Fluorous tagged organocatalysts have been used by several groups to facilitate the removal, recovery, and reuse of these important class of catalysts from post reaction mixtures. For previous F-Blog posts regarding organocatalysts, please click here. The Miura group at Gifu Pharmaceutical University have just recently published new results in fluorous tagged organocatalysis as a communication in Org. Letters. They had previously used a L-phenylalanine derived sulfonamide in the organocatalyst mediated asymmetric aldol reaction as shown below. The reaction is interesting since it is run in brine. Overall, d.e.’s and e.e.’s were generally in the good to very good range.
The researchers then wished to employ a fluorous tagged organocatalyst for recycling in order to take advantage of FSPE separation. By simply changing the trifluorosulfonamide to a perfluorooctylsulfonamide they were able render the organo catalyst fluorous. Once the fluorous organocatalyst was prepared they optimized the reaction conditions which ended up being very similar to what they used before. The only major difference being the temperature of the reaction. The observed d.e. and e.e. was essentially the same as with the non-fluorous organocatalyst. These conditions were then employed on a number of different aldehyde substrates some of which are shown below. As seen electron deficient aldehydes worked better than electron rich.
The real benefit of using the fluorous organocatalyst, however, is in the ease of isolation by FSPE. Since the reaction is run in water the crude reaction mixture can be directly applied to a FSPE cartridge. A fluorophobic wash of 70% methanol in water is used to wash all organic components off the cartridge including the product. A fluorophilic wash of 100% methanol was then employed to recover the fluorous organocatalyst in >90% recovery. It doesn’t get much simpler than that. The recovered catalyst was reused 5 times with only slight degradation in performance, which can be attributed to the slightly lower amount of organocatalyst used during each cycle.
Perfluorinated compounds (PFCs) such as perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) have been identified as bioaccumulative and environmentally persistent. The identification and detection of these compounds in environmental and other matrices is one of increasing importance. Detection of these compounds at low ppb levels is desired and several methods are in place. There are however some types of samples which are difficult to analyze, primarily those that contain solid fats. This is an important limitation since it’s known that PFCs bioaccumulate preferentially in fatty tissue. An accepted manuscript in J. Flourine Chem. describing the use of fluorous liquid-liquid extraction (FLLE) and weak anion exchange SPE addresses this shortcoming and provides a method by which PFCs can be detected at the ppb level from high fat content samples.
The researchers from the University of York and the Food and Environment Research Agency of the UK used a solvent tuning strategy in order to maximize either selectivity or recovery of the PFCs from fatty matrices. In 2005 FTI scientists first reported the use of solvent tuning rather than substrate tuning for increasing partitioning coefficients in FLLE. Clearly for a set of analytes of fixed composition such as PFCs, substrate tuning is not posible. Within that 2005 report we proposed a qualitative model to guide researchers in selecting solvents in a rational manner to increase fluorous partitioning. The UK researchers applied the model in a semi-quantitative manner using % fluorine content of the fluorous solvent and polarity index for the non-fluorous solvent. They found that changes in partition coefficients for various PFCs and solvent combinations were consistent with the model’s prediction.
The researchers examined over 40 different solvent combinations and identified 3:1 trifluoroethanol:perfluorohexane and water saturated DCM as the best as it provided over 80% recovery for 11 different PFCs. Although a non-fluorous compound TDCA also was partially recovered, it was sufficiently selective so that the fluorous phase could be evaporated, reconstituted in MeOH, and analyzed by TOF-MS. A procedure which used FLLE followed by anion exchange SPE was developed for the detection of PFCs in cheese samples. As seen in the picture below, this protocol is capable of enriching solid fat containing samples in PFOS down to 1 ppb detection levels.The authors then tested 10 different cheese samples and found that none contained PFC levels above 1 ppb.
An excellent example of not only solvent tuning to optmize of FLLE but also demonstrating how fluorous separations can be used in conjunction with other separation modes to achieve results unobtainable using only one method.
