Hypophosphorous Acid1

H3PO2

[6303-21-5]  · H3O2P  · Hypophosphorous Acid  · (MW 66.00) (tautomer)

[14332-09-3]

(reduction of aromatic diazonium salts,1,2 nitro compounds,3 and pyrrole derivatives;4 synthesis of organic derivatives of hypophosphorous acid;1,5,6 generation of selenols7)

Alternate Name: HPA.

Physical Data: mp 26.5 °C; decomposes at 140 °C; d 1.493 g cm-3 (19 °C); pKa 1.1.

Solubility: sol water, alcohol, ether, dioxane.

Form Supplied in: widely available as 50% aqueous solutions (d 1.274 g mL-1).

Preparative Methods: the anhydrous acid is prepared from the commercial solution or from inorganic salts.1,8

Handling, Storage, and Precautions: decomposes upon heating above 140 °C into H3PO4 and poisonous, spontaneously flammable PH3. Slowly decomposes at rt. Air sensitive. Use in a fume hood.

Reduction of Arenediazonium Compounds.

Hypophosphorous acid is widely accepted as the preferred reagent for the reduction of diazonium salts.1,2,9 Copper(I) Oxide is a very effective catalyst of this reaction9 (eq 1). Dediazonation with HPA can also be used in Pschorr-type cyclizations.10

Alkylphosphinic Acids.

Radical addition of HPA or its alkali salt to alkenes is initiated by organic peroxides and gives phosphinic acid derivatives in good yields (eq 2).1,6,11,12 The alkene to HPA ratio controls the formation of alkyl- or dialkylphosphinic acid.1,11 Alkyl phosphinates also add to alkenes in the presence of peroxides.1 Alkylphosphinic acids can be prepared from HPA and alcohols,13 and alkenylphosphinic acids have been obtained from enol esters.14

Hydroxyalkylphosphinic Acids.

HPA reacts with aldehydes, ketones, and 1,2-ketones to provide 1-hydroxyalkylphosphinic acids (eq 3).1 When carbonyl compounds are used in excess, bis(1-hydroxyalkyl)phosphinic acids are formed.

1,2-Alkadienylphosphinic Acids.

Reactions of HPA with alkynic alcohols are accompanied by alkyne-allene rearrangement and lead to 1,2-alkadienylphosphinic acids (eq 4).15

Aminoalkylphosphinic Acids.

HPA reacts with azomethines under mild conditions, providing good yields of 1-(alkylamino)alkylphosphinic acids.1 Synthetic possibilities of this reaction have been extended by replacing the azomethines with a mixture of aldehyde or ketone and amine or hydrazine.6,16 Thus reaction of HPA with equimolar amounts of formaldehyde and secondary amines at rt in aqueous solution gives the corresponding dialkylaminophosphinic acids (eq 5). With an excess of amine and formaldehyde, bis(dialkylaminoalkyl)phosphinic acids are formed.1,16

Alkyl Hypophosphites.

A particularly easy preparation of alkyl hypophosphites involves the reaction of crystalline HPA with orthocarbonyl compounds (eq 6).5 Treatment of HPA with diazoalkanes also gives good yields of the desired esters.17 Reaction of HPA with orthoformates in the presence of p-Toluenesulfonic Acid leads to the formation of alkyl dialkoxymethylphosphinates.18

Reduction with Hypophosphorous Acid.

Palladium on Carbon catalyzed reduction with HPA converts the nitro group of arenes into an amino group,3 and quinones into hydroquinones.19 HPA in combination with Hydrogen Iodide is used for reduction and reductive alkylation of pyrrole derivatives.4

Selenols.

A commercial 50% solution of HPA is a convenient reagent for generation of selenols from diselenides or selenic acids.7


1. Yudelevich, V. I.; Sokolov, L. B.; Ionin, B. I. RCR 1980, 49, 46.
2. Wulfman, D. S. In The Chemistry of Diazonium and Diazo Groups; Patai, S., Ed.; Wiley: New York, 1978; Part 1, p 286. Fieser, M.; Fieser, L. F. FF 1967, 1, 489.
3. Nasielski, J.; Moucheron, C.; Nasielski-Hinkens, R. BSB 1992, 101, 491. Nasielski-Hinkens, R.; Leveque, P.; Castelet, D.; Nasielski, J. H 1987, 26, 2433.
4. Gregorovich, B. V.; Liang, K. S. Y.; Glugston, D. M.; MacDonald, S. F. CJC 1968, 46, 3291; Khan, S. A.; Plieninger, H. CB 1975, 108, 2475. Corbella, A.; Gariboldi, P.; Jommi, G.; Mauri, F. CI(L) 1969, 583.
5. Baudler, M. In Organic Phosphorus Compounds; Kosolapoff, G. M.; Maier, L., Eds.; Wiley: New York, 1973; p 1. Livantsov, M. V.; Prishchenko, A. A.; Lutsenko, I. F. JGU 1985, 55, 2226.
6. Kleiner, H.-J. MOC 1982, E1, 271.
7. Labar, D.; Krief, A.; Hevesi, L. TL 1978, 41, 3967. Synthetic Methods of Organic Chemistry; Theiheimer, W., Ed.; Karger: Basel, 1968; Vol. 22, p 19.
8. Handbook of Preparative Inorganic Chemistry; Brauer, G., Ed.; Academic: New York, 1963; p 555.
9. Korzeniowski, S. H.; Blum, L.; Gokel, G. W. JOC 1977, 42, 1469.
10. Dattolo, G.; Cirrincione, G.; Almerico, A. M.; Aiello, E.; D'Asdia, I. JHC 1986, 23, 1371.
11. Nifant'ev, E. E.; Magdeeva, R. K.; Dolidze, A. V.; Ingorokva, X. X.; Samkharadze, L. O.; Vasyanina, L. K.; Bekker, A. R. JGU 1991, 83.
12. Broan, C. J.; Cole, E.; Jankowski, K. J.; Parker, D.; Pulukkody, K.; Boyce, B. A.; Beeley, N. R. A.; Millar, K.; Millican, A. T. S 1992, 63.
13. Devedjiev, I.; Ganev, V.; Stefanova, R.; Borisov, G. PS 1987, 31, 7.
14. Holt, D. A.; Erb, J. M. TL 1989, 30, 5393.
15. Belakhov, V. V.; Yudelevich, V. I.; Komarov, E. V.; Ionin, B. I.; Petrov, A. A. JGU 1984, 920. Devedjiev, I.; Ganev, V.; Borisov, G.; Zabski, L.; Jedlinski, Z. PS 1989, 42, 167.
16. Dhawan, B.; Redmore, D. JCR(S) 1988, 34. Synthetic Methods of Organic Chemistry; Theiheimer, W., Ed.; Karger: Basel, 1971; Vol. 25, p 357. Kapura, A. A.; Shermergon, I. M. JGU 1989, 1137.
17. Kabachnik, M. J.; Shipov, A. E.; Mastrjukova, T. A. IZV 1960, 146.
18. Gallagher, M. J.; Honegger, H. AJC 1980, 33, 287. Bailie, A. C.; Cornell, C. L.; Wright, B. J.; Wright, K. TL 1992, 33, 5133.
19. Entwistle, I. D.; Johnstone, R. A. W.; Telford, R. P. JCR(S) 1977, 117.

Vladimir V. Popik

St. Petersburg State University, Russia



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