Aminophosphine

In organophosphorus chemistry, aminophosphines are compounds with the formula R_3−nP(NR₂)_n where R is a hydrogen or organic substituent, and n = 0, 1, or 2. At one extreme, the parents H₂PNH₂ and P(NH₂)₃ are lightly studied and fragile. At the other extreme, tris(dimethylamino)phosphine (P(NMe₂)₃) is commonly available. Intermediate members are known, such as Ph₂PN(H)Ph. Aminophosphines are typically colorless and reactive to oxygen. Aminophosphines are pyramidal geometry at phosphorus.[1]

Parent members

Structure of P(NMe₂)₃.

The aminophosphine called the Verkade base is a superbase.

The fundamental aminophosphines have the formulae PH_3−n(NH₂)_n (n = 1, 2, or 3). Fundamental aminophosphines can not be isolated in a practical quantities but have been examined theoretically. H₂NPH₂ is predicted to be more stable than the P(V) tautomer HN=PH₃.[2]

Secondary amines are more straightforward. Trisaminophosphines are made by treating phosphorus trichloride with secondary amines:

PCl₃ + 6 HNMe₂ → (Me₂N)₃P + 3 [H₂NMe₂]Cl

Aminophosphine chlorides

Structure of Me₂NPCl₂.

The amination of phosphorus trihalides occur sequentially, with each amination proceeding slower than before:[3]

PCl₃ + 2 HNMe₂ → Me₂NPCl₂ + [H₂NMe₂]Cl

Me₂NPCl₂ + 2 HNMe₂ → (Me₂N)₂PCl + [H₂NMe₂]Cl

(Me₂N)₂PCl + 2 HNMe₂ → (Me₂N)₃P + [H₂NMe₂]Cl

Monosubstitution selectivity improves with bulky amines such as diisopropylamine.[4] Commercially available aminophosphine chlorides include dimethylaminophosphorus dichloride and bis(dimethylamino)phosphorus chloride.

Methylamine and trifluorophosphine react to give the diphosphine MeN(PF₂)₂:

2 PF₃ + 3 MeNH₂ → MeN(PF₂)₂ + 2 [MeNH₃]F

Me(PF₂)₂ is a bridging ligand in organometallic chemistry.

Aminophosphines can also made from organophosphorus chlorides and amines.[5] Chlorodiphenylphosphine and diethylamine react to give an aminophosphine:[1][6]

Ph₂PCl + 2 HNEt₂ → Ph₂PNEt₂ + [H₂NEt₂]Cl

Primary amines react with phosphorus(III) chlorides to give aminophosphines with acidic α-NH centers:[7]

Ph₂PCl + 2 H₂NR → Ph₂PN(H)R + [H₃NR]Cl

Reactions

Protonolysis

Protic reagents attack the P-N bond. Alcoholysis readily occurs:

Ph₂PNEt₂ + ROH → Ph₂POR + HNEt₂

The P-N bond reverts to the chloride when treated with anhydrous hydrogen chloride:

Ph₂PNEt₂ + HCl → Ph₂PCl + HNEt₂

Transamination similarly converts one aminophosphine to another:

P(NMe₂)₃ + R₂NH ⇌ P(NR₂)(NMe₂)₂ + HNMe₂

With tris(dimethylamino)phosphine, dimethylamine evaporation can drive the equilibrium.[8]

Since Grignard reagents do not attack P-NR₂ bond, aminophosphine chlorides are useful reagents in preparing unsymmetrical tertiary phosphines. Illustrative is converting dimethylaminophosphorus dichloride to chlorodimethylphosphine:[9]

2 MeMgBr + Me₂NPCl₂ → Me₂NPMe₂ + 2 MgBrCl

Me₂NPMe₂ + 2 HCl → ClPMe₂ + Me₂NH₂Cl

Also, illustrative is the synthesis of 1,2-bis(dichlorophosphino)benzene using (Et₂N)₂PCl (Et = ethyl). This route gives C₆H₄[P(NEt₂)₂]₂, which is treated with hydrogen chloride:[10]

C₆H₄[P(NEt₂)₂]₂ + 8 HCl → C₆H₄(PCl₂)₂ + 4 Et₂NH₂Cl

Conversion to phosphenium salts

Diaminophosphorus chlorides and tris(dimethylamino)phosphine are precursors to phosphenium ions of the type [(R₂N)₂P]^+:[11][12]

R₂PCl + AlCl₃ → [R₂P^+]AlCl₄^−

P(NMe₂)₃ + 2 HOTf → [P(NMe₂)₂]OTf + [H₂NMe₂]OTf

Oxidation and quaternization

Typical aminophosphines oxidize. Alkylation, such as by methyl iodide, gives the phosphonium cation.

