Biao|last1=Han|first2=Yutaka|last2=Ono|first3=Qing|last3=Xu|first4=Shigeru |last4=Shimada|journal=Bulletin of the Chemical Society of Japan|year=2010|volume=83|issue=9|pages=1086–1099|doi=10.1246/bcsj.20100141}}

:R₂P(O)H + CH₂=CHR → R₂P(O)CH₂CH₂R

The reaction can be effected both using metal catalysts or free-radical initiators.

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Category:Addition reactions

Category:Green chemistry

Category:Organophosphanes

Category:Stoichiometry