H-phosphonic acid

The diesters of H-phosphonic acid can be obtained by several synthetic procedures. This chapter outlines the most commonly nsed approaches in that respect. 

Saks et al. [27] used for the first time H-phosphonic add for the preparation of dialkyl H-phosphonates. There are a few patents devoted to the preparation of dialkyl H-phosphonates via direct esterification of H-phosphonic add with alcohols [28-31]. The process involves heating under reflux a mixture of H-phosphonic acid, an alcohol in excess of that reqnired... 

Diphenyl H-phosphonate is obtained by the treatment of H-phosphonic acid with a twofold excess of triphenyl phosphite [35-37]. 

Another approach employs treatment of H-phosphonic acid or its monoalkyl ester with a carboxylic acid anhydride and an alcohol at 20-50 C. Dimethyl H-phosphonate is obtained according to this procedure in quantitative yield based on the starting phospho-nic acid (See Appendix) [38]. 

Moreover, the monomethyl ester of H-phosphonic acid undergoes further rearrangement, yielding the monomethyl ester of the methyl phosphonic acid. 

Physical constants of some H-phosphonic acid diesters... 

General procedure for the preparation of dialkyl H-phosphonates by direct esterification of H-phosphonic acid (ref U.S. Pat. 3,725,515, 1973). 

Ethylhexanol-l (0.8 kg) was reflnxed while stirring. A mixtnre of 0.5 kg of H-phosphonic acid and 1.5 kg of 2-ethylhexanol-l (in a total excess of 45%) was added dropwise thereto within 4 h. The reaction temperatnre rose from 185 to 208 °C. Water (250 mL) was separated nsing a water separator. One gram of the cooled reaction mixture consumed 0.9 mL or 0.1 N sodium hydroxide solution. After the subsequent distillation, 93 g of octene (corresponding to a conversion rate of 2-ethylhexanol-l used at only 5%), unaltered excess 2-ethylhexanol-l, 150 g of di-(2-ethylhexyl)ether (corresponding to a conversion rate of 2-ethylhexanol-1 used at only 7.5%), and 1.68 kg of di-(2-ethylhexyl) H-phosphonate (boiling point 130-135 C/O.l mm Hg) were obtained in a yield of 90% of the theory. 

Although it was accepted that the diesters of H-phosphonic acid existed primarily in tautomeric form 2 [10, 11], in which the phosphorus atom is trivalent, it is now well established that these compounds exist in form 1, in which the phosphorus is tetravalent. The P H NMR spectra... 

The IR spectra of diesters of H-phosphonic acid contain characteristic bands attributed to the P=0, P-H, and P-O-C vibrations that are used in the structural characterization of this type of compound. The positions of the P-H and P=0 absorption bands depend largely on the type of the substituents R. More detailed studies of the vibrational spectra and force constants for some dialkyl H-phosphonate representatives have been reported recently [24]. 

The vibrational frequencies that correspond to these absorption bands for H-phosphonic acid diesters as well as the characteristic absorption bands of some of their representatives are summarized in Table Al. 

The second methoxy group hydrolyzes in the same way to yield H-phosphonic acid as a final product. The rate of the alkaline hydrolysis of dialkyl H-phosphonate depends on the substituent type at the phosphorus atom [18] (Table 3.2). 

Transesterification of H-phosphonate diesters is a simple synthetic route for the preparation of a variety of different esters of H-phosphonic acid (Scheme 3.1). 

It was found that the transesterification of dimethyl H-phosphonate with 1,2-propane-diol yields 4-methyl-2-hydro-2-oxo-l,3,2-dioxaphospholane [56]. Obviously, the first stage of the reaction furnished methyl-2-hydroxypropyl H-phosphonate. Subsequent intramolecular transesterification of the methyl-2-hydroxypropyl phosphonate yielded 4-methyl-2-hydro-2-oxo-l,3,2-dioxaphospholane. The specific reactivity of these esters of H-phosphonic acid is determined by the presence of a P-hydroxyl gronp. The role of the P-hydroxyl gronp may be regarded as an intramolecular catalysis. The reactivity enhancement of P-hydroxylethyl esters of H-phosphonic acid probably can be explained through hydrogen bonding, which favors the intramolecular transesterification reaction. 

Scheme 3.2 Synthesis of phosphorus-containing polyurethanes via transesterification of diesters of H-phosphonic acids with urethane diols.

The P-H group of diesters of H-phosphonic acid is highly reactive and participates in a variety of chemical reactions such as oxidation, addition to C=C double bond, C=N (Schiff bases), ketones, isocyanates, and condensation reactions. 

