Fluorous catalytic oxidation reaction

Fluorous distanoxane 7 and tin oxide 8 have been evaluated in catalytic transesterification reactions by the Panek and Porco groups for the pur-... 

A copper catalysed click (azide-alkyne cycloaddition) reaction has been used to prepare a fluorous-tagged TEMPO catalyst (Figure 7.20). TEMPO is a stable organic free radical that can be used in a range of processes. In this case, its use in metal-free catalytic oxidation of primary alcohols to aldehydes using bleach as the terminal oxidant was demonstrated. The modified TEMPO can be sequestered at the end of the reaction on silica gel 60 and then released using ethyl acetate for reuse in further reactions in this way the TEMPO was used four times with no loss in activity. 

Bayardon and Sinou have reported the synthesis of chiral bisoxazolines, which also proved to be active ligands in the asymmetric allylic alkylation of l,3-diphenylprop-2-enyl acetate, as well as cyclopropanation, allylic oxidations and Diels-Alder reactions. [62] The ligands do not have a fluorine content greater than 60 wt% and so are not entirely preferentially soluble in fluorous solvents, which may lead to a significant ligand loss in the reaction system and in fact, all recycling attempts were unsuccessful. However, the catalytic results achieved were comparable with those obtained with their non-fluorous analogues. 

The application of perfluorous polyethers in biphasic catalysis was first described by Vogt (133), who also synthesized ligands based on hexafluor-opropene oxide oligomers to create metal complexes that are soluble in the perfluorous polyethers. The solvophobic properties of the fluorous solvent were successfully incorporated in the metal complexes catalytic oligomerization and polymerization reactions with nickel and cobalt complexes were demonstrated. 

Stoichiometric oxidation with Se02 became more attractive after Sharpless showed that the reaction could be carried out with catalytic amounts of Se02 and TBHP as the (re)oxidant [106]. The reaction involves an oxometal mechanism (see Fig. 4.39). The use of fluorous seleninic acids with iodoxybenzene as oxidant introduces the possibility of recycling the catalyst [107]. 

A fluorous surfactant covalently tethered to silica provides a thin film of perfluorinated solvent for reactions and/or extractions. This material was used for the small-scale hydrocyclization of 6-bromo-1-hexene with NaBH and a catalytic amount of a fluorous tin bromide in 1-butanol. The yield of methylcyclopentane was modest, however, and this technology is a long way from being viable on an industrial scale. The same idea has been more successfully employed with fluorous silica-supported tin Lewis acid catalysts for Baeyer-Villiger oxidations. ... 

An important application of metal bis(perfluorooctanesulfonyl)amides in fluorous biphasic systems is the Baeyer-Villiger oxidation. Especially Sn[N(S02C8F17)2]4 could be reused more than four times in the Baeyer-Villiger oxidation of adamantone and cyclobutanone without significant decomposition of the catalyst (Equation 4.26). It was found that there was almost no difference for the formation rate of y-butyr-olactone between the first and fourth cycles within a 1 h reaction time, which showed that there was not only no loss of the catalyst, but also no decrease in catalytic activity during the repetition [47]. 

Both complexes were able to catalyze the oxidation of jt-substituted methyl phenyl sulfides at a substrate/catalyst molar ratio of 100, with good sulfoxide selectivities (> 90%). It should be noted that FB reactions catalyzed by Mn-14 consistently afforded higher sulfoxide yields than homogeneous reactions catalyzed by Mn-13 (Table 4). Moreover, three consecutive recyclings of the fluorous layer were performed with no appreciable loss of catalytic activity and selectivity. 

Use in Radical Reactions. Thiols are good sources for alkyl-thio radicals under oxidative conditions. The fluorous disulfide can be prepared from the thiol in excellent yield using 1.3 equiv of H2O2 and a catalytic amount of NaOH (1-2 mol %) (eq 4). After the reaction is complete, the clear reaction mixture is separated into two layers, from which pure crystalline fluorous disulfide is filtered off. 

Fluorous thiourea conveniently prepared by isothiocyanate perfluorooctyl aniline was also employed in chemoselective oxidation of the sulfide in the presence of 30% HjOj (Scheme 7.30) [46]. The catalyst could significantly shorten the reaction time and improve the product yield. As expected, the catalyst could be easily recovered by fluorous solid-phase extraction and reused for up to five times without significant loss of catalytic activity. 

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