Fluorous Salen

Formation of high-valent oxometal species from (salen)metal complexes and iodosylbenzene (PhIO) and the mechanism of the oxygen transfer from these species to alkyl aryl sulfides have been investigated in detail [22]. Fluorous salen ligands 13 and 14 were synthesized and the corresponding (salen)manganese(III) complexes Mn-13 and Mn-14 were evaluated in the oxidation of alkyl aryl sulfides with PhIO under homogeneous and FB conditions, respectively [23]. 

Only a few years after the development of the homogeneous chiral Mn(salen) complexes by Jacobsen and Katsuki, several research groups began to study different immobiUzation methods in both liquid and soUd phases. Fluorinated organic solvents were the first type of Uquid supports studied for this purpose. The main problem in the appUcation of this methodology is the low solubility of the catalytic complex in the fluorous phase. Several papers were pubUshed by Pozzi and coworkers, who prepared a variety of salen ligands with perfluorinated chains in positions 3 and 5 of the saUcyUdene moiety (Fig. 2). 

Further efficient ligands for the epoxidation of alkenes have been reported by Pozzi, but using PhIO as the oxidant and pyridine V-oxide as an additive in FBS.[7, 51-53] Chiral (salen)Mn complexes have been synthesised, which are soluble in fluorous solvents and active in the epoxidation of a variety of alkenes. The catalysts were of the form shown in Figure 6.14. 

Keeping in mind the recovery of the catalyst issue, Pozzi et al. [59] specifically tailored the salen ligand to suit its application in fluorous biphasic (FB) system (Figure 4). Accordingly, authors made modification at 5 and 5 position of Jacobsen catalyst by replacing fert-butyl group with perfluoroalkyl chain 6 or 3,5-bis heptadecafluorooctylphenyl 7, 8 [60]. 

Figure 4. Structure of fluorous chiral Co(salen) catalysts 6-8.

Cavazzini, M. Quid, S. Pozzi, G. (2002) Hydrolytic kinetic resolution of terminal epoxides eatalyzed by fluorous chiral Co(salen) complexes. Tetrahedron 58 3943-3949. 

More examples are found for varied oxidation processes mainly for various epoxidations carried out by metal catalysts bearing F-modified ligands, such as porphyrins,139 Ru perfluoroacetylacetonate salt,140 or salen complexes,141 142 or using the 3 selenium compound as catalyst.143 The potential for enantioselective transformations offering an easy way to recover precious chiral reagents and catalysts was demonstrated in enantioselective epoxidation using fluorous chiral salen... 

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). 

The use of fluorous chiral manganese salene (Jacobsen-Katsuki) catalysts (29, 30) [30] in combination with different oxidants enables enantioselective epoxidation of olefins [31] in high yields and with moderate to high enantiomeric excess (Scheme 3.12). 

Reagents involved in the oxidation of alcohols to aldehydes-ketones by oxygen under fluorous biphasic conditions are TEMPO, CuBr-SMe2, and 4,4 -bis[heptadecafluoro)-dodecyl]-2,2 -bipyridyl. The Mn(salen) complexes that mediate epoxidation of alkenes have been modified to bear polyfluoroalkyl substituents in the aromatic rings. Oxyfunc-tionalization of unactivated C—H sites is achieved with perfluorinated oj-2-butyl-3-propyloxaziridine. "... 

An asymmetric hydrogen transfer of ketones was reported using chiral perfluorinated ligands in a 2-propanol/n-perfluooctane biphasic system. Several perfluorinated salen and diamine ligands were examined for the reaction catalyzed by the [Ir(COD)Cl]2 complex diamine 16 was found to be most effective (Scheme 20). The reaction was carried out at 70" C for 30 min and then the mixture was cooled to 0°C. The perfluorooctane solution was separated and used for the next reaction. The reactivity was almost the same as that of the first mn, and the enantioselectivity was higher (79% ee). Two further recyclings of the fluorous layer yielded the product with enantioselectivities up to 59% ee, but a decrease in activity was observed. 

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]. 

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