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Hydrophosphination of an alkene.

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.^[1] Most hydrophosphinations involve reactions of phosphine (PH3).^[2]

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:^[2]

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:^[2]

PH₃ + R₂C=CH₂ → R₂(CH₃)CPH₂ (R = Me, alkyl, etc)

Bases catalyze the addition of secondary phosphines to vinyldiphenylphosphine:^[3]

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.^[2]

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.^[4] Some highly efficient hydrophosphinations appear not to proceed via radicals, but alternative explanations are lacking.^[5]

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.^[6]^[7]^[8]

Early transition metal and lanthanide catalysts

Metal complexes of d⁰ configurations are effective catalysts for hydrophosphinations of simple alkenes and alkynes.^[9]^[10] 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.^[11] 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.^[12] 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.^[7]

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.^[7]

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.^[13]^[14] 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:^[15]

R₂P(O)H + CH₂=CHR → R₂P(O)CH₂CH₂R

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

References

^ Svara, Jürgen; Weferling, Norbert; Hofmann, Thomas (2006). "Phosphorus Compounds, Organic". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a19_545.pub2. ISBN 3527306730.
^ ^a ^b ^c ^d Trofimov, Boris A.; Arbuzova, Svetlana N.; Gusarova, Nina K. (1999). "Phosphine in the synthesis of organophosphorus compounds". Russian Chemical Reviews. 68 (3): 215–227. Bibcode:1999RuCRv..68..215T. doi:10.1070/RC1999v068n03ABEH000464. S2CID 250775640.
^ King, R. Bruce (1972). "Poly(tertiary Phosphines) and Their Metal Complexes". Accounts of Chemical Research. 5 (5): 177–185. doi:10.1021/ar50053a003.
^ Quin, L. D. A Guide to Organophosphorus Chemistry; John Wiley & Sons: New York, 2000; pp 28-29.
^ Alonso, Francisco; Moglie, Yanina; Radivoy, Gabriel; Yus, Miguel (2012). "Solvent- and catalyst-free regioselective hydrophosphanation of alkenes". Green Chemistry. 14 (10): 2699. doi:10.1039/c2gc35898k. hdl:11336/95357.
^ Greenberg, S.; Stephan, D. W. (2008). "Stoichiometric and catalytic activation of P–H and P–P bonds". Chemical Society Reviews. 37 (8): 1482–1489. doi:10.1039/B612306F. PMID 18648674.
^ ^a ^b ^c Glueck, David S. (2010). "Recent Advances in Metal-Catalyzed C–P Bond Formation". C-X Bond Formation. Topics in Organometallic Chemistry. Vol. 31. pp. 65–100. doi:10.1007/978-3-642-12073-2_4. ISBN 978-3-642-12072-5.
^ Rosenberg, L. R. ACS Catal. 2013, 3, 2845.Rosenberg, L. (2013). "Mechanisms of Metal-Catalyzed Hydrophosphination of Alkenes and Alkynes". ACS Catalysis. 3 (12): 2845–2855. doi:10.1021/cs400685c.
^ Bange, Christine A.; Waterman, Rory (2016). "Challenges in Catalytic Hydrophosphination". Chemistry - A European Journal. 22 (36): 12598–12605. doi:10.1002/chem.201602749. PMID 27405918.
^ Trifonov, A. A.; Basalov, I. V.; Kissel, A. A. (2016). "Use of organolanthanides in the catalytic intermolecular hydrophosphination and hydroamination of multiple C–C bonds". Dalton Transactions. 45 (48): 19172–19193. doi:10.1039/C6DT03913H. PMID 27891536.
^ Zhao, G.; Basuli, F.; Kilgore, U. J.; Fan, H.; Aneetha, H.; Huffman, J. C.; Wu, G.; Mindiola, D. J. (2006). "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 of the American Chemical Society. 128 (41): 13575–85. doi:10.1021/ja064853o. PMID 17031972.
^ Perrier, A.; Comte, V.; Moïse, C.; Le Gendre, P. (2010). "First Titanium-Catalyzed 1,4-Hydrophosphination of 1,3-Dienes". Chemistry – A European Journal. 16 (1): 64–67. doi:10.1002/chem.200901863. PMID 19918817.
^ Shulyupin, M. O.; Kazankova, M. A.; Beletskaya, I. P. (2002). "Catalytic Hydrophosphination of Styrenes". Organic Letters. 4 (5): 761–3. doi:10.1021/ol017238s. PMID 11869121.
^ Kazankova, M. A.; Shulyupin, M. O.; Borisenko, A. A.; Beletskaya, I. P. (2002). "Synthesis of Alkyl(diphenyl)phosphines by Hydrophosphination of Vinylarenes Catalyzed by Transition Metal Complexes". Russian Journal of Organic Chemistry. 38 (10): 1479. doi:10.1023/A:1022552404812. S2CID 94929172.
^ Han, Li-Biao; Ono, Yutaka; Xu, Qing; Shimada, Shigeru (2010). "Highly Selective Markovnikov Addition of Hypervalent H-Spirophosphoranes to Alkynes Mediated by Palladium Acetate: Generality and Mechanism". Bulletin of the Chemical Society of Japan. 83 (9): 1086–1099. doi:10.1246/bcsj.20100141.