Phosphorus acid tautomerism

H3PO3 is more clearly described with the structural formula HP0(0H)2. This species exists in equilibrium with a minor tautomer P(0H)3. IUPAC recommendations, 2005, are that the latter is called phosphorous acid, whereas the dihydroxy form is called phosphonic acid. Only the reduced phosphorus compounds are spelled with an "ous" ending. Other important oxyacids of phosphorus are phosphoric acid (H3PO4) and hypophosphorous acid (H3PO2). The reduced phosphorus acids are subject to similar tautomerism involving shifts of H between O and P. 

Detailed investigations of the chemical reactivity of the diketiminato-stabilized phosphenium cations like 28 (Scheme 17) are to date rare and include only two reports dealing with the substitution and reduction of P-halogen-derivatives. Thus, reaction of 28 (X=Br) with sodium hydroxide in toluene was reported to proceed with displacement of the halide substituent at phosphorus and conservation of the heterocyclic ring to give a mixture of bromide and triflate salts containing a P-hydroxy-substituted cation, both of which were isolated in small yields [89], The products are remarkable as they represent one of very few examples of a stable phosphinous acid which does not rearrange to the tautomeric secondary phosphine oxide. Potassium reduction of the P-chloro-substituted derivative 34 produced the... 

Coordination of the pyrophosphite occurs through the phosphorus, which is preferred over the harder oxygen by the soft Pt(II) ion. Thus there is a tautomeric relationship analogous to that in phosphorous acid ... 

F. Zecchini gave for the refraction eq. 26-04 with the / -formula, and 15-17 with the /i2-formula. 0. Stelling measured the X-ray K-absorption spectrum of phosphorous acid, and of its salts and di-esters, and found that the results agree with the assumption that the phosphorus is tervalent with the solids, and tautomeric quinque- and ter-valent in soln. Phosphorous acid, the di-esters, methyl phosphorous acid, and ferric monopropyl phosphate have the structure RO)2 PO.H sodium diethyl phosphite, (C2H50)2 PO.Na silver diethyl phosphate, (C2H50)2 PO.OAg and monoacetyl phosphorous acid ... 

Fosinopril is a pharmaceutical that, in addition to other chirality centres in the molecule, has a chirality centre at the phosphorus atom. In the determination of the absolute configuration at the phosphorus atom it should be noted that contributions of the phosphorus d-orbitals to the P-0 double bond are not considered. The doubly bonded oxygen atom has therefore a lower priority than the substituted oxygen atom. Note also that fosinopril is a pro-drug whose actual active form, fosinoprilat, results from hydrolysis of the acylal function in the body. It possesses only two chirality centres since the free phosphinic acid is as a consequence of tautomerism configurationally unstable. 

Intermediate stages in the reaction have been noted, such as the production of phosphoryl monochloride, POC1, with small amounts of water.8 The solution produced at first has stronger reducing power than the final solution, which was attributed to a first production of P(OH)3 with subsequent change to the tautomeric OPH(OH)2.9 In concentrated or hot solution subsequent decomposition of the phosphorous acid may take place with the production of phosphoric acid and red phosphorus, which change has been represented by the equation... 

In the hypophosphorous and phosphorous series the phosphorus undoubtedly is in a lower state of oxidation, and may be wholly in the trivalent state, which corresponds to a symmetrical structure of the molecules. More probably, however, these acids consist of a mixture containing the more symmetrical molecules in tautomeric equilibrium with less symmetrical molecules which contain hydrogen directly united to phosphorus. In either form the unsaturated acids and their salts are powerful reducing agents and are easily oxidised to the stable phosphate series. 

Hydroxy-2-phenylquinazolin-4(3//)-one and 3-hydroxy-2-phenylquinazoline-4(3//)-thione, which can be regarded as tautomeric structures of quinazolin-4-ol and quinazoline-4-thiol 3-oxide, are deoxygenated on heating with iodine and red phosphorus in acetic acid to 2-phenylquinazolin-4(3// )-one and 2-phenylquinazoline-4(3//)-thione, respectively. ... 

When an ester reacts with a ring-carbon atom in the presence of a Friedel-Crafts catalyst, a cyclic ketone (or its tautomeric enol) is usually formed (p. 378), but in boiling phosphorus oxychloride, the oxygen function may be replaced by a chlorine atom [2634]. A carboxylic acid may react with a suitably placed ring-carbon by heating with either PPA or phosphorus oxychloride [2127] phosphorus pentoxide in xylene is also effective [2347]. When an electron-rich ring such as that of pyrrole is available, heating with acetic anhydride for I h suffices [3913]. 

The tautomeric equilibrium between secondary phosphane sulphides HR2P=S and thiophosphinic acids R2PSH lies completely on the side of the former at room temperature, and P—H bond cleavage does not occur until over 180°C. ° The proton-transfer reaction can, however, be brought about much more readily by co-ordination to transition metals, which initially takes place via the sulphur atom. On raising the temperature, quantitative conversion into the phosphorus-bonded form occurs. 

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