Ethylene phosphate

For a number of reactions of cyclic di- and triesters of phosphoric acid, there are exchange data which can be rationalized on the assumption of trigonal bipyrami-dal intermediates which readily interconvert by pseudorotation. This constitutes a strong argument that at least these cyclic esters react by an associative mechanism and is suggestive evidence that simple trialkyl phosphates also react by this mechanism. The pH dependence of exocyclic versus endocyclic cleavage of methyl ethylene phosphate is readily interpreted in terms of the effect of ionization of the intermediate on the pseudorotation of these pentacoordinate intermediates. ... 

The equilibrium interconversion between an ethylene phosphite and a bicyclic spirophosphorane is shown to proceed by the insertion of the phosphite into the labile O-H bond of the hydroxyethyl ester. The mechanism is similar to the insertion of carbenes or nitrenes. Energy relationships of reaction intermediates were studied by MO RHF, MP2(full), MP4SDTQ, and DFT calculations. In most cases, they predicted that hydroxyethyl ethylene phosphates were more stable than the strained spirophosphoranes, which is not supported by the experimental evidence. The best correspondence to experimental data was obtained by DFT calculations with Perdew-Wang correlation functions <2003JST35>. 

Ah initio calculations to map out the gas-phase activation free energy profiles of the reactions of trimethyl phosphate (TMP) (246) with three nucleophiles, HO, MeO and F have been carried out. The calculations revealed, inter alia, a novel activation free-energy pathway for HO attack on TMP in the gas phase in which initial addition at phosphorus is followed by pseudorotation and subsequent elimination with simultaneous intramolecular proton transfer. Ah initio calculations and continuum dielectric methods have been employed to map out the lowest activation free-energy profiles for the alkaline hydrolysis of a five-membered cyclic phosphate, methyl ethylene phosphate (247), its acyclic analogue, trimethyl phosphate (246), and its six-membered ring counterpart, methyl propylene phosphate (248). The rate-limiting step for the three reactions was found to be hydroxyl ion attack at the phosphorus atom of the triester. ... 

The hydrolysis in acid or base of five-membered cyclic esters of phosphoric acid proceed some 10 times faster than their acyclic analogs or the six- and seven-membered cyclic phosphates 55 56. Unlike that of dimethyl phosphate, the hydrolysis of hydrogen ethylene phosphate is accompanied by rapid oxygen exchange into the unreacted substrate57, viz-... 

Before returning to the ethylene phosphate problem, let us review several additional observations derived from the hydrolysis of five-membered phos-tonates which are pertinent to the application of the combined concepts of ring strain and pseudorotation. The rate of hydrolysis of the five-membered propylphostonate acid (18),... 

Fig, 5. pH-product profile for the hydrolysis of methyl ethylene phosphate in water at 25°C. 

Species (23) and (24) function as the intermediates for the water-catalyzed hydrolysis of methyl ethylene phosphate which gives 25% exocyclic cleavage. Ionization to generate (26) followed by pseudorotation to (27), viz. 

Whereas the phosphetanium (Sect. 2.2) derivatives undergo base-catalyzed hydrolysis with essentially complete retention of configuration, the reaction with the related thietanium salts proceeds with complete inversion of configuration and can be described simply as a direct nucleophilic substitution 35. As far as the author is aware, no exchange evidence of the type er countered in the acid-catalyzed hydrolysis of ethylene phosphate, has been found for the cyclic sulfate esters. Consequently despite the geometrical similarities between the cyclic sulfates and phosphates and the related hybridization of the central atoms, the situation in which a finite pentacovalent intermediate sulfur species exists has not been delineated (restricting the discussion to esters). 

This pathway is favorable because the lowest empty molecular orbital of the cy. lic methyl ethylene phosphate ester consists principally of lobes on the back sides of the P-0 ring bonds which can readily interact with the approaching electron pair from water. 

Several commercial products have resulted from our phosphorus oligomer research. Fyrol 99, a 2-chloroethyl ethylene phosphate oligomer, has been successfully used as a flame retardant additive in rebonded urethane foam, in thermoset resins, in intumes-cent coatings, adhesives, paper air filters (13), and related uses. This product is less volatile and has a higher flame retardant efficacy than the parent compound tris(2-chloroethyl) phosphate. A related product was developed especially for use in flexible polyurethane foams. A vinylphosphonate/methylphospho-... 

Lim, C. and Tole, P. (1992) Concerted Hydroxl Ion Attack and Pseudorotation in the Base-Catalyzed Hydrolysis of Methyl Ethylene Phosphate, J. Phys. Chem. 96, 5217-5219. 

Using a phosphodiesterase, it has been possible to measure directly the enthalpies of hydrolysis of ethylene phosphate, trimethylene phosphate, and diethyl phosphate (all as their sodium salts). As expected, that of the first was found to be appreciably more negative than the latter two. 

Associative mechanisms may include the formation of discrete trigonal bi-pyramidal addition intermediates. In the case of the hydrolysis of five-membered ring cyclic phosphodiesters and phosphotriesters, the pentavalent adducts certainly are intermediates (61). The five-membered ring cyclic esters undergo hydrolysis in acid or base much faster than noncyclic esters such as dibenzyl phosphate because the five-membered rings are strained, and this strain is relieved by the formation of trigonal bipyramidal adducts. An example of the intermediate formation of a pentavalent intermediate is the alkaline hydrolysis of ethylene phosphate shown in reaction (41). 

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