Fluorous aerobic oxidations

Figure 6.16. Aerobic oxidation of 1-phenylethanol catalysed by palladium complexes of a fluorous pyridine...

Fluorous solvents would seem to be particularly suitable for performing aerobic oxidations, based on the high solubility of oxygen in fluorocarbons, a prop-... 

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. 

Fig. 27. [Cu -TEMPO]-catalyzed aerobic oxidation of alcohols using fluorous biphasic conditions 97,98).

Copper-Catalyzed Aerobic Oxidation oe Alcohols under Fluorous Biphasic Conditions (97,98)... 

A fluorous acetylacetonate ligand, C7F15COCH2COC7F15, was introduced into metal-catalyzed aerobic oxidation reactions in fluorous media, which appears to be useful for two reasons (1) catalyst recycling is easy (2) the gas is highly soluble... 

However, these methods suffer from low activities and/or narrow scope. Uemura and coworkers [74,7 5] reported an improved procedure involving the use of Pd(OAc) 2 (5 mol%) in combination with pyridine (20 mol%) and 3 A molecular sieves (500 mg per mmol of substrate) in toluene at 80 °C. This system smoothly catalyzed the aerobic oxidation of primary and secondary aliphatic alcohols to the corresponding aldehydes and ketones, respectively, in addition to benzylic and allylic alcohols. Representative examples are summarized in Table 5.7. The corresponding lactones were afforded by 1,4- and 1,5-diols. This approach could also be employed under fluorous biphasic conditions [76]. 

Tab. 4.13 Copper-catalyzed aerobic oxidation of alcohols under fluorous biphasic conditions ...

The Ru-catalyzed epoxidation of tran -stilbene in the presence of NaI04 was carried out using a bipyridyl ligand with a fluorous ponytail at the 4 and 4 positions. As illustrated by the first equation in Scheme 8, a triphasic system comprising water, dichloromethane and perfluorooctane was employed in the reaction. The reaction was complete in 15 min at 0°C and tran -stilbene oxide 5 was obtained from the dichloromethane layer in a 92% yield. The fluorous layer, containing the catalyst, could be recycled for four further runs without any addition of RuCls. The same perfluoroalkyl-substituted bipyridyl ligand was used successfully in the copper(i)-catalyzed TEMPO (2,2, 6,6 -tetramethylpiperidine (V-oxyl)-oxidation of primary and secondary alcohols under aerobic conditions (Scheme 8, second equation). ... 

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