Monophasic organic solvents

Semiconductor nanoclusters trapped in AOT w/o microemulsions are reported to exhibit longer excited state lifetimes (about 10-100 ns) than those in aqueous solution or in monophasic organic solvents [213]. Clearly the surfactant-nanoparticle interaction is very important not only in restricting growth but also in extending the hfetimes of the excited states. Tata et al. [214] have shown that the removal of water from the micelles leads to a strong increase in fluorescence intensity, and the addition of specific quencher, 4-hydroxythiophenol, leads to variations in quenching efficiencies. 

Fig. 31 Preparation of optically active compounds employing HLADH and NADH, which are codeposited onto glass beads in a monophasic organic solvent, (a) Reduction reaction to produce chiral alcohols in the presence of ethanol for NADH regeneration, (b) Oxidation reaction to produce enantiomerically pure alcohol or a ketone out of the racemic mixture coupled with the reduction of isobutyraldehyde to regenerate NAD+...

Dordick, J. S., Enzymatic catalysis in monophasic organic solvents, Enzyme Microb. TechnoL, 11, 194-211, 1989. 

In peptidase-catalyzed peptide synthesis the solubility of the starting components dramatically influences the course of the synthesis. From the ideal medium, water, the spectrum of solvents ranges from water-miscible organic solvents and aqueous-organic biphasic systems to monophasic organic solvents with trace amounts of... 

Lowering the water-activity of the medium [1565] by using water-miscible organic cosolvents such as ethanol or methanol. Alternatively, the reaction can be carried out in a biphasic aqueous-organic system or in a monophasic organic solvent (e.g., ethyl acetate, di-/-propyl, or methyl t-butyl ether) which contains only traces of water to preserve the enz3mie s activity. 

Monophasic organic solvents in which the catalyst is suspended constitute the most widely used way of obtaining the proper conditions to foster... 

Dordick, J. S. 1989. Enzymatic Catalysis in Monophasic Organic Solvents. Enzyme and Microbial Technology 11 (4) 194-211. 

Figure 12.1 Scanning electron micrographs of polymer formed by enzymatic synthesis in different synthetic conditions, a) AOT 1.5 M, p-ethylphenol 0.3 M, b) Monophasic organic solvent system of 85% dioxane and 15% water (by volume), c) AOT 0.5 M, p-ethylphenol 0.15 M, d) AOT 1.5 M, p-ethylphenol 0.15 M, e) AOT 0.5 M, p-ethylphenol 0.3 M, and f) AOT 1.5 M, p-ethylphenol...

In a comparative study of two systems (Table 12.7), (1) the monophasic organic solvent systems of dioxane plus water and (2) the reverse micellar system, the distinction in polymerisation lies in the oligomer-to-polymer ratio (soluble-to-insoluble product ratio). 

Lin and coworkers disclosed that, at room temperature, nonenzymatic chemical addition was still observed in a water-organic solvent biphasic reaction system, though the volume of aqueous phases was relative small. Lin developed a method of preparing an active enzyme meal that contained essential water to retain its power for catalysis and found a new catalytic reaction system by application of the prepared meal in a nonaqueous monophasic organic medium (Figure 5.7). There was no problem over a wide range of temperature (from 0-30 °C) when the reactions were carried out under micro-aqueous conditions [50]. 

Most of the oxidants used with these catalysts tend to be water soluble. However, the use of tetrabutylammonium monopersulfate (5) has been reported to provide relatively smooth oxidation with good ee s e.g, 4 -> 6) under monophasic conditions in organic solvents. The enantioselectivity is both substrate and catalyst dependent, with best results being obtained from electron-rich alkenes and Katsuki-type catalysts (c/. 2) <99TL1001>. Che and co-workers <99CC1789> have also reported on the use of an immobilized chromium binaphthyl catalyst, which offers the advantage of simple work-up and catalyst recovery. 

The insolubility of enzymes in monophasic organic systems has a controlling influence on the kinetics of enzymatic catalysis in organic media. Insolubilized enzymes are subject to intraparticle and external diffusional limitations which can mask the true, intrinsic kinetics of catalysis. These limitations are particularly severe for highly active and purified enzymes such as horseradish peroxidase. One way to overcome this problem is to increase the surface area of the enzyme in contact with the organic solvent. 

Biphasic systems proved to be advantageous as well in the biocatalytic synthesis of (-)-l-trimethylsilylethanol which was performed by asymmetric reduction of acetyltrimethylsilane with an isolate from Rhodotorula sp. AS2.2241 [144]. Immobilized cells were employed due to the easy separation of the product as well as the improved tolerance against unfavorable factors. In an aqueous/organic solvent biphasic system higher product yield and enantiomeric excess were achieved as compared to an aqueous monophasic system. Several organic solvents were examined, and isooctane was found to be the most suitable organic phase for the reaction. 

Fluorous biphasic catalysis was pioneered by Horvath and Rabai [54, 55] who coined the term fluorous , by analogy with aqueous , to describe highly fluori-nated alkanes, ethers and tertiary amines. Such fluorous compounds differ markedly from the corresponding hydrocarbon molecules and are, consequently, immiscible with many common organic solvents at ambient temperature although they can become miscible at elevated temperatures. Hence, this provides a basis for performing biphasic catalysis or, alternatively, monophasic catalysis at elevated temperatures with biphasic product/catalyst separation at lower temperatures. A number of fluorous solvents are commercially available (see Fig. 7.16 for example), albeit rather expensive compared with common organic... 

While the monophasic reaction in organic solvents is known to suffer from product inhibition, the continuous reaction in the liquid-liquid biphasic system allowed to overcome this limitation by in situ product extraction from the ionic catalyst phase. To avoid metal leaching out of the ionic liquid phase, the catalyst... 

When either the organic solvent or the ionic liquid is used as a pure solvent, the control of the water content, or rather the water activity, is of crucial importance as a minimum amount is necessary to maintain the enzyme activity. For ionic liquids, the same methods can be used to operate a reaction at constant water activity as those established for organic solvents [17,70,72]. As pure solvents and in biphasic systems [BMIM][PF6] or [BMIM][(CF3S02)2N], for example, are used. Water-miscible ionic liquids can be used in monophasic systems, e.g. [BMIM][Bp4] or [MMIM][MeS04]. 

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