Catalysis continued solvents

CsF added to tetra- -butylammonium bromide or Aliquat 336, the mixture stirred for 5 min, n-octyl bromide added, stirring continued for 5 min, and allowed to react for 40 h - n-octyl fluoride. Y 77%. Work-up is easy and reagents readily available yields are high from satd. chlorides and bromides (incl. benzylic), but aromatic bromides were more resistant. F.e. and with tetra-n-butylammonium fluoride, also from tosyla-tes, and comparison with other heterogeneous methods s. G. Bram et al., Synth. Commun. 18, 1661-7 (1988) review of solid-liq. phase transfer catalysis without solvent s. Bull. Soc. Chim. France 1989, 247-51. 

The combination of ionic liquids with supercritical carbon dioxide is an attractive approach, as these solvents present complementary properties (volatility, polarity scale.). Compressed CO2 dissolves quite well in ionic liquid, but ionic liquids do not dissolve in CO2. It decreases the viscosity of ionic liquids, thus facilitating mass transfer during catalysis. The separation of the products in solvent-free form can be effective and the CO2 can be recycled by recompressing it back into the reactor. Continuous flow catalytic systems based on the combination of these two solvents have been reported [19]. This concept is developed in more detail in Section 5.4. 

Lewis add catalysis has been and continues to be of great interest in organic synthesis.111 While various kinds of Lewis add-promoted reactions have been developed and many have been applied in industry, these reactions must generally be carried out under stridly anhydrous conditions. The presence of even a small amount of water stops the reaction because most conventional Lewis adds read immediately with water, rather than with the substrates, and decompose. This destrudive reaction has restrided the use of Lewis acids in organic synthesis. From a viewpoint of today s environmental consciousness, however, it is desirable to use water instead of organic solvents as a reaction solvent.1231... 

Ionic liquids represent a unique class of reaction media for catalytic processes, and their application in catalysis has entered a period of exploding growth. The number of catalytic reactions involving ionic liquids continues to increase rapidly. These liquids offer promising solutions to the problems associated with conventional organic solvents the potential advantages may include enhanced reaction rates, improved chemo- and regioselectivities, and facile separation of products and catalyst recovery. 

Hardacre et al. report the Friedel-Crafts benzoylation of anisole with benzoic anhydride to yield 4-methoxybenzophenone with various ILs and zeolite catalysts (USY, HZSM-5, H-beta, and H-mordenite). The rates of reaction were found to be significantly higher using ionic liquids compared with organic solvents.Continuous-flow studies of successful ionic liquid systems indicate that the bulk of the catalysis is due to the formation of an acid via the ion exchange of the cation with the protons of the zeolite as shown in the following reaction. Scheme 8. 

T n continuation of a study of the uncatalyzed reactions between MDI (4,4 -diphenylmethane diisocyanate) and alcohols in DMF (N,N-dimethylformamide) (J), the effect of dibutyltin dilaurate on the same reactions has been studied. The results were compared with those found in studies on the mechanism of catalysis of urethane formation in apolar solvents (2-6). 

The selective oxidation of C—H bonds in alkanes under mild conditions continues to attract interest from researchers. A new procedure based upon mild generation of perfluoroalkyl radicals from their corresponding anhydrides with either H2O2, m-CPBA, AIBN, or PbEt4 has been described. Oxidation of ethane under the reported conditions furnishes propionic acid and other fluorinated products.79 While some previously reported methods have involved metal-mediated functionalization of alkanes using trifluoroacetic acid/anhydride as solvent, these latter results indicate that the solvent itself without metal catalysis can react as an oxidant. As a consequence, results of these metal-mediated reactions should be treated with caution. The absolute rate constants for H-abstraction from BU3 SnH by perfluorinated w-alkyl radicals have been measured and the trends were found to be qualitatively similar to that of their addition reactions to alkenes.80 a,a-Difluorinated radicals were found to have enhanced reactivities and this was explained as being due to their pyramidal nature while multifluorinated radicals were more reactive still, owing to their electrophilic nature.80... 

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