Hydrolysis of 4-nitrophenyl diphenyl phosphate

Menger et al. synthesized a Ci4H29-attached copper(II) complex 3 that possessed a remarkable catalytic activity in the hydrolysis of diphenyl 4-nitrophenyl phosphate (DNP) and the nerve gas Soman (see Scheme 2) [21], When 3 was used in great excess (ca. 1.5 mM, which is more than the critical micelle concentration of 0.18 mM), the hydrolysis of DNP (0.04 mM) was more than 200 times faster than with an equivalent concentration of the nonmicellar homo-logue, the Cu2+-tetramethylethylenediamine complex 9, at 25°C and pH 6 (Scheme 4). The DNP half-life is calculated to be 17 sec with excess 1.5 mM 3 at 25°C and pH 6. The possible reasons for the rate acceleration with 3 were the enhanced electrophilicity of the micellized copper(II) ion or the acidity of the Cu2+-bound water and an intramolecular type of reaction due to the micellar formation. On the basis of the pH(6-8.3)-insensitive rates, Cu2+-OH species 3b (generated with pK3 < 6) was postulated to be an active catalytic species. In this study, the stability constants for 3 and 9 and the thermodynamic pvalue of the Cu2+-bound water for 3a —> 3b + H+ were not measured, probably because of complexity and/or instability of the metal compounds. Therefore, the question remains as to whether or not 3b is the only active species in the reaction solution. Despite the lack of a detailed reaction mechanism, 3 seems to be the best detoxifying reagent documented in the literature. 

Recently, Bunton et al. synthesized the Ci6H33-attached triamine copper(II) complex 8b [26], which promoted the hydrolysis of diphenyl 4-nitrophenyl phosphate (DNP) at alkaline pH. The catalytic activity was almost the same as that for Menger s previously reported comicellar system with 3b. The active species was proposed to be the hydroxide-bound copper(II) complex 8b. The pvalue of the copper(II)-bound water molecule was speculated to be about 8 from the fact that the nonalkylated and tetradecyl homologous copper(II) complexes have a pvalue of 8 (determined by DNP hydrolysis kinetics). Since the micellar metal complex 8b was not fully characterized either in the solid state or in a micellar solution, its hydrolysis mechanism remains to be elucidated. 

Attachment of a cyclodextrin unit to an N-CH3 form of the CR macrocycle (Fig. 39) yielded a zinc complex with only modestly improved effectiveness (k2 = 21.7x 10 2M 1s 1) over the mononuclear system (k2 — (3.80 + 0.05) x 10 2M 1s 1) for the hydrolysis of diphenyl 4-nitrophenyl phosphate under identical conditions.109 This enhancement is proposed to be due to binding of 4-nitrophenolate moiety in the cyclodextrin cavity. 

Several multinuclear zinc complexes that exhibit phosphate diester and triester reactivity have been reported. A binuclear analog of [(CR)Zn]2+ (Fig. 6a) in which two [(CR)Zn]2+ units are linked by an aromatic spacer was found to be 4.4 times more effective in terms of the hydrolysis of diphenyl 4-nitrophenyl phosphate in CH3CN H20 at 25 °C than its mononuclear analog.204... 

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 reactivity of zinc complexes supported by a variety of tridentate amine donor ligands (Fig. 44) with diphenyl 4-nitrophenyl phosphate in 20% (v/v) acetonitrile-water, and with 2,4-dinitrophenyl diethyl phosphate in 1 % (v/v) in methanol-water, has been investigated. 5 For the former reaction, the second-order rate constant for the hydrolysis of 2,4-dinitrophenyl diethyl phosphate correlates linearly with the —AH value for the formation of the [(ligand)Zn(OH2)]2 + complex from free chelate ligand and aqueous zinc ion. This indicates that in this series of complexes, faster hydrolysis of 2,4-dinitrophenyl diethyl phosphate corresponds to weaker chelate... 

The effectiveness of three isomers, 2-, 3- and 4-hydroxyiminomethyl-l-pyridinium iodide (61) as agents to catalyse the hydrolysis in cationic micellar media of p-nitrophenyl acetate (PNPA) and p-nitrophenyl diphenyl phosphate (PNPDPP) were studied at pH 8. The 2-isomer was found to be the most effective for the hydrolysis of the carboxylic ester PNPA, but the 4-isomer for the hydrolysis of the triphosphate ester PNPDPP. ... 

Several studies have dealt with alkaline hydrolysis of 4-nitrophenyl diphenyl phosphate, and it was demonstrated that the reaction was very fast in microemulsions based on cationic surfactants and much slower in a system based on a nonionic surfactant [19,20]. 

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