Diphosphorus tetraiodide explained

Diphosphorus tetraiodide is an orange crystalline solid with the formula P₂I₄. It has been used as a reducing agent in organic chemistry. It is a rare example of a compound with phosphorus in the +2 oxidation state, and can be classified as a subhalide of phosphorus. It is the most stable of the diphosphorus tetrahalides.

Synthesis and structure

Diphosphorus tetraiodide is easily generated by the disproportionation of phosphorus triiodide in dry ether:

It can also be obtained by treating phosphorus trichloride and potassium iodide in anhydrous conditions.^[1]

Another synthesis route involves combining phosphonium iodide with iodine in a solution of carbon disulfide. An advantage of this route is that the resulting product is virtually free of impurities.^[2]

The compound adopts a centrosymmetric structure with a P-P bond of 2.230 Å.^[3]

Reactions

Inorganic chemistry

Diphosphorus tetraiodide reacts with bromine to form mixtures PI_3−xBr_x. With sulfur, it is oxidized to P₂S₂I₄, retaining the P-P bond. It reacts with elemental phosphorus and water to make phosphonium iodide, which is collected via sublimation at 80 °C.^[2]

Organic chemistry

Diphosphorus tetraiodide is used in organic synthesis mainly as a deoxygenating agent.^[4] It is used for deprotecting acetals and ketals to aldehydes and ketones, and for converting epoxides into alkenes and aldoximes into nitriles. It can also cyclize 2-aminoalcohols to aziridines^[5] and to convert α,β-unsaturated carboxylic acids to α,β-unsaturated bromides.^[6]

As foreshadowed by the work of Bertholet in 1855, diphosphorus tetraiodide can convert glycols to trans alkenes.^{[4] [7]} This reaction is known as the Kuhn–Winterstein reaction, after the chemists who applied it to the production of polyene chromophores.^{[4] [8]}

Notes and References

1. H. Suzuki . T. Fuchita . A. Iwasa . T. Mishina . December 1978 . Diphosphorus Tetraiodide as a Reagent for Converting Epoxides into Olefins, and Aldoximes into Nitriles under Mild Conditions . Synthesis . 1978. 12 . 905–908 . 10.1055/s-1978-24936 .
2. Brown. Glenn Halstead. 1951. Reactions of phosphine and phosphonium iodide. PhD. Iowa State College. 5 Oct 2020.
3. Z. Žák. M. Černík. Diphosphorus tetraiodide at 120 K. Acta Crystallographica Section C. 1996. C52. 2 . 290–291. 10.1107/S0108270195012510.
4. Encyclopedia: Encyclopedia for Reagents in Organic Synthesis 2009. 2009. Krief. Alain. Telvekar. Vikas N.. Diphosphorus Tetraiodide . Diphosphorus Tetraiodide. 10.1002/047084289X.rd448.pub2. 978-0471936237 .
5. H. Suzuki . H. Tani . 1984 . A mild cyclization of 2-aminoalcohols to aziridines using diphosphorus tetraiodide . Chemistry Letters . 13 . 12 . 2129–2130 . 10.1246/cl.1984.2129 .
6. Vikas N. Telvekar . Somsundaram N. Chettiar . June 2007 . A novel system for decarboxylative bromination . Tetrahedron Letters . 48 . 26 . 4529–4532 . 10.1016/j.tetlet.2007.04.137 .
7. Über konjugierte Doppelbindungen I. Synthese von Diphenyl-poly-enen . de . Conjugated double-bonds I: Synthesis of diphenyl-polyenes . Richard . Kuhn . Richard Kuhn . Alfred . Winterstein . . 11 . 1 . 87–116 . 1928 . 10.1002/hlca.19280110107.
8. Inhoffen. H. H.. Radscheit. K.. Stache. U.. Koppe. V.. 1965. Untersuchungen an hochsubstituierten äthylenen und Glykolen, II. Synthese des 3.4-Bis-[4-oxo-cyclohexyl]-hexens-(3) mit Hilfe der Kuhn-Winterstein-Reaktion. de. Experiments on highly-substituted ethenes and glycols II: Synthesis of 3,4-bis-[4-oxo-cyclohexyl]-3-hexane via the Kuhn-Winterstein reaction. Justus Liebigs Ann. Chem.. 684. 24–36. 10.1002/jlac.19656840106.