Three-phase extraction

In addition to SLM, which is the most commonly used three-phase extraction principle, at least in analytical chemistry, also other ways of placing an organic phase between two aqueous phases are known. In the classical bulk liquid membrane (BLM) setups, U-tubes or similar devices are used to confine bulk volumes of organic liquids between two aqueous phases. This type of devices is very little used for sample preparation in analytical chemistry, as the extraction process becomes slow and the enrichment factors possible are very limited. 

A large number of fluorous tin reagents for use in palladium-catalyzed Stille cross-coupling reactions with organic halides or triflates have been developed. Reaction times can be reduced to just a few minutes when conducted with microwave heating. Three-phase extraction between water (top), dichloromethane (middle), and perfluorohexanes (FC-72) (bottom) allow the coupled product to be isolated from the dichloromethane and the resulting fluorinated tin chloride to be recovered from the FC-72 phase and recycled (Scheme 5.8.8). 

Preparation of 26 [14] Allyl alcohol 22 (0.91 mmol) and triethylamine (1 equiv.) were dissolved in dry tetrahydrofuran (THF) (2 mL) under argon. A solution of bromo tris(2-perfluorohexylethyl)silane 23 (0.25 equiv) in THF (2 mL) was slowly added to the reaction mixture at 25 °C. The resulting mixture was stirred at 25 °C for 3 h. After removal of the solvent, the residue was purified by three-phase extraction with FC-72 (10 mL), dichloromethane (10 mL), and water (10 mL). The organic/aqueous biphase was extracted twice more wdth FC-72 (10 mL). After concentration of the combined fluorous extracts, the residue was purified by flash chromatography (hexane/diethyl ether, 50 1) to yield a colorless oil. 

Fluorous trialkyl silyl protecting groups have also been used to simplify the purification of complex reaction mixtures [5] (Scheme 3.16). Separation of the reaction products can be achieved by means of a simple three-phase extraction (aqueous/or-ganic/fluorous) instead of the usual chromatography. In this respect, the concept of using fluorous protecting groups has some parallels with the solid-phase-sup-ported chemistry which also was primarily developed to simplify multiple workup operations. 

Scheme 3.17 Fiuorous variants of the Ugi (above) and Bigineiii muiti-component reactions (beloiv) enabie the purification of the primary fluorous condensation products (not shown in this scheme) by simple two- or three-phase extraction, followed by "traceless" cleavage of the fluorous silyl tag with TBAF [5].

One early example of microwave-assisted fluorous synthesis involved palladium-catalyzed Stifle couplings of fluorous tin reagents with aryl halides or triflates (Scheme 16.59) [90]. The desired biaryl products were isolated in good yields and purity after a three-phase extraction. Similar results were also achieved by use of so-called F-21 fluorous tags (CFl2CFl2CioF2i) on the tin reagent [91]. 

A highly fluorous alkoxy ethyl ether was synthesized and used as a protective group of alcohols. Treatment of an ether solution of 1 equivalent of a primary alcohol and 3 equivalents of the fluorous vinyl ether 29 with 5 mol% of camphorsulfonic acid (CS A) for 3 h at room temperature provided the desired protected alcohols 30 in 84-93% yields (Scheme 33). The products and the excess fluorous vinyl ether were extracted with FC-72 and then separated by column chromatography. Secondary and even tertiary alcohols were similarly protected in good yields in THF at 65°C for 30 5 min. The protected fluorous acetals were treated in a 1 1 solution of ether and methanol with 5 mol% of CSA for 1 h for deprotection. After completion of the reaction, the products were isolated in pure form by simple three-phase extraction (reaction mixture/saturated aqueous NaHC03/FC-72). 

The reaction of fluorous tin such as ArSn(CH2CH2C6Fi3)3 makes product isolation easier, because the organotin by-product formed after the reaction is easily separable by a three-phase extraction method.f f ° ... 

A three-phase extraction system has been described. The method depends on the separation of a mixed organic solvent into two phases when diantipyryl-methane is present as the extracting agent 489). In one phase, the so-called third phase, the extracting elements are concentrated to 95-98% yield. The small volume of this third phase permits it to be transferred quantitatively and directly to the electrodes of the emission spectrograph. The system was described for the separation and analysis of Hg, Se, Sn, Cd, B, Zr, and Hf. 

Different extraction techniques have been developed. These techniques have been classified as porous and nonporous, based on their structure, as a flat (like a paper sheet with less than 1 pm of thickness) or hollow fiber (200-500 pm i.d.) configuration. Other classification refers to the number of phases involved in the extraction (one-, two-, or three-phase extraction techniques) [186]. A distinction can be based on the nature of the acceptor phase liquid membrane extractions, where the acceptor phase is a liquid, such as supported liquid membrane (SLM) extraction, microporous membrane liquid-liquid... 

Olive oil is nowadays produced by either two or three-phase extraction systems. The three phase method in which a solid fraction (wet pomace or alperujo), a liquid residue ( alpechin ), and the olive oil are obtained. Two phase method in which a solid fraction, wet pomace and a liquid one, the olive oil, are obtained. The main advantage of the two-phase system is the reduction of the olive mill wastewater amoimts compared to that originated from the three phase technology, in which 0.6-1.31 water/kg olives are used at the centrifugation step, by eliminating the production of alpechin thus avoiding its consequences environmental... 

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