Ligands perfluoroalkyl-substituted

Perfluoroalkyl-substituted BINAPHOS ligand 29c was also developed for the asymmetric hydroformylation of vinyl arenes in SCCO2. With this ligand regio-and enantio-selectivity (ee s up to 93.6%) were high without the need for hazardous organic solvents [63,64],... 

The C02-philic perfluoroalkyl-substituted (R,S)-3-H F -BINAPHOS ligand [34] was successfully applied to enantioselective hydrogenation in the inverted SCCO2/H2O system. The complex [Rh(cod)2]BARF was chosen as metal source and the active catalyst was formed in situ. Using the same procedure as above, similar activities and more than 98% ee were obtained consistently over five subsequent cycles in the hydrogenation of methyl 2-acetamido acrylate. The results demonstrate the potential of the inverted SCCO2/H2O system for asymmetric synthesis of chiral biologically active products. 

Kainz S, Koch D, LeitnerW etal (1997) Perfluoroalkyl-substituted arylphosphanes as ligands for homogenous catalysis in supercritical carbon dioxide. Angew Chem Int Ed 36(15) 1628-1630... 

Francio G, Wittmann K, Leitner W (2001) Highly efficient enantioselective catalysis in supercritical carbon dioxide using the perfluoroalkyl-substituted ligand (R, S)-3-H2F6-BINAPHOS. J Organomet Chem 621(1-2) 130-142... 

The use of Cu in combination with TEMPO also affords an attractive catalyst [200, 201]. The original system however operates in DMF as solvent and is only active for activated alcohols. Knochel et al. [202] showed that CuBr.Me2S with perfluoroalkyl substituted bipyridine as the ligand and TEMPO as cocatalyst was capable of oxidizing a large variety of primary and secondary alcohols in a fluorous biphasic system of chlorobenzene and perfluorooctane (see Fig. 4.69). In the second example Ansari and Gree [203] showed that the combination of CuCl and TEMPO can be used as a catalyst in l-butyl-3-methylimidazolium hexafluorophosphate, an ionic liquid, as the solvent. However in this case turnover frequencies were still rather low even for benzylic alcohol (around 1.3 h 1). 

Numerous perfluoroalkyl-substituted arenes have been prepared by the coupling of aryl iodides or bromides with perfluoroalkyl iodides in the presence of copper bronze in a dipolar aprotic solvent at elevated temperatures. Perfluoroalkyl bromides and chlorides can be utilized in similar reactions provided a bidentate ligand such as 2,2 -bipyridyl or 1,10-phenanthroline is added 199, 278). Perfluoroalkylcopper reagents are believed to be intermediates in the couplings 200, 201). 

Neutral catalysts or catalyst precursors based on fluorinated ligand systems have been applied in compressed CO2 to a broad range of transformations such as Zn- and Cr-catalyzed copolymerization of epoxides and CO2 [53, 54], Mo-catalyzed olefin metathesis [9], Pd-catalyzed coupling reactions [43, 55, 56] and Pd-catalyzed hydrogen peroxide synthesis [57]. Rhodium complexes with perfluoroalkyl-substituted P ligands proved successful in hydroformylation of terminal alkenes [28, 42, 44, 58], enantioselective hydroformylation [18, 59, 60], hydrogenation [61], hydroboration [62], and polymerization of phenylacetylene... 

Chiral cationic Rh catalysts for the hydrogenation of prochiral C-C double bonds require the use of BARF or related anions even with potentially CO2-S0IU-ble ligands such as Et-DuPHOS (DuPHOS l,2-bis(2,5-dialkylphospholano)-benzene) [70] or a perfluoroalkyl-substituted aryl phosphonite [19]. The ligand... 

For a general review on the syntheses of perfluoroalkyl-substituted ligands see L. P. Barthel-Rosa, J.A Gladysz, Coord. Chem. Rev. 1999, 190-192, 587. 

Further catalyst development has occurred based on the improved regioselectivity of the perfluoroalkyl-substituted ligands for use in homogeneous hydro-formylations in Kainz et al. developed a... 

Kainz, S. Koch, D. Baumann, W. Leitner, W. Perfluoroalkyl-substituted arylphosphines as ligands for homogeneous catalysis in supercritical carbon dioxide. Angew. Chem., Int. Ed. Engl. 1997, 36 (15), 1628-1630. 

The required perfluorinated phospane ligand can easily be prepared in three steps starting from 4-iodoaniline. The fluorinated chain was introduced via an Ullmann-type reaction with F13C6I in presence of copper-bronze in DMSO at 120°C for 1 h leading to the perfluoroalkyl substituted aniline 35 in 86% yield... 

Fig. 4. A general concept for solubilizing catalyticaUy relevant ligands for the use m SCCO2, exemplified for a bidentate aryl phosphine ligand. Perfluoroalkyl chains or other solubitoers waved line) are fixed at the periphery of the ligand. The position of the substitution and the use of appropriate spacers bold line) are used to tune the stereo-electronic properties of the ligand...

An in situ catalyst formed from palladium acetate and the perfluoroalkyl-substituted ligand [F(CF2)6(CH2)2 2PPh resulted in the formation of methylcin-namate in 91 % isolated yield after 64 h at 100 °C and an estimated CO2 density of ca. 0.7 g mL [62]. The ligand 3,5-H°F -tpp gave almost identical yield after 24 h at 90 °C under otherwise very similar conditions [63]. Tris(2-furyl)phos-phine also lead to active catalysts [63], especially when used together with fluorinated Pd-sources like palladium trifluoroacetate [64]. Most of the above systems lost their activity if metallic palladium was formed under non-optimum reaction conditions. However, the use of a commercial heterogeneous Pd/C catalyst was reported to give up to 85% yield of methyl cinnamate in the presence of CO2 at temperatures and pressures beyond the critical data [65]. The CO2 density in the latter experiments can be estimated to ca. 0.22 g mL" and was... 

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). ... 

An insoluble rhodium catalyst [(nbd)Rh(acac)] has been used to synthesize polyphenylacetylene in the presence of liquid and SCCO2 (82). The catalyst is solubilized in the reaction mixture by addition of a perfluoroalkyl-substituted triph-enylphosphine ligand. Polyphenylactylene formed in good yields (60-75%) had a molecular weight of about 4 x 10 g/mol for THF soluble fractions, and molecular weight distributions in the range of 8-10. 

Use of Perfluoroalkyl Substituted Phosphorus Compounds as Ligands for Homogeneous Catalysis in Supercritical Carbon Dioxide... 

When the reaction was promoted by the unmodified catalysts [(cod) Rh(hfacac)] (hfacac = hexafluoroacetylacetonate CF3COCHCOCF3, cod = cyclooctadiene), the superiority of SCCO2 in the hydroformylation rates was demonstrated very well. The modified catalytic systems formed with perfluoroalkyl-substituted triarylphos-phine and triaryl phosphite ligands exhibited higher regioselectivities in scCOj than in conventional solvents. The olefin isomerization is a typical side reaction for phosphite-modified systems in conventional solvents, while it is suppressed effectively in SCCO2. 

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