What Separation Methods Exist ?

Fluorous Technology Primer - Part II

Liquid-Liquid Extractions

Perfluorinated or very highly fluorinated solvents are called "fluorous solvents" and they are typically immiscible with organic solvents and water. They are used in liquid-liquid extractions to quickly separate fluorous compounds from organic compounds in a two phase liquid-liquid extraction, or from organic and inorganic (or water soluble organic) compounds in a three-phase liquid-liquid extraction. The most popular fluorous solvent is 3M's FC-72®, but a number of related solvents are available and these are all comparably priced.

A photograph of a typical three-phase liquid-liquid extraction is shown on the left. Such extractions are readily automated, and can be used to quickly partition reaction mixtures into organic (yellow), inorganic (blue) and fluorous (clear) fractions. In many cases, the crude organic products are pure enough to be taken on to the next reaction, and the fluorous products can usually be recycled, if desired.

Liquid-liquid extractions work well when fluorous domains are relatively large. In the best cases, only a single separation is needed. With lower partition coefficients, the organic fraction is washed several times with the fluorous solvent. Thanks to the exceedingly low solubilities of organic compounds in fluorous solvents, the washing process can be conducted repeatedly without extractive loss of the organic product. Liquid-liquid extractive methods are typically used when the desired product is organic and some other reaction component (reactant, reagent, catalyst, scavenged product) is fluorous.

Solid-Liquid Extractions

FTI sells a complete line of fluorous reverse phase silica gel products under the FluoroFlash® name. In addition to loose silica with a SiCH₂CH₂C₈F₁₇ bonded phase, FluoroFlash® TLC plates, solid phase extraction and flash chromatography cartridges and HPLC columns are available. Silica gel with a fluorocarbon-bond phase can be used to adsorb fluorous molecules and free them from non-absorbed organic molecules by the simple process of solid-liquid extraction illustrated below. In the separation stage, a crude reaction mixture is charged to a suitable amount of fluorous silica gel and the silica is eluted first with a "fluorophobic" solvent to remove the organic compounds while leaving the fluorous compounds adsorbed. In cases where the fluorous products are desired, a second elution with a "fluorophilic" solvent then provides this material.

These fluorous solid phase extractions are different from traditional chromatographies, and this is advantageous in a parallel setting. In solid phase extractions relatively high loadings of substrate/silica are used, and all of the mixtures in the synthesis behave identically. No fractionation is needed. In traditional chromatographies, each mixture behaves differently and lower loadings and carefully monitoring of fractions are needed.

The sequence below shows the separation of a fluorous dye (F-orange, FTI) from a non-fluorous one (Solvent Blue-35, Aldrich) by Fluorous SPE over a 2 gram FluoroFlash® silica gel cartridge. These aminoanthraquinone dyes have about the same polarity and cannot be separated by regular or reverse phase silica gel. The cartridges can be reused. For detailed instructions, along with a list of frequently asked questions, please download our F-SPE application note.

The solid-liquid extractions are operationally filtrations and they are easy to conduct in parallel either manually (see the manual solid-phase extraction apparatus above) or by using various automated techniques. In addition to the operational convenience, solid-liquid extractions succeed with many fewer fluorines in the fluorous domain compared to liquid-liquid extractions. For this reason, solid-liquid extractions are especially useful when the desired product of the reaction bears a fluorous tag. The solid phase extraction is applicable in essentially all areas from traditional synthesis through parallel synthesis, and is especially useful for parallel synthesis of intermediates.

Solid-liquid extraction over FluoroFlash® silica is currently the most general and most easily implemented fluorous-organic phase separation technique. It is useful for the gamut of fluorous methods. Fluorous solvents are rarely needed for the extractions, and they are used only to wash the silica prior to reuse, if desired.

Fluorous Chromatography

The separation of fluorous molecules from each other can sometimes be accomplished by standard chromatographic techniques, including traditional or reverse phase chromatography. However, the best way to separate fluorous compounds from each other is usually by chromatography over FluoroFlash® fluorous silica. These separations capitalize on the unique feature of fluorous solid phases, which is their ability to separate molecules primarily by fluorine content. An illustrative example of this is shown below with a family of fluoroacyl-tagged amides. The control compound lacking the fluorous tag (C₇H₁₅) comes off with the solvent front, as do most other non-fluorinated organic compounds under these conditions. The fluorinated homologs then emerge strictly in order of fluorine content, and a solvent gradient is needed to push the more highly fluorinated members of the series off the column.

Many popular fluorous techniques involve fluorous-organic separations, so preparative fluorous chromatography is not needed. However, fluorous chromatography still has two major uses. First, it can be used in methods development experiments to select suitable solvents for fluorous-organic solid phase extractions, thereby ensuring in advance that separation conditions are suitable. Second, it can be used to analyze the purity of essentially any kind of fluorous component, and it provides information that is largely complementary to traditional chromatographic analyses. In contrast to other methods, fluorous mixture synthesis techniques rely heavily on fluorous chromatography for the separation of tagged compounds by the fluorine content of the tag.

Our Technology Primer - Table of Contents

• Liquid-Liquid Extractions
• Solid-Liquid Extractions
• Fluorous Chromatography

• Advantages at a Glance
• For Further Reading