Owing to the presence of the P-H group, diesters of H-phosphonic acid can be oxidized by molecular oxygen to the corresponding phosphates [69]. 

Diesters of H-phosphonic acid react with sulfur to give dialkyl thiophosphates [78]. 

Direct azidation, cyanation, and thiocyanation of diesters of H-phosphonic acid were accomplished readily with sodium pseudohalides in acetonitrile under mild modified Atherton-Todd conditions [124]. 

According to NMR data, pyrocatechol H-phosphonic acid (R = 411 ) forms with unsaturated compounds addition products of the type I having phosphite character with phosphorus chemical shifts in the range 126-130 ppm, typical for such compounds. The... 

The only products formed in the reaction between dialkyl H-phosphonates and amines are alkylammonium salts of the monoalkyl ester of H-phosphonic acid I resulting from alkylation of the corresponding amine [279-285]. 

Alkylammonium salts of monoalkyl H-phosphonic acid, having at least one ethyl group in the cation, undergo Hoffman s elimination [283],... 

Hydrogen halides react with dialkyl H-phosphonates to form monoesters of H-phosphonic acid and alkyl halides as a result of nucleophiUc attack of the halide anion at the a-carbon atom of the alkoxy group [300]. 

The reaction between dialkyl H-phosphonates and Ce (HI) or La (III) chlorides furnishes the corresponding metal salts of the monoalkyl esters of H-phosphonic acid [302],... 

C H NMR data for some salts of H-phosphonic acid monoalkyl esters with general formula [RO-PHOO] A" ". Chemical shift for the a-carbon atoms of the corresponding dialkyl H-phosphonates are given in parentheses... 

It has been established that salts of monoalkyl H-phosphonic acids add more easily to double bonds than do the corresponding acids [306]. The addition takes place in the presence of different catalysts, and depending on their type, the reaction mechanism is either ionic or radical. Addition of alkali and ammonium salts of monoalkyl H-phosphonates to unsaturated compounds or ketones results in the formation of alkyl monoalkyl phosphonates. 

It was established that some ammonium halides dealkylate dialkyl H-phosphonates with the formation of the ammonium salts of the corresponding monoalkyl ester of H-phosphonic acid [307]. 

The addition of H-phosphonic acid diesters neat or in solvent to the aryUdenalkylamines [45] or to the Schiff base precursors (readily available by the condensation of primary with aldehydes) is one of the best synthetic routes to a-aminophosphonic acids [15,46-50],... 

AT-(phosphonomethyl) glycine has been obtained from diketopiperazine, formaldehyde, and H-phosphonic acid diesters followed by hydrolysis [85]. 

Heating various H-phosphonic acid diesters with a hydroxyalkylcarbamate mixture [87,88] yielded 3-ethyl-2-hydroxy-2-oxo-l,4,2-oxazaphosphorinane 1, which is water--soluble and hydrolytically stable. 

Bisphosphonates have been prepared by the reaction of a carboxylic acid with H-phosphonic acid and phosphorus trichloride [134]. 

Different methods are known for the synthesis of dialkyl 1,2- epoxyalkylphosphonates. There are excellent reviews by Redmore [250], Yamamoto [251], and loga [252] describing the present methods for the synthesis of epoxyphosphonates. In this book, the following methods for the preparation of epoxyphosphonates based on diesters of H-phosphonic acid and their derivatives will be included (i) reaction of sodium dialkyl H-phosphonates with a-halo ketones, (ii) reaction of dialkyl halohydrinphosphonates with bases, (iii) the Darens reaction of dialkyl chloromethylphosphonates with carbonyl compounds, (iv) oxidation of 1,2-unsaturated phosphonates with a peroxide. 

The sodium salts of dialkyl phosphite, except for the dimethyl homologue, react with dialkyl H-phosphonates to form dialkyl alkylphosphonate and the corresponding sodium salt of monoaUcyl H-phosphonic acid [372]. 

Although there are no X-ray structural data published yet for the above-type phosphonate salts, the structure of the closely related monoUthium salt of H-phosphonic acid has been already reported [391]. In this structure, the tetrahedral environment of the phosphorus center is completed by three oxygens and one hydrogen atom. The P-H bond length is 1.17 A and this hydrogen atom does not take part in a hydrogen-bond network. The specific NMR characteristics of these compounds are in most respects identical to those for the... 

A useful comparison is also the P NMR data for H-phosphonic acid and its mono- and disodium salts, summarized in Table 4.4. 

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