A Novel Fluorous Biphasic System: Werner-type Complexes in Fluorous Media

Abstract

Fluorous chemistry has seen a number of advances since its birth in the early 1990s. One of the most attractive characteristics of fluorous solvents is their unique solubility properties depending on temperature. This phenomenon has led to the development of a wide range of catalysts that are modified with fluorous tags and are used in biphasic catalysis and easily recovered. Many fluorous phase transfer catalysts are confined to bringing small ions into fluorous media by using fluorous onium or crown ether vehicles. The most popular method to bring transition metal complexes into fluorous media is quite limited, usually resulting in ligand tuning and thus a change in reactivity at the metal center. This can be circumvented by pairing a cationic transition metal with a highly fluorous anion rendering the neutral species highly fluorophilic. To achieve this goal, we chose to use fluorous BArf?, [B(3,5-C?H?(Rf?)?)?]?, as the mode of transport and pair it with classic Werner-type complexes that recently have been shown act as organocatalysts in enantioselective Michael additions. The literature synthesis of Na[B(3,5-C?H?(Rf?)?)?] (3) was improved and through salt metathesis two new fluorophilic salts were made. The Werner-type trication [Co(en)?]?? was solubilized in PFMC (perfluoromethylcyclohexane) to generate [Co(en)?][B(3,5-C?H?(Rf?)?)?]? (4). This fluorophilic salt was found to be preferentially soluble in fluorous media with a partition coefficient in PFMC/H?O of 99.0:1.0 and in PFMC/CH?C?H? of >99.3:<0.7. Another Werner-type trication, [Co(R,R-chxn)?]??, was also paired with [B(3,5-C?H?(Rf?)?)?]? to afford [Co(R,R-chxn)?][B(3,5-C?H?(Rf?)?)?]? (5), whose partition coefficients in PFMC/H?O and PFMC/CH?C?H? were the same as 4. Within the scope of Werner-type complexes, this work constitutes a significant stride toward developing a series of compounds that bring the concept of organocatalysis into fluorous media. The new compounds 3-5 show high preferences for the fluorous phase and provide a baseline for future Werner-type salt metathesis with fluorous BArf?.