Acetate p-nitrophenyl

When Jencks reacted hydroxylamine with p-nitrophenyl acetate, p-nitrophenolate ion was released at a rate faster than that at which acetohydroxamic acid was formed. This burst effect is evidence for a two-step reaction. In this case the intermediate is O-acetylhydroxylamine, which subsequently reacts with hydroxylamine to form the hydroxamic acid. 

Figure 6-8 is a pH-rate profile for the hydrolysis of p-nitrophenyl acetate. The slopes of the straight-line portions are —1,0, and -L 1, reading in the acid to base direction, and this system can be described by... 

These Br nsted-type plots often seem to be scatter diagrams until the points are collated into groups related by specific structural features. Thus, p-nitrophenyl acetate gives four separate, but parallel, lines for reactions with pyridines, anilines, imidazoles, and oxygen nucleophiles.Figure 7-4 shows such a plot for the reaction of trans-cmmm c anhydride with primary and secondary aliphatic amines to give substituted cinnamamides.All of the primary amines without substituents on the a carbon (R-CHi-NHi) fall on a line of slope 0.62 cyclopentylamine also lies on this line. If this line is characteristic of normal behavior, most of the deviations become qualitatively explicable. The line drawn through the secondary amines (slope 1.98) connects amines with the structure R-CHi-NH-CHi-R. The different steric requirements in the acylation reaction and in the model process... 

The curvature may be an artifact of a selection of nucleophiles of mixed structural types chosen to display a wide range in pAo. Buncel et al. ° varied pK by changing the solvent composition over a limited range rather than by changing the structure. They studied the reaction between X-C6H4-CT and p-nitrophenyl acetate in 40-90 mol% dimethylsulfoxide—water mixtures with just three X substituents... 

Figure 7-5. Bry4nsted-type plot for nucleophilic reactions of p-nitrophenyl acetate. Key , simple imidazoles in 28.5 ethanol at JO°C. p = 0.80 (data from Ref. 197] O, oxygen anions, in water at 25°, P = 0.95 for linear portion [data from Ref. 119, 198] O, a effect nucleophiles. Several of the nucleophiles are identified.

These data are for the nucleophilic catalysis of the hydrolysis of p-nitrophenyl acetate by imidazoles and benzimidazoles at pH 8.0. Tbe apparent second-order catalytic rate constants are defined by... 

The importance of the proximity effect in cyclodextrin catalysis has been discussed on the basis of the structural data. Harata et al. 31,35> have determined the crystal structures of a-cyclodextrin complexes with m- and p-nitrophenols by the X-ray method. Upon the assumption that m- and p-nitrophenyl acetates form inclusion complexes in the same manner as the corresponding nitrophenols, they estimated the distances between the carbonyl carbon atoms of the acetates and the adjacent second-... 

An artificial metalloenzyme (26) was designed by Breslow et al. 24). It was the first example of a complete artificial enzyme, having a substrate binding cyclodextrin cavity and a Ni2+ ion-chelated nucleophilic group for catalysis. Metalloenzyme (26) behaves a real catalyst, exhibiting turnover, and enhances the rate of hydrolysis of p-nitrophenyl acetate more than 103 fold. The catalytic group of 26 is a -Ni2+ complex which itself is active toward the substrate 1, but not toward such a substrate having no metal ion affinity at a low catalyst concentration. It is appearent that the metal ion in 26 activates the oximate anion by chelation, but not the substrate directly as believed in carboxypeptidase. 

The hydrolysis of p-nitrophenyl acetate and bis(p-nitrophenyl phosphate) are frequently used to probe hydrolytic activity. A problem with some other dinuclear systems is that the Zn units are held together by bridging ligands which can be cleaved on reaction with the substrate.440 This is not the case in a ditopic ligand such as those designed by Lippard and co-workers based on Kemp s triacid imide with a xylyl spacer.441,442 Both zinc dimers and mixed metal dimers were formed and a structure characterized with a bridging phosphodiester (Figure 6). 

Hua I, Hochemer RH, Hoffmann MR (1995) Sonolytic hydrolysis of p-nitrophenyl acetate The role of supercritical water. J Phys Chem 99 2335-2342... 

Fig. 6. The variation of the pseudo first-order rate constant for release of p-nitro-phenolate ion from p-nitrophenyl acetate at pH 10.6 with the concentration of added cycloheptaamylose (VanEtten et al., 1967a).

