Fluorous ligand recycling

Fig. 7.6 Recycling ofthe fluorous ligand in aminocarbonylations (reproduced with permission from [100]).

The turnover frequency was found to be greatest for the two fluorous ligands and the control ligand with the trimethylsilyl function was found to be less effective than triphenylphosphine. These fluorous derivatives of Wilkinson s catalyst were used to hydrogenate 1-octene in perfluoromethylcyclohexane (PP2). The reaction was carried out a number of times in order to evaluate the efficiency of the system in terms of recycling and reuse [16], It was found that as the number... 

Diamine-derived chiral copper complex 254 was used in the asymmetric cyclopropanation (Scheme 1.120) [177]. Perlluorinated diamine ligand 255 was developed and showed moderate levels of enantioselectivity for the cyclopropanation of diazoacetate (Scheme 1.121) [178]. The fluorous ligand was readily separated by the simple decantation of the fluorous phase. Although the recycling of the catalyst was expected, reuse was difficult because of its partial decomposition. 

A fluorous 1,4-disubstituted [l,2,3]-trizole 26 was prepared from fluoroalkyl tosylate to replace the air- and moisture-sensitive phosphine ligand as a recyclable ligand for the palladium-catalyzed Suzuki-Miyaura reaction and Mizoroki-Heck reaction (Scheme 7.23) [36]. As expected, the fluorous ligand, together with palladium acetate, promoted these coupling reactions and the fluorous ligand was conveniently... 

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. 

Studies of the phase behaviour at ambient temperature within the separator [43] show that there is significant solubility of the product nonanal within the fluorous phase and vice versa. Although this does not present a problem for the nonanal (it will simply be recycled to the reactor and create a steady state, it does mean that fluorous solvent is always being lost. The loss of the fluorous solvent (2.8 mol% into pure nonanal), as for the catalyst and the free ligand [41] is much more significant at low conversion, so... 

A cationic complex, formed in situ from 5 and [Rh(COD)2]OTf, was also active in biphasic hydrogenation [14]. No preference for the fluorous phase was found for ligands containing only one perfluoroalkyl tail, but neutral and cationic complexes, containing mono- and bidentate 4a or 5, respectively, were selectively dissolved in the fluorous phase. No leaching and recycling studies were performed. 

To solve the issue of ligand leaching that was encountered in some of the examples above, fluorous polymeric phosphine ligands 15a-c [28] were developed. The rhodium complexes prepared from 15a-c using a 3 1 ratio of P Rh [28b, 29] displayed good turnover frequencies (TOFs) in the case of 15 a, but reaction rates for 15b,c were lower. The catalyst derived from 15 a was recycled seven times without loss of activity, although leaching was not studied quantitatively. 

When reactions are carried out in a fluorous phase or for that matter in any biphasic system, the products can often be recovered by simple phase separation. If required, the fluorous phase can be washed with further organic solvent to recover any residual product that remains in the fluorous phase. However, in catalysed reactions, efficient recycling of the catalyst is critical to the success of the reaction. The cost of derivatizing the modifying ligands means that any unrecovered catalyst has serious implications for the economics of the process. 

A similar reaction has been conducted under fluorous biphasic conditions, using a perfluoroalkylated bipyridine as ligand to ensure that the copper species resides in the fluorous phase [22], The oxidation of a range of primary alcohols to the corresponding aldehydes was found to be possible, an example of which is shown in Scheme 9.11. The catalyst could be successfully recycled by phase separation, with analytically pure products being isolated even after... 

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