Phase transfer catalysis (PTC) has been an active area of interest for over 50 years and have now been incorporated into hundreds of industrial processes. The primary role of these catalysts are to transfer an inorganic reagent from a non-organic phase into the organic phase containing an organic substrate. Onium salts, ammonium, phosphonium, etc., are among the most popular of phase transfer catalysts. Crown ethers are another important class of PTC’s, which are widely used. After reaction, the catalyst is often removed by aqueous extraction, which separates the product from the phase transfer catalyst. This, of course, leaves the problem of how to treat and remove the PTC from the wastewater. A more sustainable approach would be to be able to remove, recover, and reuse the catalyst without effecting the activity of the catalyst.
Fluorous modified PTC’s have been prepared and tested by several groups including Gladysz, Pozzi, and Horvàth. Prof. Alison Stuart’s group at the University of Leicester has also been active in this area, specifically fluorous crown ethers. They report their latest results in a recently accepted manuscript in J. Fluorine Chem. In this case they made fluorous dibenzo-crown ethers as pictured below. They also prepared the corresponding alkyl substituted dibenzo-crown ethers as controls. Their goal was to produce PTC’s which are recoverable by fluorous solid phase extraction (FSPE) and easily recyclable.
The preparation of these fluorous PTC’s is relatively straightforward, so I won’t detail that here, but rather concentrate on the activity and recyclability. The first study was to compare cation coordinating capability of each of the crown ethers. They found that all four crown ethers were essentially similar when it came to pottassium ion coordination, but they found large differences in sodium ion coordination with the fluorous crown ethers coordinating sodium ions better than the alkyl analogs. They interpreted these results to mean that the electron withdrawing effects of the perfluoroalkyl chain were minimal due to the ethylene spacer or the nitrogen, but that conformational changes due to the perfluoroalkyls led to increased coordination of the smaller sodium ion.
The authors then looked at several nucleophilic substitution reactions; two Finkelstein reactions and a nucleophilic aromatic substitution reaction. In the Finkelstein reactions they found that the fluorous crown ethers were the most catalytically active while in the nucleophilic aromatic substitution they found that all of the PTC’s worked similarly.
When it came to recoverability, however, the fluorous versions when combined with FSPE were clearly superior providing recoveries in the 92-94% range. The recovered fluorous crown ethers were used up to four times. Using conventional silica gel of the non-fluorous crown ethers, recoveries were in the 80% range demonstrating the advantages of using fluorous tagged molecules in conjunction with fluorous separation media.
The authors have developed fluorous crown ethers which were not only more active than their non-fluorous analogs, but also were more effectively recovered and recycled using FSPE.