4- Nitrophenyl diphenyl

There has been a useful review of phase-transfer catalysis in nucleophilic aromatic substimtion. A comparison has been reported of the reactions with nucleophiles of l-chloro-2,4-dinitrobenzene (substimtion) and 4-nitrophenyl diphenyl phosphate (dephosphorylation) in neutral micelles of dodecyl (10) and (23) polyoxyethylene glycol. In the substimtion reaction considerable amounts of ether may be formed by reaction with alkoxide ions at the micellar surface. Differences in reactivity of the two substrates are probably due to differences in their location in the micellar structures. ... 

FIGURE 12. Effect of CTABr on reactivity for reactions of 4-nitrophenyl diphenyl phosphate with 4-nitrobenzaldoximate ( ) and 2-quinoUnealdoximate (O) ions. Reprinted with permission from Bunton and Ihara, J. Org. Chem., 42, 2865. Copyright (1977) American Chemical Society... 

Substituted phenolate anions react with 4-nitrophenyl diphenyl-phosphinate in a quasi-symmetrical reaction so that the effective charges can be determined for both forming and breaking bonds from the two quantities and Peq without any special experiments made on the leaving group substituents. The transition structure for the reaction, which has a concerted mechanism, has ... 

A commonly observed form of curve is bell-shaped. Figure 3.6 compares the effect observed in the reaction of a single alpha nucleophile (Ox ) at several common electrophilic centers C=0 (PNPA, 4-nitrophenyl acetate), P=0 (PNPDDP, 4-nitrophenyl diphenyl phosphinate), and SO (PNPBS, 4-nitrophenyl diphenyl sulfonate). Each of these shows the characteristic bell-shaped plot however, the magnitude of the alpha effect covers a 10-fold range. 

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]. 

The results are in accordance with those obtained by Taft and colleagues80,81. These authors have measured the 19F NMR chemical shifts of p-FC6H4OH in the presenceof 60 bases including sulphoxides (DMSO, methyl phenyl, methyl p-nitrophenyl, diphenyl, tetramethylene sulphoxide) and determined the association constants K for the hydrogen bond shown in equation 15. 

Fig. 1 Micellar effects upon reaction of p-nitrophenyl diphenyl phosphate with benzimidazolide ion (solid points) open points are for reaction in the absence of benzimidazole , 10 4 M benzimidazole, pH 10.7 , 1.2 x 10-4 M benzimidazole, pH 11 O, pH 10.7, and 11 respectively. The solid lines are theoretical. (Reprinted by permission of the American Chemical Society)...

Reactions of hydrophilic anions in microemulsions can be treated in terms of the ion-exchange formalism. The extent of ion binding to microemulsion droplets has been estimated conductimetrically, and the rates of reactions of OH- and F with p-nitrophenyl diphenyl phosphate are consistent with the extent of ion binding and competition between reactive and inert anions. Alternatively the data could be accommodated to variations in the estimated surface potential of the droplet (Mackay and Hermansky, 1981 Mackay, 1982). 

Dephosphorylation of p-nitrophenyl diphenyl phosphate by arenimidazo-lide ions is very strongly accelerated by the tri-n-octylethylammonium bromide or mesylate (TEABr or TEAMs respectively) (Scheme 9). The... 

Fig. 6 Reaction of p-nitrophenyl diphenyl phosphate in non-micellar aggregates of tri-n-octyl ethylammonium mesylate (TEAMs) at pH 10.7 , 10 4M naphth-2,3-imidazole and O, 10-4 and 2 x KT4 M benzimidazole, respectively. (Reprinted with permission of the American Chemical Society)...

Nitrophenyl diphenyl phosphate + aldoximate and aryloxide ions 4-02NC6H4CH(0Et)2 + H30 + PhNH.NHPh + 2H30 +... 

Rate constants of bimolecular, micelle-assisted, reactions typically go through maxima with increasing concentration of inert surfactant (Section 3). But a second rate maximum is observed in very dilute cationic surfactant for aromatic nucleophilic substitution on hydrophobic substrates. This maximum seems to be related to interactions between planar aromatic molecules and monomeric surfactant or submicellar aggregates. These second maxima are not observed with nonplanar substrates, even such hydrophobic compounds as p-nitrophenyl diphenyl phosphate (Bacaloglu, R. 1986, unpublished results). 

