Diesters of 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. 

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]. 

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]. 

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 properties of polymers are hardly affected by the incorporation of phosphorus. Diesters of H-phosphonic acid and their inunediate derivatives have found a number of applications in polymer synthesis such as flame retardants, antioxidants, heat and Ught stabilizers, catalysts, degrading agents, and alkylating agents. They are also used as corrosion inhibitors, scale inhibitors, and lubricants (antiwear and load-carrying additives). 

This observation gives grounds for the assumption that diesters of H-phosphonic acid can participate in exchange reactions with urethane, carbonate, and amide groups. 

The book contains chapters dealing with physical and spectral properties ( H, C, and NMR data) characteristic reactions important classes of compounds based on the esters of H-phosphonic add their application as physiologically active substances, flame retardants, catalysts, heat, and light stabilizers lubricants scale inhibitors polymer carriers of drugs preparation of diesters of H-phosphonic acid, and general procedures for conducting the most important reactions. 

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). 

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],... 

Internal salts of H-phosphonic acid diesters with the general formula given below possess plant preservative properties against pathogenesis caused by fungicides [12]. 

The H-phosphonate approach entails phosphitylation of a hydroxylic component with an activated unprotected derivative of phosphonic acid to furnish H-phosphonate (a stable tetra-coordinate form of the unprotected monophosphite) followed by oxidation on phosphorus. The H-phosphonate approach is not generally used for the synthesis of monophosphates due to the difficulty of oxidation of the H-phosphonate monoesters and the necessity either to render the phosphorus atom of H-phosphonate into the three-coordinate form by silylation (to form disilylphosphite) or to introduce a protecting group (to form H-phosphonate diester) prior to oxidation. Nevertheless, in rare cases, the H-phosphonate procedure turns out to be a method of choice if, for instance, the simultaneous oxidation of functional groups other than H-phosphonate is required.51... 

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

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. 

This implies that these compounds have P-H type acidity and are therefore considerably less acidic than the corresponding P-OH type acids. It was established by means of P NMR spectroscopy that in dimethoxy ethane, the acidity of dibutyl hydrogen phosphonate is close to that of ethanol, and that these P-H acids are stronger than the corresponding N-H acids [3]. The pK values of a series of H-phosphonate diesters have been calculated by the so-called premetallization method [4], according to the following equation ... 

The transesteiification is carried out at elevated temperatures, usually in the range between 95 and 180 °C, depending on the type of H-phosphonate diesters, and proceeds both in the absence and in the presence of a catalyst (an acid or a base). The commonly used basic catalysts are alkah metals, alkaU alkoxides, and tertiary amines, whereas H3P04and CH3COOH are employed in the acid catalysis. 

Scheme 5.1 Chemical degradation of microporous polyurethane elastomer with diesters of the H-phosphonic acid.

In 2011, Zhuang et al. [133] reported a Chan-Lam-Evans version of the arylation of H-phosphonate diesters using CUjO/phen with arylboronic acids. The reaction was conducted in air... 

Diesters of phosphorous acid are in general neutral because the phosphorous acid exists mostly in the phosphonate form with one hydrogen directly attached to the phosphorus. But with alkali metals the H can be changed against the alkali and reactive intermediates formed. Such alkali metal derivatives of dialkyl phosphites react with alkyl halides to give dialkyl alkanephosphonates, according to Eqs. (45) and (46). 

In turn, for the phosphonic acid diester 95, stereoselectivity of the native phosphodiesterase was enhanced by over three orders of magnitude by alteration of the pA a values of the leaving group phenol. Eor example, for X = C02Me, Y = H the stereoselectivity was 5000 higher than for X = NO2, Y = 2-F. ... 

Phosphonamidates are usually prepared by coupling an activated phosphonate monoester with the amine of interest. Since most phosphonate amino add analogues are prepared as their diesters, this procedure first requires selective cleavage to the monoester. Activation from the phosphonic acid oxidation level typically involves preparation of the phosphono-chloridate, but it can also be accomplished by chlorination of an H-phosphinic acid analogue. 

The addition of dialkyl hydrogenphosphonates to tetrafluoroethene occurs in the presence of di- r butyl peroxide to give the series of polyfluoroalkyl phosphonic diesters, H(CF2CF2) P(0)(0R)2 (n = 1-3) the free acids with n = 1-5 and the corresponding phosphonic dichlorides with n = 1-9 have also been recorded ... 

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