Metaphosphate ion

Niecke et al. have prepare polyimido analogues of the metaphosphate ion, PO3, bylithiation ofthe corresponding amido compounds [16]. Thus the monomeric solvent-separated ion pair [(THF)4Li][P(NMes )3] (10) is obtained by treatment of (Mes N)2P(NHMes ) with "BuLi [16]. A monomeric contact ion pair (11) containing the unsymmetrical anion [P(N Bu)2(NMes )]" has also been reported [16]. By contrast the dilithium derivative of the trisimidometaphosphate [P(N Bu)3]" forms a dimer (12) [17], with a cubic structure reminiscent of that of (7). 

The present survey is concerned exclusively with short-lived compounds of quinquevalent phosphorus with coordination number 3, which are not yet isolable in a classical sense with few exceptions they all possess at least one P/O double bond. Specifically, these are the metaphosphinates /, the metaphosphonates 2, and the metaphosphates 3. Studies of the methyleneoxophosphoranes 1 (X = O) and the monomeric metaphosphate ion 3 (Z = Oe, X = Y = O) have been especially thorough. 

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. 

The rate maximum at pH 4 is assigned to a specific reaction of the monoester anion 104 which exists exclusively under these conditions. Westheimer 57) first advanced a metaphosphate ion mechanism in which 102 is formed via a six-membered monoester-anion/water complex (103). An intramolecular proton transfer via a four-membered ring according to 105 m is also conceivable, as is the formation of a zwitterion 106 in a prior protonation equilibrium. 

The possible mechanisms for solvolysis of phosphoric monoesters show that the pathway followed depends upon a variety of factors, such as substituents, solvent, pH value, presence of nucleophiles, etc. The possible occurrence of monomeric metaphosphate ion cannot therefore be generalized and frequently cannot be predicted. It must be established in each individual case by a sum of kinetic and thermodynamic arguments since the product pattern frequently fails to provide unequivocal evidence for its intermediacy. The question of how free the PO ion actually exists in solution generally remains unanswered. There are no hard boundaries between solvation by solvent, complex formation with very weak nucleophiles such as dioxane or possibly acetonitrile, existence in a transition state of a reaction, such as in 129, or SN2(P) or oxyphosphorane mechanisms with suitable nucleophiles. 

More recent studies have shown that monomeric metaphosphates such as 147 are just as unisolable as the metaphosphate ion 102, and are even more electrophilic. Generation of metaphosphates is accomplished mainly in two ways, i.e. by thermal or photochemical fragmentation reactions, on the one hand, and by decomposition of suitably activated phosphates on the other. 

There are several important chemical species that consist of four atoms and have a total of 24 valence-shell electrons. Some of the most common isoelectronic species of this type are C032-, N03 , S03, and P() j (known as the metaphosphate ion). Because four atoms would require a total of 32 electrons for each to have an octet, we conclude that eight electrons must be shared in four bonds. With four bonds to the central atom, there can be no unshared pairs on that atom if the octet rule is to be obeyed. Therefore, we can draw the structure for CO, 2 showing one double C=0 bond and two single C-O bonds as... 

Such studies on creatine kinase (Eq. 12-31) utilized both a bound Mn2+ ion and a nitroxide spin label to estimate distances of various protons from the nitroxide.683 Together with EPR measurements (Box 8-C), which gave the Mn2+-nitroxide distance, a model of the ATP Mn2 complex in the active site was constructed. Additional EPR experiments on Mn2+ complexes with ATP and ADP containing 170 in the a, (3, or y phospho groups showed that in the enzyme ATP creatine complex the metal ion is bound to all three phospho groups of ATP. It remained coordinated with the two phospho groups of ADP and also that of the phospho-creatine product in the enzyme ADP creatine-P complex as well as in the transition state, which is pictured occurring via a metaphosphate ion.684... 

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. 

When the fragmentations yielding monomeric methyl metaphosphate (8,9.) or metaphosphate ion (10) are carried out in the presence of 2,2,6,6-tetramethylpiperidine in acetophenone as solvent, the major products are the enol phosphates. Presumably the processes take place by initial attack of the monomeric metaphosphates on the carbonyl group of the ketone. 

Monomeric metaphosphate ion will also activate the carbonyl group of esters (10). In particular, it promotes the reaction of ethyl acetate with aniline to yield O-ethyl-N-phenylacetimidate. The reaction is complete within seconds, whereas, in the absence of monomeric metaphosphate, the reaction between ethyl acetate and aniline is slow, and yields acetanilide. 

The hydrolysis of monoesters of phosphoric acid presumably proceeds by way of the presently unknown monomeric metaphosphate ion, P03. These hydrolyses show a maximum rate at pH 4, where the monoester monoanion is present in largest concentration. More than two decades ago, C. A. Bunton and his co-workers (64) at University College, London, and Walter Butcher (65) at the University of Chicago interpreted the pH-rate profile in terms of Equation 6. The postulated... 

Both Sn044- and Sn032- are stannates because they contain Sn(IV). The former is sometimes known as orthostannate and the latter as metastannate in the same way that P043- and PO3- are known as the orthophosphate and metaphosphate ions, respectively (see Chapter 13). 

Non-metal imides can also be obtained. Thus the metaphosphate ion, POj has its analogue in [Li(THF)4]+[P(NAr)3] , where the three NAr groups are propellerlike with the PN3 core planar.151... 

These contain hybrid ions intermediate between the pyrophosphate ion and the infinite iinear metaphosphate ion, in which the terminal PO4 groups share one 0 and the intermediate groups two 0 atoms. The normal sodium triphosphate. 

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). 

Hydrolysis of Phosphates. The conclusion that phosphoryl transfer between amines is concerted does not exclude the formation of a metaphosphate intermediate in reactions with weaker nucleophiles, including water. Pyridine is a much stronger nucleophile than water and it is possible that phosphoryl transfer between pyridines is concerted because the metaphosphate ion does not have a significant lifetime in the presence of pyridine the metaphosphate ion might exist for a short time in water. However, it is much more difficult to determine whether or not the mechanism is concerted for a reaction with water than for a reaction with pyridine. 

We can conclude that phosphate esters and other phosphate compounds react with water through bimolecular substitution in a concerted 5 2, or A D, mechanism with no metaphosphate intermediate. The appearance of the transition state is that it resembles metaphosphate monoanion, but the reality of the mechanism is that the reaction is a one-step bimolecular substitution. The metaphosphate ion can be formed in the gas phase (25) and there is evidence that metaphosphate can exist briefly in non-nucleophilic solvents 11,24. The reason that it is not an intermediate in water is presumably that there is no significant barrier for its reaction with water. 

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