One of the earliest reports on the use of dendrimers in catalysis is the unimolecu-lar decarboxylation of 6-nitro-benzisoxazole-3-carboxylate in the presence of a dendrimer comprising ether dendrons which are functionalized at their periphery with tetra-alkylammonium cations (e.g. 20, Scheme 21) [30]. In aqueous media, the quaternary ammonium groupings promote the reactivity of organic anions which presumably bind in high concentration to the polycationic periphery of the dendrimer. The latter species enhances the rate of the bimolecular hydrolysis of p-nitrophenyl diphenyl phosphate catalyzed by o-iodosobenzoate ion. 

Two types of amphiphilic quaternary 3-pyridinium ketoximes (253a, b) with different positioning of the hydrophobic alkyl chain have been synthesized and tested as hydrolytic micellar catalysts. A considerable positive deviation from the expected first-order curve was observed in the absorbance vs time plot when p-nitrophenyl diphenyl phosphate (252 R = Ph) and p-nitrophenyl diethyl phosphate... 

The reactions of a phosphate triester, p-nitrophenyl diphenyl phosphate with hydroxide and fluoride ions has been demonstrated to be catalyzed strongly by cationic surfactants and inhibited by NaLS and a non-ionic surfactant (Bunton and Robinson, 1969a Bunton et al., 1969, 1970). Hexadecyltrimethylammonium bromide (CTAB) increased the second-order rate constant for the reaction ofp-nitrophenyl diphenyl phosphate with hydroxide ion by a maximum factor of approximately 11 and that with fluoride ion by a maximum factor of approximately 33 at CTAB concentrations of 3 x 10 m and 2 x 10 m respectively. At higher detergent concentrations the catalysis became progressively less pronounced (Fig. 11). This behavior does not fit equation (10) (Bunton and Robinson, 1969a). However, a number of other micelle-catalyzed reactions between anions and neutral molecules have been found to... 

Fig. 11. Catalysis of the reaction of p-nitrophenyl diphenyl phosphate with 0 01 m sodium hydroxide by CTAB , at 25-0 , at 15-4 , at 5-0° Bunton and Robinson, 1969a).

Robinson, 1969a). It is probable that the hydrophobic nature of the phenyl groups of p-nitrophenyl diphenyl phosphate results in deep penetration of the neutral ester in the Stern layer, thus shielding the phosphoryl group from nucleophilic attack. Unlike other reactions between nucleophiles and neutral substrates catalyzed by cationic micelles (Bunton and Robinson, 1968, 1969a) and the hydrolysis of dinitrophenyl phosphate dianions in the presence of cationic micelles (Bunton et al., 1968), the catalysis of the hydrolysis of -nitrophenyl diphenyl phosphate by CTAB arises from an increase in the activation entropy rather than from a decrease in the enthalpy of activation. The Arrhenius parameters for the micelle-catalyzed and inhibited reactions are most probably manifestations of the extensive solubilization of this substrate. However, these parameters can be composites of those for the micellar and non-micellar reactions and the eifects of temperature on the micelles themselves are not known. Interpretation of the factors which affect these parameters must therefore be carried out with caution. In addition, the inhibition of the micelle-catalyzed reactions by added electrolytes has been observed (Bunton and Robinson, 1969a Bunton et al., 1969, 1970) and, as in the cases of other anion-molecule reactions and the heterolysis of dinitrophenyl phosphate dianions, can be reasonably attributed to the exclusion of the nucleophile by the anion of the added salt. 

The CTAB-catalyzed reaction between p-nitrophenyl diphenyl phosphate and hydroxide or fluoride ion is also inhibited by phenyl, diphenyl, and p-t-butylphenyl phosphates (Bunton et al., 1969). The inhibition by these bulky anions decreases, however, with decreasing pH, and at lower pH values, where the hydroxide ion reaction becomes negligible, the reaction of p-nitrophenyl diphenyl phosphate with p-t-butylphenyl, phenyl, and inorganic phosphate ions is enhanced significantly by CTAB (Fig. 12 and Table 8). The order of the micellar rate enhancement for these nucleophilic reactions (p-t-BuC6H40P03 >C6H60P0 > HOPOf ). The cationic micellar catalysis is thus not dependent on the nucleophilicity of the anions but is explicable in terms of hydrophobic interactions between the nucleophiles and the micelle (Bunton et al., 1969). 

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