Metaphosphate, intermediates

The intermediate metaphosphate/phosphonate can then be made to react with ethylene oxide to effect the insertion of ethyleneoxy units into the P-O-P linkages. If a limited amount of water or an alcohol is added to the meta intermediate, the resultant oligomer can be produced with a controlled hydroxyl functionality (11, 12). An example of a functional oligomer made this way is the following ... 

Some examples of rate and binding constants for these micelle-assisted reactions are in Table 6. There are very large differences in k j /k y/ for these reactions, but the rate effects on decarboxylation are large and depend upon the charge on the head group. Reaction of 2,4-dinitrophenyl phosphate is often written as generating intermediate metaphosphate ion, but this species is so short-lived that reaction follows an enforced association mechanism (Buchwald and Knowles, 1982). 

Reactions of phosphate monoester dianions and of phosphorylated pyridines with water and other nucleophiles proceed through a mechanism that shows many of the characteristics expected for reaction through an intermediate metaphosphate monoanion, and several investigators (3-5) have suggested that these and related reactions proceed through such an intermediate. However, recent work has shown that metaphosphate, if it is formed, has too short a lifetime in several hydroxylic solvents to diffuse through the solvent before reaction, so that the reaction must occur by a stepwise or concerted preassociation mechanism. Methanolysis of the 2,4-dinitrophenyl phosphate dianion occurs with inversion, for example (6). 

TrialkylPhosphates. Triethyl phosphate [78-40-0] C H O P, is a colorless Hquid boiling at 209—218°C containing 17 wt % phosphoms. It may be manufactured from diethyl ether and phosphoms pentoxide via a metaphosphate intermediate (63,64). Triethyl phosphate has been used commercially as an additive for polyester laminates and in ceHulosics. In polyester resins, it functions as a viscosity depressant as weH as a flame retardant. The viscosity depressant effect of triethyl phosphate in polyester resins permits high loadings of alumina trihydrate, a fire-retardant smoke-suppressant filler (65,66). 

Phosphate esters have a variety of mechanistic paths for hydrolysis. Both C-O and P-0 cleavage are possible depending on the situation. A phosphate monoanion is a reasonable leaving group for nucleophilic substitution at carbon and so 8 2 or SnI reactions of neutral phosphate esters are well known. PO cleavage can occur by associative (by way of a pentacoordinate intermediate), dissociative (by way of a metaphosphate species), or concerted (avoiding both of these intermediates) mechanisms. 

J. P. Guthrie, Hydration and Dehydration of Phosphoric Acid Derivatives Free Energies of Formation of the Pentacoordinate Intermediates for Phosphate Ester Hydrolysis and of Monomeric Metaphosphate, J. Am. Chem. Soc. 1977, 99, 3391. 

Many mechanistic aspects of the hydrolysis of phosphate esters in protic media remain uncertain. In spite of predictions that racemization at phosphorus should be the final outcome if indeed the (hypothetical) metaphosphate intermediate is involved in the solvolysis of monoesters, the results of several studies on the methanolysis of appropriately O-isotopically labelled compounds are consistent with reactions proceeding with inversion of configuration, as observed for all enzymic and non-enzymic systems so far examined this has resulted in the suggestion that if metaphosphate is actually formed, then it must be in a masked form. 

Compound (32) phosphorylates hindered alcohols in the presence of EtOH consistent with a dissociative mechanism involving a metaphosphate-like intermediate,since it proceeds with considerable racemization at phosphorus(Scheme 6). The extent of phosphoryl transfer which proceeds with retention of configuration is ca. 357, (i e. ca. 707. racemization) the excess of (S)p configuration would arise from transfer with configurational inversion, and might indicate a relatively free ... 

The involvement of monomeric metaphosphate in the phosphoryl transfer from phosphate monoesters, and of pentaco-ordinate intermediates from phosphotriesters represent two extremes in the mechanistics of the phosphoryl transfer process. Between the extremes are the (S 2)p processes involving transition states having various bond orders, but no true... 

The monomeric metaphosphate ion itself commands a fair amount of attention in discussions of metaphosphates. It is postulated as an intermediate of numerous hydrolysis reactions of phosphoric esters 52 S4,S5) and also of phosphorylation reactions S6> kinetic and mechanistic studies demonstrate the plausibility of such an assumption. In addition, the transient formation of ester derivatives of meta-phosphoric acid — in which the double-bonded oxygen can also be replaced by thio and imino — has also been observed they were detected mainly on the basis of the electrophilic nature of the phosphorus. 

Doubts have recently been expressed regarding the validity of the metaphosphate pathway for hydrolysis of the monoanion of 2,4-dinitrophenyl phosphate (111) 70,71,72) since the basicity of the 2,4-dinitrophenolate group is insufficient to produce a zwitterion corresponding to 106 or even a proton transfer via intermediates of type 103 or 105 (pKa values in water 4.07 for 2,4-dinitrophenol, 1.0 and 4.6 for 2,4-dinitrophenyl phosphate). Instead, hydrolysis and phosphorylation reactions of the anion 111 are formulated via oxyphosphorane intermediates according to 114. 