Values of /c2 and Kd for the reactions of the cycloamyloses with a variety of phenyl acetates are presented in Table IV. The rate constants are normalized in the fourth column of this table to show the maximum accelerations imposed by the cycloamyloses. These accelerations vary from 10% for p-f-butylphenyl acetate to 260-fold for m-f-butylphenyl acetate, again showing the clear specificity of the cycloamyloses for meta-substituted esters. Moreover, these data reveal that the rate accelerations and consequent specificity are unrelated to the strength of binding. For example, although p-nitrophenyl acetate forms a more stable complex with cyclohexa-amylose than does m-nitrophenyl acetate, the maximal rate acceleration, h/kan, is much greater for the meta isomer. 

The most effective catalyst for the hydrolysis of p-nitrophenyl acetate was reported to be a cycloheptaamylose derivative containing approximately two imidazole groups per cycloheptaamylose molecule (Cramer and Mackensen, 1970). At pH 7.5 and 23°, this material accelerates the rate of release of phenol from p-nitrophenyl acetate by a factor of 300 when compared with the hydrolysis of this substrate in the absence of catalyst. However, when compared with an equivalent concentration of imidazole, which is an effective catalyst for ester hydrolysis at neutral pHs, the rate accelerations imposed by this cycloheptaamylose derivative are only two- to threefold. Cramer and Mackensen attributed this rate enhancement to nucleophilic displacement of phenol from the included ester by a cycloheptaamylose hydroxyl group, assisted internally by the attached imidazole group... 

In analogy to the derivative prepared by Breslow and Overman, the cyclohexaamylose-iV-methylhydroxamic acid displays a pronounced specificity for p-nitrophenyl acetate as opposed to n-nitrophenyl acetate. This specificity is probably again derived from the geometry of the inclusion complex i.e., a more favorable location of the reactive center of the para-isomer relative to the hydroxamate function within the inclusion complex. 

The hydrolysis of esters by the nickel derivative (271) provided an early example of the use of a metal-capped cyclodextrin as a catalyst (shown here as its p-nitrophenyl acetate inclusion complex) (Breslow Overman, 1970 Breslow, 1971). The synthesis of this host involves the following steps (i) covalent binding of the pyridine dicarboxylic acid moiety to cyclodextrin, (ii) coordination of Ni(n) to this species, and (iii)... 

Thiolysis of p-nitrophenyl acetate Synthetic vesicles prepared by injection. Rates compared with those in aq.CTABr Cuccovia et al., 1979... 

Fig. 2 Speciation diagram for 2.0 x 10" 3 M La3+ in methanol as a function of pH. Data superimposed on the figure as ( ) are second-order rate constants for La3 +-catalyzed methanolysis of p-nitrophenyl acetate (2) as a function of pH.

At the start of this section the cleavage of meta- and para-substituted phenyl acetates by a- and )S-CD was discussed in detail and a variety of evidence was cited that is consistent with the mechanisms A and B, in Scheme 2. Further support for the view that para-substituents tend to force the phenyl group out of the cavity (Scheme 2B) comes from the different effects that neutral additives (potential inhibitors) have on the cleavage of m- and p-nitrophenyl acetate (mNPA and pNPA). In brief, species which bind to CDs, and inhibit the reaction of mNPA, do not necessarily inhibit that of pNPA (Tee and Hoeven, 1989 Tee et al., 1993b). 

Table A5.12 Cleavage of p-nitrophenyl acetate and its thio analogue by cyclodex-trins."...

Table A5.13 Constants for the cleavage of p-nitrophenyl acetate by presence of potential inhibitors (PI)." a-CD in the...

Many of these reactions are not observed at all when the relevant groups are allowed to come together in bimolecular processes in aqueous solution. For mechanistic work involving intermolecular reactions, therefore, it is necessary to use activated substrates. Much of what we know about the relevant reactions of esters, for example, comes from studies using aryl esters like p-nitrophenyl acetate, or acyl-activated compounds like ethyl trifluoroacetate (Bruice and Benkovic, 1966 Jencks, 1969 Bender, 1971). 

A catalytic effect that may be of the same type as the mimicry of acyl transferase enzymes is reported by Gandour et al. (1978). They describe the rate enhancement of the aminolysis of p-nitrophenyl acetate in the presence of bis(2,6-pyridinyl) crown ethers [328] and [329] in chlorobenzene. The rate of... 

Catalytic rate constants for butylaminolysis of p-nitrophenyl acetate in chlorobenzene at 25°Ca... 

Allosteric behavior of the heterotropic variety is seen in the interaction between polymer and detergent or polymer and polymer (Shirahama, 1974 Arai et al., 1973 Tsuchida and Osada, 1973). Shinkai et al. (1977b) observed a sigmoid profile of rate constant vs. concentration of cationic detergents in the acyl transfer reaction from p-nitrophenyl acetate (PNPA) to copolymers (7). 

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