Micro-aqueous organic reaction system

Iin and co-workers [40] have developed a so-called micro-aqueous organic reaction system. In contrast to former preparations of almond meal, the almond kernels are soaked in water prior to grinding. After the defatting step, the meal contains 8-10% water (w/w), making it unnecessary to add the amount of water needed for enzyme activity. The reactions are carried out in buffer-saturated organic solvents to avoid a possible drying effect of the solvent on the biocatalyst. Further addition of water to the reaction results in lower conversions and values,... 

The conversion of co-hydroxyalkanals to the corresponding cyanohydrins in moderate enantioselectivities could also be accomplished by transhydrocyanation with acetone cyanohydrin as the cyanide source. These substrates are considered difficult because of their high solubility in water. Through the employment of an almond meal preparation in a micro-aqueous organic reaction system, the ee-values could be significantly improved [54]. 

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]. 

A selective heating in liquid/liquid systems was exploited by Strauss and coworkers in a Hofmann elimination reaction using a two-phase water/chloroform system (Fig. 2.10) [32]. The temperatures of the aqueous and organic phases under micro-wave irradiation were 110 and 55 °C, respectively, due to the different dielectric properties of the solvents (Table 2.3). This temperature differential prevented decomposition of the final product. Comparable conditions would be difficult to obtain using traditional heating methods. A similar effect has been observed by Hallberg and coworkers in the preparation of /3,/3-diarylated aldehydes by hydrolysis of enol ethers in a two-phase toluene/aqueous hydrochloric acid system [33],... 

The measurement of fast interfacial reactions is very difficult, since the reaction has started just after contact of the two phases. The HSS method could measure a reaction several seconds after the contact, and the two-phase stopped-flow method could measure the reaction in the range from a few tenths of a millisecond to several hundred milliseconds. The micro two-phase sheath flow method can measure reactions as fast as < 1 ms [10]. A schematic drawing of the laser-induced fluorescence measurement in the sheath flow system is shown in Fig. 3. An inner organic phase and an outer aqueous phase flow with the same line velocity. The fluorescence at the interface is observed as a function of the distance from the end of the fused-silica capillary. The distance is converted into time. 

The AuNPs can also be prepared by the two-phase micro-emulsion method in which, first the metal-containing reagent is transferred from an aqueous to an organic phase. After the addition of a surfactant solution to this system, a micro-emulsion, i.e., a dispersion of two imntiscible liquids, is formed. In micro-emulsion methods, alkane thiols are often added to the reaction solution, and tliese additives form dense self-assembled monolayers on the gold surface. This method was employed for the preparation of self-assembled two- and three-dimensional ensembles of AuNPs [12,13,50]. 

Hydrogen addition across the C=C bond of dimethyl itaconate was performed with sodium formate in aqueous solution as an H-transfer reagent (Scheme 9.4) [54]. In this reaction, the substrate is in the organic layer whereas the catalyst and the formate are in the aqueous phase where the reaction takes place. In such a liquid-liquid biphasic system, mass transfer may be an issue. A 43% conversion was obtained in the micro-structured reactor while benchmarking with a traditional batch vessel afforded quantitative conversion (>99%). This was attributed to the poor mass transfer coefficient in the microdevice, which was originally designed to resolve heat transfer issues. 

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