Hydrolysis of phosphate triester

Other kinetically allowed mechanistic models, i.e. hydroxide ion attack on the monoanion, can be rejected on the grounds that the required rate coefficients far exceed that found for alkaline hydrolysis of phosphate triesters. At pH > 9 two new reactions appear, one yielding a 1,6-a.nhydro sugar by nucleophilic attack through a five-membered transition state of the 1-alkoxide ion upon C-6 with expulsion of phosphate trianion. The second is apparently general-base catalysis by 1-alkoxide of water attack on C-6 or phosphorus through greater than six-membered cyclic transition states. 

Figure 11.43 Selected mechanistic features for hydrolysis of phosphate triesters and diester anions.

Figure 11.46 Two anomeric effects cooperate in hydrolysis of phosphate triesters.

Mora, J. R., Kirby, A. J., Nome, F. (2012). Theoretical Study of the Importance of the Spectator Groups on the Hydrolysis of Phosphate Triesters. The Journal of Organic Chemistry, 77, 7061-7070. 

The factors involved and mechanistic pathways in the hydrolysis of phosphate esters, particularly those of a cyclic nature, continue to be the source of much speculation. A further study of the simplest cyclic triester, ethylene methyl phosphate, seems only to have served to consolidate already polarized views. The original, experiments of Westheimer s group employed gc and H nmr spectroscopy and demonstrated that ethylene methyl phosphate (26) hydrolyses under alkaline conditions by... 

The first term, representing acid-"catalyzed" hydrolysis, is important in reactions of carboxylic acid esters but is relatively unimportant in loss of phosphate triesters and is totally absent for the halogenated alkanes and alkenes. Alkaline hydrolysis, the mechanism indicated by the third term in Equation (2), dominates degradation of pentachloroethane and 1,1,2,2-tetrachloroethane, even at pH 7. Carbon tetrachloride, TCA, 2,2-dichloropropane, and other "gem" haloalkanes hydrolyze only by the neutral mechanism (Fells and Molewyn-Hughes, 1958 Molewyn-Hughes, 1953). Monohaloalkanes show alkaline hydrolysis only in basic solutions as concentrated as 0.01-1.0 molar OH- (Mabey and Mill, 1978). In fact, the terms in Equation(2) can be even more complex both elimination and substitution pathways can operate, leading to different products, and a true unimolecular process can result from initial bond breaking in the reactant molecule. 

In no small measure because of the importance of such substances and processes as those just mentioned, the hydrolysis of phosphate esters has received much fundamental study. Triesters are attacked by OH at phosphorus and by H20 at carbon ... 

Cryptate complexes with macrobicychc hgands containing three bipy units, in which the Ln + ion is contained within a hgand cavity, have been synthesized. Such hgands will complex Ln + ions, such as Eu + and Sm +, under conditions where Ln + ions are not. An application has been using lanthanide cryptates of the early lanthanides (La, Ce, Eu) as catalysts in the hydrolysis of phosphate monoesters, diesters, and triesters. Schiff base complexes can be synthesized by the reaction of a lanthanide salt with a diamine and a suitable carbonyl derivative such as 2,6-diacetylpyridine. 

Phosphoryl transfer reactions have essential roles throughout biochemistry. The enzymes that catalyze these reactions result in tremendous rate enhancements for their normally unreactive substrates. This fact has led to great interest in the enzymatic mechanisms, and debate as to whether the mechanisms for enzyme-catalyzed hydrolysis of phosphate esters differ from those of uncatalyzed reactions. This review summarizes the uncatalyzed reactions of monoesters, diesters and triesters. A selection of enzymatic phosphoryl transfer reactions that have been the most studied and are the best understood are discussed, with examples of phosphatases, diesterases, and triesterases. 

Me2pyo[14]trieneN4 (CR) ligand (Fig. 6a) catalyzes the hydrolysis of the triester diphenyl 4-nitrophenyl phosphate in aqueous acetonitrile solution.221 This reaction is first-order in zinc complex and phosphate ester. On the basis of pH-rate studies, which revealed a kinetic pifa value of 8.7,40 the active zinc complex is proposed to be [(CR)Zn-OH]+. A hybrid mechanism in which the zinc center of [(CR)Zn-OH]+ serves to provide the hydroxide nucleophile, and also electrophilically activates the phosphoryl P O bond, is favored for this system. This type of bifunctional mechanism was proposed based on the fact that the second-order rate constant for the [(CR)Zn-OH]+-catalyzed reaction (2.8 x 10 1 M-1 s 1) is an order of magnitude larger than that of free hydroxide ion-catalyzed hydrolysis (2.8 x 10 2M 1 s 1). As OH- is a better nucleophile than the zinc-coordinated hydroxide, Lewis acid activation of the substrate is also operative in this system. 

