Fluorous porphyrin

Scheme 3.13 Photosensitized oxidation of allylic alcohols and cyclohexene with singlet oxygen ( O2) in the presence of a fluorous porphyrin sensitizer (31) [32a].

The issues of sensitizer degradation and purification of the oxidation products were taken into account by DiMagno et al., who used the electron-deficient fluorous porphyrin 15 (5,10,15,20-tetrakis(heptafluoropropyl)porphyrin) as a sensitizer in the photooxidation of cyclohexene and allylic alcohols in CHjCN/perfluorohexanes [26]. 

A fluorous version of the cobalt-porphyrin-catalyzed aerobic epoxidation of alkenes was examined in the presence of 2-methylpropanal (Scheme 9). A fluorous porphyrin ring containing four 3,5-diperfluorooctylphenyl groups was prepared and complexed with Co(OAc)2 to form the cobalt catalyst 6. The reaction was carried out with vigorous stirring at room temperature in a biphasic system comprising... 

Oxidation reactions are not limited to those that occur at a carbon centre. The perfluorinated Ni(F-acac)2-benzene-CgFi7Br system described above was also active for the oxidation of sulfides to sulfoxides and sulfones [28], A sacrificial aldehyde is required as co-reductant, but the reaction may be tuned by changing the quantity of this aldehyde. If 1.6 equivalents of aldehyde are used, the sulfoxide is obtained, whereas higher quantities (5 equivalents) lead to sulfones. Fluorous-soluble transition metal porphyrin complexes also catalyse the oxidation of sulfides in the presence of oxygen and 2,2-dimethylpropanal [29],... 

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... 

Tetrakis(heptafluoropropyl)porphyrin (TPFPP) in a fluorous biphasic medium is effective for 02( Ag) generation and possesses good stability for... 

For typical fluorous biphase catalysis the most important aspect is the simple recycling and re-use of the catalyst. Fluorous solvents have one special advantage over hydrocarbon solvents, however. Their very high oxygen dissolving capacity, combined with their extreme resistance to oxidative decomposition makes perfluorocarbons in combination with fluorous catalysts the optimum choice for oxidation reactions. Thus, the biomimetic oxidation of olefins with molecular oxygen and 2-methylpropanal as a co-reductand has been achieved with a fluorous cobalt porphyrin catalyst (22) [23], and also even without catalyst [24] (Scheme 3.7). 

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],... 

In a CCI4 solution, porphyrin 15 showed increased chemical stability toward O2 and hydroperoxides with respect to TPPo. However, physical segregation into the fluorous phase was found to be the most important factor in reducing the incidence of degradation processes. In addition, the FB approach ensured the easy separation of the hydroperoxides from 15 at the end of the reaction. The fluorous layer containing the sensitizer (57-94% of the starting material-depending on reaction conditions) could be re-used without further treatments. 

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