Epoxidation fluorous catalysis

In this context it is interesting to note the recent reports of fluorous catalysis without fluorous solvents [68]. The thermomorphic fluorous phosphines, P[(CH2)m(CF2)7CF3]3 (m=2 or 3) exhibit ca. 600-fold increase in n-octane solubility between -20 and 80 °C. They catalyze the addition of alcohols to methyl propiolate in a monophasic system at 65 °C and can be recovered by precipitation on cooling (Fig. 7.20) [68]. Similarly, perfluoroheptadecan-9-one catalyzed the epoxidation of olefins with hydrogen peroxide in e.g. ethyl acetate as solvent [69]. The catalyst could be recovered by cooling the reaction mixture, which resulted in its precipitation. 

Cobalt tetraarylporphyrins with fluorine-containing substituents were active in epoxidation of alkenes using fluorous catalysis in the presence of oxygen and 2-methylpropanal [167,170-171]. Manganese and cobalt complexes of perfluorinated tetraazocyclonone catalyzed allylic oxidation of alkenes with r-BuOOH/Oa [172]. The complex with the salen ligand 57 was active in alkene epoxidation under Mikayama s conditions, and indene was epoxidated at a high stereospecificity [173]. 

The possibility of asymmetric induction under the fluorous biphase conditions was first speculated upon by Horvath and Rabai [10], and this year has seen the first report of asymmetric catalysis in a fluorous biphase [69]. Two, C2 symmetric salen ligands (29a, b) with four C8Fi7 ponytails have been prepared (Scheme 5) and their Mn(II) complexes evaluated as chiral catalysts for the aerobic oxidation of alkenes under FBS-modified Mukaiyama conditions. Both complexes are active catalysts (isolated yields of epoxides up to 85%) under unusually low catalyst loadings (1.5% cf. the usual 12%). Although catalyst recovery and re-use was demonstrated, low enantioselectivities were observed in most cases. 

Catalytic oxidahon reachons were among the earliest explored applicahons of the FBS concept because of the chemical and thermal stabihty of perfluorocarbons, the convenient partihon of the final polar products into the organic phase, and the possible increased lifetime of the catalysts confined in the fluorous phase [4—6]. The first example of enantioselechve catalysis under FBS condihons, reported in 1998, also dealt with an oxidation process, namely the epoxidahon of prochiral alkenes [26]. Mn(III)-complexes of salen hgands 2 and 3 were found to catalyze the asymmetric epoxidation of indene in a two-phase system CH2Cl2/perfluoroc-tane at 20 °C under an atmospheric pressure of oxygen in the presence of pivalal-dehyde (Scheme 5.1). 

Except for transitions from heterogeneous to homogeneous catalysis, there is also common groimd for the various methodologies described here the application of SCCO2 is described in the presence of ionic liquids [32a] or of fluorous solvents [32b,c] as well in aqueous operation [33a-d] and aerobic epoxidations have been attained in fluorous biphasic systems using ionic liquids [33e]. On the other hand, ionic liquids [34] or fluorous solvents [35a,c] have been used together with aqueous operations and water-soluble polymers are the focal point of the application of... 

---