Reactions in functional micelles

Most of the reactive groups are effective nucleophiles and in many cases the initial step of the reaction involves formation of a covalent intermediate, which subsequently breaks downj regenerating reactive surfactant in the micelle. Several techniques have been used to separate the initial nucleophilic attack and subsequent 

RCO NHOH-PR CO 0CtH4N02 - RCO NHOCO R +O C6H4NO2-4 Ueolca, R. and Ohkubo, K. (1978) Tetrahedron Lett., 4131. 

Reaction could involve acylation of the imidazole moiety, followed by A O-acyl 

The acylimidazole intermediate was detected spectrophotometrically, showing that reaction was stepwise rather than concerted, and both steps of the reaction were separated kinetically. 

The nucleophilicity of the hydroxyethyl group in deacylation at high pH has also been demonstrated in transesterification [105]. 

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Several workers have attempted to draw analogies between reactions in functional micelles and in enzymes, but in general this seems to be unjustified because there is generally little substrate- or stereo-specificity in the micellar reactions and usually limited turnover rates. 

For a number of reactions in functional micelles and comicelles second-order rate constants are similar in micelles and in water. Except for aromatic nucleophilic substitution they are slightly smaller in the micelles than in water, and the pattern of behavior is exactly that found for reactions of organic nucleophilic anions in non-functional micelles. Some examples of these comparisons are in Table 9. 

It is evident that for bimolecular reactions in non-functional micelles in water the key factor in rate enhancement is the increased concentration of the two reactants in the micellar pseudophase (Table 3 and 4) and the same effect should be at work for reactions in functional micelles. The problem is simply that of estimating the concentration of functional groups in the micellar pseudophase. For the simplest case, that of a functional micelle, not involving deprotonation equilibria, with a nucleophilic head group denoted as N, there will be one reactive group per micellar head group, and if the substrate is fully micellar bound we can apply Eqn. 12, derived for reaction in non-functional surfactants, where ... 

Relative rate constants of reactions in functional micelles and comicelles ... 

Several reasons can be adduced for concerted reactions apparently being relatively unimportant in functional micelles. 

A quantitative assessment of the effects of head group bulk on Sn2 and E2 reactions in cationic micelles has been made.148 The kinetics of the acid-catalysed hydrolysis of methyl acetate in the presence of cationic, anionic, and non-ionic surfactants has been reported on.149 The alkaline hydrolysis of -butyl acetate with cetyltrimethylammonium bromide has also been investigated.150 The alkaline hydrolysis of aromatic and aliphatic ethyl esters in anionic and non-ionic surfactants has been studied.151 Specific salting-in effects that lead to striking substrate selectivity were observed for the hydrolysis of />-nitrophenyl alkanoates (185 n = 2-16) catalysed by the 4-(dialkylamino)pyridine-functionalized polymer (186) in aqueous Tris buffer solution at pH 8 and 30 °C. The formation of a reactive catalyst-substrate complex, (185)-(186), seems to be promoted by the presence of tris(hydroxymethyl)methylammonium ion.152... 

The determination of the enzyme activity as a function of the composition of the reaction medium is very important in order to find the optimal reaction conditions of an enzyme-catalyzed synthesis. However, the correlation between the reaction media properties and their effects on enzymatic reactions in reverse micelles is still unclear,... 

Figrue BE 16.20 shows spectra of DQ m a solution of TXlOO, a neutral surfactant, as a function of delay time. The spectra are qualitatively similar to those obtained in ethanol solution. At early delay times, the polarization is largely TM while RPM increases at later delay times. The early TM indicates that the reaction involves ZnTPPS triplets while the A/E RPM at later delay times is produced by triplet excited-state electron transfer. Calculation of relaxation times from spectral data indicates that in this case the ZnTPPS porphyrin molecules are in the micelle, although some may also be in the hydrophobic mantle of the micelle. Furtlier,... 

The problem of slow turnover has been noted. Initial reaction of a nucleophilic group, e.g. imidazole or oximate, in a functional micelle, with a carboxylic or phosphoric ester, for example, gives an acylated or phosphory-lated imidazole or oxime, and these derivatives hydrolyze slowly to regenerate the nucleophile. Kunitake and Shinkai (1980) discuss a number of reactions in micelles which contain both nucleophilic and basic groups which are potentially capable of acting as bifunctional reagents (Tonellato, 1979, Kunitake and Shinkai, 1980 Bunton, 1984)... 

Another example of rapid turnover in a micellar system is the cleavage of carboxylic and phosphate esters by o-iodosobenzoate in cationic micelles. This reaction was not studied with a functional micelle, but it is useful to note it in this context (Moss et al., 1983, 1986). 

In a functional micelle in which the reactive group is fully deprotonated there is a 1 1 relationship between the concentrations of reactive nucleophile and micellar head group in the micellar pseudophase. If under these conditions the substrate is fully micellar bound, (5) or (6) take the very simple form (19). This rate constant, kM, can then be converted into the second-order rate constant, k in M 1s 1, estimating the volume element of reaction, VM, which can be assumed to be that of the micelle or of its Stem layer, and these second-order rate constants can be compared with reaction in water of a chemically similar, non-micellized, nucleophile. 

It is easy to explain the large rate of enhancements which have been observed with some functional micelles, relative to reaction in water. 

Reactions in most functional micelles involve nucleophilic attack by an anionic moiety, e.g. oximate, hydroxamate, thiolate or alkoxide. Therefore it may be necessary to take into account the acid-base equilibrium which generates the anionic moiety. The simplest approach is to work at a pH such that deprotonation is essentially quantitative, but if this cannot be done the extent of deprotonation has to be measured directly or estimated. 

Section 6) can be applied to reactions of the non-micellizing ions. The second-order rate constants at the surface of a functionalized micelle and tri-n-octylammonium ion are very similar (Biresaw et al., 1984 Bunton and Quan, 1984). Some examples are given in Table 11. In some reactions mixtures of functional and nonfunctional amphiphile were used and allowance was made for the dilution. The second-order rate constants in Table 11 are calculated using nucleophile concentrations written as mole ratios of nucleophile to quaternary ammonium ion because it is not obvious how a molar volume element of reaction can be estimated for the tri-n-octylammo-nium ions. 

Stereoselectivity is small, or non-existent when rates of reaction of enantiomers are compared in micelles of chiral, but chemically inert surfactants (Moss and Sunshine, 1974). However, reactions of enantiomeric substrates in chiral functionalized micelles are stereoselective as are the corresponding reactions with chiral reagents in chemically inert micelles. 

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