Addition to carbonyls

In diazaphospholenes the polarity of the P-H bond is inverted compared to traditional secondary phosphines. They have some hydridic character. One manifestation of this polarity is their reactivity toward benzophenone in yet another way.[13]

Diazaphospholene phosphine hydride

References

1. Mateo Alajarín; Carmen López-Leonardo; Pilar Llamas-Lorente (2005). "The Chemistry of Phosphinous Amides (Aminophosphanes): Old Reagents with New Applications". Top. Curr. Chem. Topics in Current Chemistry. 250: 77–106. doi:10.1007/b100982. ISBN 978-3-540-22498-3.
2. Sudhakar, Pamidighantam V.; Lammertsma, Koop (1991). "Nature of Bonding in Phosphazoylides. A Comparative Study of N₂H₄, NPH₄, and P₂H₄". Journal of the American Chemical Society. 113pages=1899–1906 (6): 1899–1906. doi:10.1021/ja00006a005.
3. Morse, J. G.; Cohn, K.; Rudolph, R. W.; Parry, R. W. (1967). "Substituted Difluoro- and Dichlorophosphines". Inorganic Syntheses. Inorganic Syntheses. Vol. 22. pp. 147–156. doi:10.1002/9780470132418.ch22. ISBN 9780470132418.
4. Denmark, Scott; Ryabchuk, Pavel; Min Chi, Hyung; Matviitsuk, Anastassia (2019). "Preparation of a Diisopropylselenophosphoramide Catalyst and its Use in Enantioselective Sulfenoetherification". Organic Syntheses. 96: 400–417. doi:10.15227/orgsyn.096.0400. PMC 8439352. PMID 34526731.
5. Agbossou, Francine; Carpentier, Jean-François; Hapiot, Frédéric; Suisse, Isabelle; Mortreux, André (1998). "The aminophosphine-phosphinites and related ligands: Synthesis, coordination chemistry and enantioselective catalysis1Dedicated to the memory of Professor Francis Petit". Coordination Chemistry Reviews. 178–180: 1615–1645. doi:10.1016/S0010-8545(98)00088-5.
6. Smith, Craig R.; Mans, Daniel J.; RajanBabu, T. V. (2008). "(R)-2,2'-Binaphthoyl-(S,s)-Di(1-Phenylethyl) Aminophosphine. Scalable Protocols for the Syntheses of Phosphoramidite (Feringa) Ligands". Organic Syntheses. 85: 238–247. doi:10.15227/orgsyn.085.0238. PMC 2719905. PMID 19655040.
7. Fei, Zhaofu; Dyson, Paul J. (2005). "The chemistry of phosphinoamides and related compounds". Coordination Chemistry Reviews. 249 (19–20): 2056–2074. doi:10.1016/j.ccr.2005.03.014.
8. Schmidt, H.; Lensink, C.; Xi, S. K.; Verkade, J. G. (1989). "New Prophosphatranes: Novel intermediates to five-coordinate phosphatranes". Zeitschrift für Anorganische und Allgemeine Chemie. 578: 75–80. doi:10.1002/zaac.19895780109.
9. Burg, Anton B.; Slota, Peter J. (1958). "Dimethylaminodimethylphosphine". Journal of the American Chemical Society. 80 (5): 1107–1109. doi:10.1021/ja01538a023.
10. Reetz, Manfred T.; Moulin, Dominique; Gosberg, Andreas (2001). "BINOL-Based Diphosphonites as Ligands in the Asymmetric Rh-Catalyzed Conjugate Addition of Arylboronic Acids". Organic Letters. 3 (25): 4083–4085. doi:10.1021/ol010219y. PMID 11735590.
11. Cowley, A. H.; Kemp, R. A. (1985-10-01). "Synthesis and reaction chemistry of stable two-coordinate phosphorus cations (phosphenium ions)". Chemical Reviews. 85 (5): 367–382. doi:10.1021/cr00069a002. ISSN 0009-2665.
12. Krannich, Larry K.; Kanjolia, Ravindra K.; Watkins, Charles L. (1985-09-02). "Synthesis and characterization of some aminophosphonium chlorides". Inorganica Chimica Acta. 103 (1): 1–8. doi:10.1016/S0020-1693(00)85202-0. ISSN 0020-1693.
13. Burck, S.; Gudat, D.; Nieger, M.; Du Mont, W.-W. (2006). "P-Hydrogen-Substituted 1,3,2-Diazaphospholenes: Molecular Hydrides". Journal of the American Chemical Society. 128 (12): 3946–3955. doi:10.1021/ja057827j. PMID 16551102.