Ytterbium methides

Benzaldehyde (81 mg) and methyl trimethoxysilyl dimethylketene acetal (165 mg) were added to a mixure of 3 ml perfluorooctane and 4 ml toluene. To this mixture was added Imol % (based on benzaldehyde) ytterbium (tris(trisperfluorooctanesulfonyl)methide) and the reaction stirred 15 minutes at 40°C. Mixing and heating were stopped and the mixture separated into upper toluene layer and lower perfluorooctane layer. Each layer was analyzed by gas chromatography 99% of the product was detected in the lower layer. Atomic emission spectrometry indicated that at least 99% of the catalyst was also present in the lower layer. 

Preparation of ytterbium tris(tris(perfluorobutanesulfonyl)-methide)... 

Tris(perfluorobutanesulfonyl)methide (3.0 g) was added to a solution of 15 ml acetonitrile, 15 ml water, and ytterbium carbonate (0.39 g). The mixture was stirred 7 hours at ambient temperature and was then heated to 50 °C one hour. The mixture was filtered and the product isolated by vacuum drying at 50°C at 1-10 mm Hg followed by drying at 90°C at 0.01 mm Hg for 24 hours. Elemental analysis supplied. 

Ytterbium(lll) tris(perfluoroalkanesulfonyl)methides 34 are effective catalysts (10% mol) for the Friedel-Crafts acylation of arenes with anhydrides. Compounds 34 can be prepared as described in Scheme 3.7. Trimethylsilylmethyl lithium 31 is reacted with commercially available per-fluoroalkanesulfonyl fluorides, giving intermediates 32 that can similarly produce tris-perfluoroalkanesulfonyl derivatives 33. These compounds are converted into the catalysts 34 by reaction with ytterbium oxide. It is shown that the highly fluorinated catalyst (34, Ri = C5F13, = C8F17) (10% mol)... 

Barrett, A. G. M., Braddock, D. G., Gatterick, D., Chadwick, D., Henschke, J. P., and McKinnell, R. M. 2000. Pluorous biphase catalytic Priedel-Crafts acylation ytterbium tris(perfluoroalkanesulfonyl)methide catalysts. Synlett 847-849. 

D. A. 2002. Highly active ytterbium(lll) methide complex for truly catalytic Priedel-Crafts acylation reactions. Synlett 1653-1656. 

The Friedel-Crafts acylation of arenes with acetic anhydride was efficiently catalyzed by ytterbium tris(per luoroalkanesulfonyl)methides 29a-29c [32]. Itwas demonstrated that catalyst 29c could be recovered in 96% yield by extraction of the reaction mixture with hot perfluoromethyldecalin and could be re-used in a second run. 

Mikami s group has also demonstrated the advantage of the fluorous super-Lewis acids such as lanthanide tris(perfluorooctanesulfonyl)methide and perfluorooctane-sulfonimide complexes with respect to temperature-dependent solubility [13bj. For example, these complexes can be re-used for the Friedel-Crafts acylation reaction without fluorous solvents [Eq. (11)]. After the reaction mixture of anisole has been heated with acetic anhydride in 1,2-dichloroefhane in the presence of ytterbium perfluorooctanesulfonimide (10 mol%) at 80 °C for 6 h, the mixture is allowed to stand at -20 °C for 30 min to precipitate the ytterbium complex. The liquid phase is decanted and the residual lanthanide complex is re-used without isolation. No loss of activity is observed for the catalyst recovered. The total isolated yield of the product, which is combined from the three runs, is 78%. 

Ytterbium(III) tris(trifluoromethanesulfonyl)methide (ytterbium(III) triflide) was applied in a catalytic amount to catalyse FC processes. Only 1 mol% of ytterbium (III) triflide at 80 C allowed acylation of thiophene and 3-methylthiophene with acid anhydrides [35]. Acylation of thiophene led exclusively to 2-substitution 3-methylthiophene gave a mixture of 2-acylated (8) and 5-acylated (9) products (Scheme 15) [35]. 

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