Diphenyl-2-pyridylphosphine explained

Diphenyl-2-pyridylphosphine is an organophosphorus compound with the formula P(C₆H₅)₂(2-C₅H₄N). It is the most widely used mono-pyridylphosphine ligand.^[1] Other mono-pyridylphosphines ligands (3-, 4-) are not common in chemical literature; however, tris-pyridylphosphines have been thoroughly investigated as ligands in transition metal complexes used for catalysis. Pyridylphosphines, including diphenyl-2-pyridylphosphine, may bind transition metals as monodentate or bidentate ligands4. Diphenyl-2-pyridylphosphine behaves as a P-bound monodentate ligand, or a P,N-bound bidentate ligand. Diphenyl-2-pyridylphosphine is a sought after ligand for its ability to relay protons to transition metals such as palladium(II) in homogeneous catalysis.^[2]

Synthesis

Diphenyl-2-pyridylphosphine is prepared from 2-lithiopyridine with chlorodiphenylphosphine:^[3]

Diphenyl-2-pyridylphosphine is an integral ligand in the Pd(II) catalyzed carbonylation of alkynes. The pi-donor ability of one bidentate P,N-coordinated ligand is highly stabilizing to the metal center.^[4] While a second monodentate, N-protonated ligand transfers protons to the metal to be used in catalysis.^[4] The role of the pyridyl group in this catalytic cycle is evident when the ligand is replaced by triphenyl phosphine, and rates of catalysis a greatly decreased. This catalytic process is an important step in the production of polymers, and other fine chemicals.

RC₂H + CO + XH + Pd cat → RCCH₂COX + RCHCHCOX Pd cat = Pd(OAc)₂/Ph₂PPy/ CH₃SO₃HR=alkyl, arylX=OH, OR’, NR₂’Scheme 1: Carbonylation of alkynes by cationic Pd(II) catalyst with a diphenyl-2-pyridylphosphine ligand.

Further reading

• 10.1016/j.jorganchem.2008.09.063 . Protonation of palladium(II)-allyl and palladium(0)-olefin complexes containing 2-pyridyldiphenylphosphine . 2009 . Scrivanti . A. . Bertoldini . M. . Beghetto . V. . Matteoli . U. . Venzo . A. . Journal of Organometallic Chemistry . 694 . 131.

Notes and References

1. 10.1016/j.tetlet.2007.02.127 . Improved synthesis of monodentate and bidentate 2- and 3-pyridylphosphines . 2007 . Kluwer . Alexander M. . Ahmad . Irshad . Reek . Joost N.H. . Tetrahedron Letters . 48 . 17 . 2999.
2. 10.1016/0022-328X(94)84007-5 . Homogeneous catalysis by cationic palladium complexes. Precision catalysis in the carbonylation of alkynes . 1994 . Drent . E. . Arnoldy . P. . Budzelaar . P.H.M. . Journal of Organometallic Chemistry . 475 . 57.
3. 10.1021/om900324a . Investigation of the Dynamic Solution Behavior of Chloro(diene)rhodium(I) Phosphine Complexes with a Pendant Unsaturated Heterocycle at Phosphorus (2-pyridyl, 2-imidazyl; diene = COD, NBD) . 2009 . BrüCk . Andreas . Ruhland . Klaus . Organometallics . 28 . 22 . 6383.
4. Doherty, S . Knight, J . Bentham, M. . Chem. Commun. . 2006 . 88–90 . 10.1039/b512556a . The first insoluble polymer-bound palladium complexes of 2-pyridyldiphenylphosphine: Highly efficient catalysts for the alkoxycarbonylation of terminal alkynes.