Catalysis in Nonaqueous Solvents

Interactions between polar head groups in nonaqueous solvents provide the primary driving force for the formation of micellar aggregates in such media. The nature of such reversed micellar cores is such that they provide a unique location for the solubilization of polar substrates. While keeping in mind the potentially dramatic effects of additives on the properties of micellar solutions, it is obvious that such nonaqueous systems hold great potential from a catalytic standpoint. They are especially of interest as models of enzymatic reactions. 

The fundamental principles controlling activity in nonaqueous systems are the same as those for aqueous solutions, except that the specificity of the micellar core for the solubilization of polar substrates is much greater than for the aqueous situation. The popularity of reversed micelles as models for enzyme catalysis stems from the fact that the micellar core is capable of binding substrates in concentrations and orientations that can be very specific to certain functionalities, much as an enzyme would do. As a result, reaction rate enhancements can be obtained comparable (with luck) to those of the natural systems, and far in excess of what can be explained on the basis of partitioning or availability of substrate. 

Work in the area of micellar catalysis in both aqueous and nonaqueous solvent systems is certain to continue to grow in importance as a tool for better understanding the chemistry and mechanics of enzymatic catalysis, as a probe for studying the mechanistic aspects of many reactions, and as a route to improved yields in reactions of academic interest. Of more practical significance, however, may be the expanding use of micellar catalysis in industrial applications as a method for obtaining maximum production with minimum input of time, energy, and materials. 

Microemulsions are composed of two mutually immiscible liquid phases, one spontaneously dispersed in the other with the assistance of one or more surfactants and cosurfactants. While microemulsions of two nonaqueous liquids are theoretically possible (e.g., fluorocarbon-hydrocarbon systems), almost all of the reported work is concerned with at least one aqueous phase. The systems may be water continuous (o/w) or oil continuous (w/o), the result being determined by the variables such as the surfactant systems employed, temperature, electrolyte levels, the chemical nature of the oil phase, and the relative ratios of the components. 

Most microemulsions, especially those employing an ionic surfactant, require the addition of a cosurfactant to attain the required interfacial properties 

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Zaks, A. and Klibanov, A. (1988) Enzyme catalysis in nonaqueous solvents. J. Biol. Chem., 263,3194-3201. 

A. Zaks and A. M. Klibanov, Enzymatic catalysis in nonaqueous solvents,... 

Zaks A and Klibanov AM. Enzymatic Catalysis in Nonaqueous Solvents. JBio Cheni 1988a 263 3194-3201. 

Swain. C tj Brown. JFJr. Concerted displacement reactions. VII, The mechanism of acid-base catalysis in nonaqueous solvents. Journal of the American Chemical Society. 1952 74.2534-2537. 

Variations of resistance with frequency can also be caused by electrode polarization. A conductance cell can be represented in a simplified way as resistance and capacitance in series, the latter being the double layer capacitance at the electrodes. Only if this capacitance is sufficiently large will the measured resistance be independent of frequency. To accomplish this, electrodes are often covered with platinum black 2>. This is generally unsuitable in nonaqueous solvent studies because of possible catalysis of chemical reactions and because of adsorption problems encountered with dilute solutions required for useful data. The equivalent circuit for a conductance cell is also complicated by impedances due to faradaic processes and the geometric capacity of the cell 2>3( . 

For some recent reviews on the use of enzymes in nonconventional media, see (a) Dreyer, S., Lembrecht, J., Schumacher, J. and Kragl, U., Enzyme catalysis in nonaqueous media past, present, and future in biocatalysis in the pharmaceutical and biotechnology industries, 2007, CRC Press, pp. 791-827 . (b) Torres, S. and Castro, G.R., Non-aqueous biocatalysis in homogeneous solvent systems. Food Technol. BiotechnoL, 2004, 42, 271-277 (c) Carrea, G. and Riva, S., Properties and synthetic applications of enzymes in organic solvent. Angew. Chem. Int. Ed., 2000, 39, 2226-2254. 

A primary reason for low yields of 5-HMF is its rapid conversion to levulinic acid in aqueous media. However, catalysis of the transformation with lanthanides has led to dramatic increases in the yield of 5-HMF. The effectiveness of different lanthanide cations has been surveyed.490,491 In nonaqueous solvents, such as DMSO, almost quantitative yields of 5-HMF have been reported.492 Performing the dehydration in the presence of activated carbon (to adsorb the generated 5-HMF) has also been reported as effective.493... 

To sum up, acid-base catalysis may often follow normal kinetics in nonaqueous solvents, which may then be used for investigating reaction mechanisms in the ordinary way. On the other hand, more complicated kinetics may sometimes occur, especially in aprotic solvents, and caution must be exercised in drawing conclusions without investigating the kinetic behavior experimentally. 

Before we discuss characteristic features of a metal sulfate catalyst, it is to be noted that the model reaction should be one which has as straightforward a mechanism as possible, preferably in a homogeneously catalyzed reaction. This is the only way we can critically evaluate the efficiency of the present solid catalyst system. The depolymerization of paraldehyde was most extensively studied in view of the foregoing criterion. For the homogeneous acid catalysis of the depolymerization of paraldehyde, there are ample data given by Bell and his associates in nonaqueous solvent (by proton acid as well as Lewis acid) and also in aqueous solution (55,56). Since most Hammett indicators change their color when adsorbed on the surface acids of both Bronsted and Lewis type, it is fortunate that this depolymerization proceeds easily by acids of both types. Evidently the dotted line in Fig. 2 shows excellent... 

Enzyme-catalyzed asymmetric syntheses involve two types of reactions (1) the asymmetric reduction of a prochiral center and (2) the resolution of a racemic material by selective reaction of one enantiomer. Both types arc demonstrated in the syntheses of chiral insect phermones reviewed by Sonnet (1988). Enzymes that have broad substrate specificity and still retain other selectivity features can be versatile and powerful catalysts. In addition, enzyme catalysis is applicable not only in aqueous media but also in nonaqueous solvents, including supercritical fluids (20-22), In all cases, however, enzymes require water to function as catalysts. A small amount of water, corresponding to a monolayer on the enzyme molecule, is usually sufficient (20),... 

If counter ions have an effect on the electron-transfer rates, then Marcus theory would have a problem because these ions are not included in the theory. For most cationic reactants, such as those in Table 6.2, anions affect the rate through normal ionic strength effects and ion pairing. The latter has been observed generally to inhibit reaction in nonaqueous solvents. The Co(phen)3 " system is somewhat unusual in that N03 seems to have some catalytic effect. For anionic reactants the situation is quite different and cations often provide significant catalysis. One of the most widely studied of these is the Fe(CN)g system for which Wahl and... 

Oxidized Fe-TAML could be the iron(V)oxo complex 6, which as noted above can be produced from la and m-chloroperox-ybenzoic acid at low temperatures (—60°C) in a nonaqueous solvent (51). Presumably such an iron(V)oxo complex can behave in a substrate-dependent way as both a two-electron or one-electron oxidant. In the former case, it is returned in one step to the iron(III) state. In the latter, it must first pass through an iron(IV) intermediate. At pH>12, the likely iron(IV) species would be the same compound as is formed from la and H202, (48) namely the iron(IV)-oxo complex 7, which has similar features with [(H20)sFeIV = 0]2 +, (54) or its water adduct 7". At other pHs, other iron(IV) compounds are known to be formed (48). Both the iron(V)-oxo and iron(IV)-oxo complexes as well as the other iron(IV) species could be involved in catalysis by 1 (see Section V.B). The possible involvement of complexes that are in a higher oxidation state than 6 cannot be ruled out. 

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