The mononuclear zinc complexes [([12]aneN3)Zn(0H2)](C104)2 (Fig. 4) and [([12]aneN4)Zn(0H2)](C104)2 (Fig. 5) promote the hydrolysis of the triester tris(4-nitrophenyl) phosphate and the diester bis(4-nitrophenyl) phosphate.225 For these... 

Figure 11,45 (a) dependence of the rates of spontaneous hydrolysis of 4-nitrophenyl triesters and one diester on the sum of the pK/s of the non-leaving groups ROH. (b) Effects of leaving group capability on the energy surface for the reaction of phosphate triesters with nucleophiles according to Kirby and Nome. ... 

The rate of hydrolysis of arsenate(V) triesters OAs(OR)3 is much faster than that of phosphate triesters, and decreases in the order R = Me, Et, -pentyl, and isopropyl. Rate constants for the first hydrolysis have been measured, and estimated for the second stage. The third step could not be followed by use of the stopped flow technique. Studies in solvent-water mixtures with varying water concentrations confirm the involvement of water in the rate law. An associative mechanism is postulated with a five-coordinate intermediate OAs(OR)3(OH2). 

The mechanism of phosphate ester hydrolysis by hydroxide is shown in Figure 1 for a phosphodiester substrate. A SN2 mechanism with a trigonal-bipyramidal transition state is generally accepted for the uncatalyzed cleavage of phosphodiesters and phosphotriesters by nucleophilic attack at phosphorus. In uncatalyzed phosphate monoester hydrolysis, a SN1 mechanism with formation of a (POj) intermediate competes with the SN2 mechanism. For alkyl phosphates, nucleophilic attack at the carbon atom is also relevant. In contrast, all enzymatic cleavage reactions of mono-, di-, and triesters seem to follow an SN2... 

An early attempt to achieve metal-metal cooperation on the nonen-zymatic hydrolysis of a phosphate triester failed, probably due to the long and flexible linker connecting two imidazole-containing metal-binding sites [41]. 

The use of a lipophilic zinc(II) macrocycle complex, 1-hexadecyl-1,4,7,10-tetraazacyclododecane, to catalyze hydrolysis of lipophilic esters, both phosphate and carboxy (425), links this Section to the previous Section. Here, and in studies of the catalysis of hydrolysis of 4-nitrophenyl acetate by the Zn2+ and Co2+ complexes of tris(4,5-di-n-propyl-2 -imidazolyl)phosphine (426) and of a phosphate triester, a phos-phonate diester, and O-isopropyl methylfluorophosphonate (Sarin) by [Cu(A(A(A/,-trimethyl-A/,-tetradecylethylenediamine)l (427), various micellar effects have been brought into play. Catalysis of carboxylic ester hydrolysis is more effectively catalyzed by A"-methylimidazole-functionalized gold nanoparticles than by micellar catalysis (428). Other reports on mechanisms of metal-assisted carboxy ester hydrolyses deal with copper(II) (429), zinc(II) (430,431), and palladium(II) (432). 

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. ... 

Various esterases exist in mammalian tissues, hydrolyzing different types of esters. They have been classified as type A, B, or C on the basis of activity toward phosphate triesters. A-esterases, which include arylesterases, are not inhibited by phosphotriesters and will metabolize them by hydrolysis. Paraoxonase is a type A esterase (an organophosphatase). B-esterases are inhibited by paraoxon and have a serine group in the active site (see chap. 7). Within this group are carboxylesterases, cholinesterases, and arylamidases. C-esterases are also not inhibited by paraoxon, and the preferred substrates are acetyl esters, hence these are acetylesterases. Carboxythioesters are also hydrolyzed by esterases. Other enzymes such as trypsin and chymotrypsin may also hydrolyze certain carboxyl esters. 

Substitutionally inert Co(m) or Ir(m) complexes have been used to measure directly the effect of Lewis acid activation on the hydrolysis of an amide [35-37], a nitrile [38] and a phosphate triester [39] (Figure 6.4). The p/C, of the cobalt-bound water molecule in 5 is 6.6 [40], Thus the upper limit for the rate-acceleration due to Lewis acid activation with this metal in the hydrolysis of esters, amides, nitriles and phosphates should be close to 109-fold. Although the observed rate accelerations for the hydrolysis reac-... 

Figure 10.17 Alkaline hydrolysis of RNA, showing only a single diester bond. The intermediate cyclic triester can be hydrolyzed at any two of the three locations indicated by dotted lines and labeled a, b, and c. If cleavage occurs at positions a and b, the first set of products is obtained. Cleavage at a and c produces the second set. A third set may be obtained by cleaving at b and c. P designates esterified phosphate.

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