Micellar catalysis, nucleophilic

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). 

Most of the work concerned with micellar catalysis of nucleophilic substitution refers to reactions of the Aac2 and SN2 types and will not be reviewed here. To date only a few systems have been examined in which a micellar medium affects the partitioning of solvolytic reactions between unimolecular and bimolecular mechanisms. The effects of cationic (hexadecyltrimethylammonium bromide = CTAB) and anionic (sodium lauryl sulfate = NaLS) micelles on competitive SN1 and SN2 reactions of a-phenylallyl butanoate 193) have been investigated189. The rate of formation of the phenylallyl cation 194) is retarded by both surfactants probably as a consequence of the decreased polarity of the micellar pseudo phase. The bimolec-... 

The unexpected observation was that for a fixed concentration of areneimidazole, at constant pH, the first-order rate constants, went through maxima with increasing concentration of (6b). This observation suggests that there is a cooperative interaction between substrate, nucleophile and ammonium ion, and measurements of the substrate solubility and acid dissociation of benzimidazole allowed calculation of the amounts of the two reactants which were bound to each ammonium ion. Therefore the rate constant, for reaction in hydrophobic aggregate, could be calculated, using the treatment which was applied to micellar catalysis (Eqn. 6). 

It has been suggested that micellar catalysis could be exploited in analytical chemistry to increase the rate of derivative formation prior to spectroscopic measurement of the product [223, 224]. This has been attempted in the assay of amino acids and peptides following reaction with l-fluoro-2,4-dinitrobenzene (this undergoes aromatic nucleophilic substitution by amines to give arylated amines) [225]. For this reaction some amines require up to 20 min. In the presence of cetrimonium bromide, catalysis was achieved, although absorbances some 10% higher were obtained in the presence of surfactant. 

Ouarti, N., Marques, A., Blagoeva, I., Rausse, M.-F. Optimization of micellar catalysis of nucleophilic substitution in buffered cetyltrimethylammonium salt solutions. 1. Buffers for the 9-10 pH range. Langmuir 2000, 76(5), 2157-2163. 

Broxton, T.J. Micellar catalysis of organic reactions. VII. The effect of the micellar counter ion in nucleophilic reactions of hydroxide and nitrite ions. Aust. J. Chem. 1981, 34(11), 2313-2319. 

Catalysis, enzymatic, physical organic model systems and the problem of, 11,1 Catalysis, general base and nucleophilic, of ester hydrolysis and related reactions, 5,237 Catalysis, micellar, in organic reactions kinetic and mechanistic implications, 8,271 Catalysis, phase-transfer by quaternary ammonium salts, 15,267 Catalytic antibodies, 31,249... 

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. ... 

Hydrophobic species bearing hydrocarbon chains present vitamin B12 or vitamin B6 type activity [5.37]. Such systems lend themselves to inclusion in membrane or micellar media. They thus provide a link with catalysis in more or less organized media such as membranes, vesicles, micelles, polymers [5.39-5.41] (see Section 7.4). Water soluble cyclophanes showing, for example, transaminase [5.42], acetyl transfer [5.43], pyruvate oxidase [5.44] or nucleophilic substitution [5.45] activity have been described. 

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. 

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