The reaction of 151 with methanol to give dimethyl phosphate (154) or with N-methylaniline to form the phosphoramidate 155 and (presumably) the pyrophosphate 156 complies with expectations. The formation of dimethyl phosphate does not constitute, however, reliable evidence for the formation of intermediate 151 since methanol can also react with polymeric metaphosphates to give dimethyl phosphate. On the other hand, reaction of polyphosphates with N-methylaniline to give 156 can be ruled out (control experiments). The formation of 156 might encourage speculations whether the reaction with N,N-diethylaniline might involve initial preferential reaction of monomeric methyl metaphosphate via interaction with the nitrogen lone pair to form a phosphoric ester amide which is cleaved to phosphates or pyrophosphates on subsequent work-up (water, methanol). Such a reaction route would at least explain the low extent of electrophilic aromatic substitution by methyl metaphosphate. 

The fact that the 3,P-NMR signal of 183a can only be observed in pyridine-containing solution provides food for thought124). Viewed in conjugation with the idea that alkyl metaphosphates could form adducts such as 173 and 174 U9,120) as discussed above, formulation as a zwitterionic pyridine/metaphosphate adduct (188) seems reasonable. Similar adducts have also been found in the reaction of TPS with dinucleotides and trinucleoside diphosphate 126). In any case, the reactions of 183 or 188 are in full accord with the expected properties of a monomeric metaphosphate and its reactivity towards alcohols is far greater than that of all other reactive phosphorylation intermediates which can arise on reaction of TPS with oligonucleotides 126). 

When monomeric metaphosphate anion POf (102) is generated form the phos-phonate dianion 170 in the presence of the hindered base 2,2,6,6-tetramethylpiper-idine, it undergoes reaction with added carbonyl compounds147), Thus, it phosphoryl-ates acetophenone to yield the enol phosphate, whereas in the presence of acetophenone and aniline the Schiff base is formed from both compounds, probably by way of the intermediate C6H5—C(CH3) (OPO e) ( NH2C6HS). This reactivity pattern closely resembles that of monomeric methyl metaphosphate 151 (see Sect. 4.4.2). 

This phosphotransferase [EC 2.7.2.1] catalyzes the thermodynamically favored phosphorylation of ADP to form ATP Aeq = [ATP][acetate]/ [acetyl phosphate] [ADP] = 3000). GDP is also an effective phosphoryl group acceptor. This enzyme is easily cold-denatured, and one must use glycerol to maintain full catalytic activity. Initial kinetic evidence, as well as borohydride reduction experiments, suggested the formation of an enzyme-bound acyl-phosphate intermediate, but later kinetic and stereochemicaT data indicate that the kinetic mechanism is sequential and that there is direct in-line phosphoryl transfer. Incidental generation of a metaphosphate anion during catalysis may explain the formation of an enzyme-bound acyl-phosphate. Acetate kinase is ideally suited for the regeneration of ATP or GTP from ADP or GDP, respectively. 

Metaphosphate or POs . This ion, which is stable in the gas phase, is quickly converted to H2PO4 in aqueous solutions (note that the rate of conversion is slower at low temperatures). Metaphosphate has been postulated to be an intermediate in a number of enzyme-catalyzed reactions. See Metaphosphate... 

In the present case, alkaline solvolysis forms almost completely racemic product in contrast with that from the neutral hydrolysis which is stereospecific. It is clear that the alkaline solvolysis involves a symmetrical intermediate, a metaphosphorimidothioate. which reasonably may be expected to be planar (isoelectronic with S03). The close similarity in reactions of thiophosphoryl and phosphoryl centers is well established for bimolecular displacements on phosphorus and the resemblance apparently extends to the metaphosphate eliminations. 

Although the metaphosphate mechanism for hydrolysis is well documented, such a pathway remains to be demonstrated in a biological system. Our present knowledge of many enzymic reactions allows, at best, the formulation of a preliminary mechanism, i.e. the chemical identity of substrates and enzymic intermediates and the minimal kinetic scheme. For example, much recent attention has been focused on the remarkable stability of the covalent phos-phoryl-enzyme (an O-phosphoryl serine) derived from E. coii alkaline phosphatase28 and inorganic phosphate, and on a systematic kinetic study of the enzyme s substrate specificity (O-, N- and S-monoesters) -9. Dephosphorylation of the enzyme, however, does not appear to be via a metaphosphate mechanism30. 

The problem of oxidative phosphorylation has been approached through model studies that utilize the phosphorylating potential of metaphosphate. In the mitrochondrial process inorganic phosphate and adenosine diphosphate are converted to adenosine triphosphate. Wieland, in a series of papers (e.g. ref. 31), has shown that a variety of thiolactones can activate inorganic phosphate in the presence of bromine for transfer to adenosine diphosphate (ADP). The intermediate may be an acyl phosphate or a sulfonium salt similar to that postulated by Higuchi and Gensch32 and by Lambeth and Lardy33, viz. 

Although inversion was not observed with the E. colt alkaline phosphatase, it has been observed for ribonucleases and many other hydrolytic enzymes and for most kinases transferring phospho groups from ATP. The difference lies in the existence of a phospho-enzyme intermediate in the action of alkaline phosphatase (see Eq. 12-38). Each of the two phosphotransferase steps in the phosphatase action apparently occurs with inversion. The simplest interpretation of all the experimental results is that phosphotransferases usually act by in-line -like mechanisms which may involve metaphosphate-ion-like transition states that are constrained to react with an incoming nucleophile to give inversion. An adjacent attack with pseudorotation would probably retain the original configuration and is therefore excluded. 

So far, all evidence is consistent with the interpretation that enzymatic reactions at phosphorus proceed with inversion by an in-line associative mechanism. There has been no need to invoke adjacent mechanisms, metaphosphate intermediates, or pseudorotation. Results are summarized in Table 8.1. 

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