The 239th American Chemical Society National Meeting will be held in San Francisco from March 21-25, 2010. There are a number of presentations and posters that may be of interest to readers of F-Blog, beyond the ones that would result from a simple search on the word “fluorous”. There are over 40 presentations which have perfluoro compounds within the title or abstract. We’ve listed some of them below which we viewed as relevant to fluorous separations, immobilizations, or preparations. To view the abstracts and authors other than the presenting author, you can click on the titles. If you are going to be attending the ACS Meeting please keep these in mind or add them to your schedule.
Late last year Prof. Qisheng Zhang at the University of North Carolina published a paper describing his group’s efforts at developing fluorous diazirines as photoaffinity labeling reagents. For more details on Prof. Zhang’s work, you can view the presentation he gave at ISoFT ‘09. Such reagents appended to appropriate substrates could then be used to investigate various intermolecular interactions such as those that occur between a protein and a small molecule. The strategy is that photoactivation of the substrate in the presence of the protein will result in a covalent bind being formed between the two molecules. Digestion, enrichment by FSPE, and analysis by MS can then provide information regarding the active site of the protein. Previous methods for identifying the cross-linked molecules include radioisotope tagging and biotin based enrichment, each of which has some disadvantages. Hence the application of fluorous techniques.
Prof. Zhang isn’t the only person thinking along these lines as researchers from die Universität Tübingen have now also reported using fluorous diazirines as photoaffinity labels. The approaches were essentially the same in term of designing and synthesizing the fluorous diazirines. The researchers, led by Prof. Stefanie Grond, then attached the fluorous photoaffinity tag to two V-ATPase inhibitors, 21-deoxyconcanolide A and bafilomycin A. The modified inhibitors were then purified by FSPE.
With the fluorous photoaffinity lavels in place they then compared the inhibitory activity of the compounds with unlabeled concanolide A and bafilomycin A in a V-ATPase holoenzyme assay. In the case of concanolide A they found a slight decrease in IC50 with the fluorous photoaffinity tag compared to unmodified concanolide, but the labeled compounds maintain enough inhibitory potential to be useful probes. This was not the case with bafilomycin where the addition of the photoaffinity label substantially changes the inhibitory activity of the substrate to the point where the labeled balfilomycin is no longer a useful compound.
The authors conclude by stating that the first studies using fluorous photoaffinity labeled concanolide A to separate and identify labeled protein fragments using FSPE and MS are underway.
An Org. Letters ASAP paper from the Merck Research Labs in Boston reports the formation of ureas and carbamates using CO2. The researchers reacted amines and anilines with CO2 catalyzed by DBU to form a carbamic acid intermediate. This intermediate was then reacted with amines or alcohols under Mitsunobu conditions, i.e. PBu3 and DBAD, to provide carbamates and ureas. They then used a modified Bohdan Miniblock to run reactions in parallel. Using this set up they were able to conduct 24 reactions at a time. Each of these were then purified using automated flash chromatography on a Biotage.
The chemistry itself is interesting from a mechanistic standpoint. When starting from a primary amine, the carbamic acid is dehydrated under the Mitsunobu conditions to form the isocyanate. Support for this mechanism is found in that secondary amines do not form any urea product. When forming ureas, however, the reaction seem to take place through standard Mitsunobu mechanism rather than dehydration. This is supported by the fact that secondary amines, incapable of dehydration to the isocyanate, are good substrates for the reaction and when using a chiral alcohol inversion at the stereocenter was observed. A nice one-pot methodology which avoids using phosgene or other active acylating agents.
From a fluorous perspective, this is the type of reaction that could easily be renedered fluorous using fluorous tagged phosphine and fluorous DIAD. The reactions could then be run in parallel with parallel purification. This could have several advantages. First, if this reaction were in the middle of synthesis it would allow for quick intermediate purification without having to use chromatography. Even automated, 24-48 purifications takes time, particularly instrument time that others could use. Second, for the final step this would allow for a quick pre-purification prior to final HPLC resulting in greater recoveries and purities.
At FTI we’ve successfully implemented fluorous methods into our library syntheses and production. Through a NIH Pilot-Scale Libraries grant the value of using fluorous techniques has been demonstrated by the number and quality of the libraries we have submitted to the Molecular Libraries Small Molecule Repository (MLSMR).
