Perfluorocarbon soluble catalysts

M. Cavazzini, F. Montanari, G. Pozzi, S. Quia, Perfluorocarbon-Soluble Catalysts and Reagents and the Application of FBS (Fluorous Biphase System) to Organic Synthesis , J. Fluorine Chem. 1999, 94,183. 

The first example of FB oxidation of sulfides dates back to 1995 dibenzothiophene and diphenyl stdfide gave the corresponding sulfones in low yields (1.4% and 10%, respectively) upon treatment with O2 at 100 °C in the presence of a not fully characterized perfluorocarbon-soluble iron—phthalocyanine [19]. Following this earlier report, Co(ll)—tetraarylporphyrin Co-5 and Co(I I [—phthalocyanine Co-12 (cf Stmcture) were tested as catalysts for the FB oxidation of methyl phenyl sulfide and para-substituted aryl methyl sulfides with O2 and a sacrificial aldehyde (Table 3) [20]. 

Alkane and Alkene Oxidation with Perfluorocarbon Soluble Metalloporphyrins as Catalysts... 

An exciting recent development in this field is the application of the unusual solubility properties of perfluorocarbons as solvents in so-called fluorous biphase chemistry [7]. These novel synthetic procedures enable ready separation of suitably designed catalysts and reagents by simple phase separation of many hydrocarbon solvents (containing the product) from the fluorous phase containing the catalyst. Success in these procedures is of course contingent on the inert nature of the fluorous phase under the reaction conditions. 

The manganese catalyst is selectively soluble in perfluorocarbons and has been tested in the enantioselective epoxidation of styrene derivatives under flu-orous biphasic conditions (C8F18/CH2CI2) at 20°C. In most cases, good yields have been observed, however, only indene was epoxidized with high enantiose-lectivity (92% ee) while all other olefins gave low enantiomeric excess Eq. (24). 

Metal complexes of fluorous tetraarylporphyrins (1-5) have been used as catalysts in the epoxidation of alkenes under FB [9] or more traditional conditions [10], depending on their affinity for perfluorocarbons. Free base porphyrins 1-5 were readily metaUated with transition metal cations under standard conditions normally employed for their nonffuorous coimterparts. In particular, porphyrins 1-4 were metalated with Mn(OAc)2 4 H2O in boiling DMF to give their respective Mn(III) complexes Mn-l-Mn-4 [10], whereas the perffuorocarbon-soluble porphyrin 5 was similarly converted into the cobalt(II) complex Co-5 by treatment with Co(OAc)2 -4H20 [9],... 

Another attractive line of application of macromolecular complexes as catalysts is the so-called fluorous catalysis, whose idea was proposed in the early 1990s [163-168]. The main idea of such catalysis is that a catalyst is soluble (immobilized) in the perfluorocarbon phase, whereas the product is soluble in an organic solvent. If a suitable solvent for substrates, e.g. toluene or benzene, is used, then the system at sufficiently high temperature is homogeneous, and on cooling, there is phase separation. As a result, the solution of the metal complex in fluorocarbon is readily separable from the reaction products and can be reused. 

Another FBC approach was developed by Knochel and co-workers to oxidize olefins, sulfides, and aldehydes to epoxides, sulfoxides, sulfones, and carboxylic acids (79). They used the Ru and Ni complexes of a perfluorinated 1,3-diketone, obtained in one step in 80 % yield by the condensation of a perfluoromethyl ester and a perfluoromethyl ketone, as catalysts for these oxidation reactions. More importantly, these Ru and Ni complexes were found to be highly soluble in perfluorocarbons (Figure 3). 

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