Metaphosphates formation

Scheme 3. The Role of A jAf -Dicyclohexylcarbodiimide in the Formation of an Active Phosphorylating Species [Metaphosphates (76) or (76), or both] from Phosphoric Ester (69). (Dotted arrows indicate electronic shifts associated with metaphosphate formation.)...

Amides of nucleotides are, like carboxamides, more stable than the corresponding esters. Their hydrolysis rate depends on the pH of the medium and the nitrogen substituents. Aliphatic amine derivatives are unstable below pH 4, hydrolyze slowly at pH 4-6, and are virtually stable at or above pH 7. The conjugated acid is thus readily hydrolyzed the neutral forms and the anion are not. Aromatic amines react slower in acid media than the alkyl derivatives. In both cases, one frequently observes S l mechanisms corresponding to metaphosphate formation (Scheme 8.5.11). There is no selectivity toward the nucleophile. 

P-0 bond dissociation with metaphosphate formation In both enzymes, that... 

Lima FS, ChAmovich H, Cuccovia IM. Kinetics and product distribution of p-nitrophenyl phosphate dianion solvolysis in ternary DMSO/Acohol/water mixtures are compatible with metaphosphate formation. J Phys Org Chem. 2012 25 9-13. 

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. 

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. 

A kinetic isotope effect 160/180 of 2% in the spontaneous hydrolysis of the 2,4-dinitrophenyl phosphate dianion, whose ester oxygen is labeled, suggests a P/O bond cleavage in the transition state of the reaction, and thus also constitutes compelling evidence for formation of the metaphosphate 66,67). The hydrolysis behavior of some phosphoro-thioates (110) is entirely analogous 68). 

As for the acetyl phosphate monoanion, a metaphosphate mechanism has also been proposed 78) for the carbamoyl phosphate monoanion 119. Once again, an intramolecular proton transfer to the carbonyl group is feasible. The dianion likewise decomposes in a unimolecular reaction but not with spontaneous formation of POf as does the acetyl phosphate dianion, but to HPOj and cyanic acid. Support for this mechanism comes from isotopic labeling proof of C—O bond cleavage and from the formation of carbamoyl azide in the presence of azide ions. 

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. 

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. 

Another instructive example is provided by a series of a-phenyl-a,P-dibromo-phosphonates 170, 171, 172. While the phosphonate dianion 170 fragments instantaneously at room temperature with formation of the POf ion (see also Sect. 4.1.3), the analogous reaction of the phosphonic monoester anion 171 leading to methyl metaphosphate 151 requires more drastic conditions and is at least 1000 times slower the diester 172 is essentially stable under the reaction conditions described for 171 addition of triethylamine leads to slow demethylation H0). The behavior of 171 contrasts with that of simple (3-haloalkylphosphonic monoesters which merely eliminate HHal on treatment with bases94. Thus it is the possibility of formation of a phenyl-conjugated double bond which supports the fragmentation of 171 to olefin + 151. 

The formation of 151 from the phosphonate 171 could be proved only by indirect means. Electron-rich aromatic compounds such as N,N-diethylaniline and N,N,N, N -tetraethyl-m-phenylenediamine U0 1I9> and N-methylaniline 120> are phosphorylated in the para- and in the ortho- plus para-positions by 151. Furthermore, 151 also adds to the nitrogen lone pair of aniline to form the corresponding phosphor-amidate. Considerable competition between nucleophiles of various strengths for the monomeric methyl metaphosphate 151 — e.g. aromatic substitution of N,N-diethylaniline and reaction with methanol or aromatic substitution and reaction with the nitrogen lone pair in N-methylaniline — again underline its extraordinary non-selectivity. 

The extent to which 151 phosphorylates the aromatic amine in the phenyl ring is highly dependent upon the solvent. For instance, aromatic substitution of N-methylaniline is largely suppressed in the presence of dioxane or acetonitrile while pho.sphoramidate formation shows a pronounced concomitant increase. The presence of a fourfold excess (v/v) or pyridine, acetonitrile, dioxane, or 1,2-di-methoxyethane likewise suppresses aromatic substitution of N,N-diethylaniline below the detection limit. It appears reasonable to assume that 151 forms complexes of type 173 and 174 with these solvents — resembling the stable dioxane-S03 adduct 175 — which in turn represent phosphorylating reagents. They are, however, weaker than monomeric metaphosphate 151 and can only react with strong nucleophiles. 

As early as 45 years ago, G. Schramm et al. (1962) carried out the synthesis of AMP, ADP and ATP using ethyl metaphosphate as the phosphorylating agent. These successful syntheses led to the formation of longer nucleotide chains however, they by no means correspond to the conditions present on the primordial Earth. Thus, the question as to the source of the phosphates remains paramount. According to Schwartz (1998), the following phosphate sources deserve consideration ... 

Earlier claims for the preparation of methyl metaphosphate, if only in situ, have received further support by the observation that the species, thought to be (119), will attack AW-diethylaniline at low temperatures to give methyl 4-AW-diethylamino-phenylphosphonate.89 The formation of 005-triethyl phosphorodithioate together with triethyl thionoformate when triethyl orthoformate is treated with P2S5 or diethyl hydrogen dithiophosphate is interpreted in terms of the species (120).00... 

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. 

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