Fluorous biphase techniques

The first chemical modification of EMFs, the cycloaddition of disilacyclo-propane to La C82> was reported in 1995 [1]. Several other groups also reported cycloadducts of EMFs [2,3], but unambiguous structural characterization was achieved only in one study [4]. More recently, water-soluble EMFs, the Gd C6o(C(COOH)2)io cycloadduct reported by Bolskar et al. [5], have been prepared to explore their potential use as MRI contrast agents [5,6], and Shinohara et al. used fluorous biphase techniques to prepare La C82(C8F]7)2 [7]. Until this work, however, La C82(C8Fi7)2 was the only reported exohedral derivative of EMFs with atomic substituents such as H, F, Cl, or Br, or with organic substituents R having only R-CEmf single bonds. 

Recently, the fluorous biphasic separation technique has been enriched with two new developments, both of which were demonstrated in hydrogenation. The need for a fluorous solvent can be eliminated by using fluorous silica as a fluorous catalyst scavenger. In liquid-liquid biphasic systems, reversible expan-... 

Despite their vast untapped potential, techniques like fluorous biphasic catalysis still have limitations. These limitations center on solubility and emanate directly from the strengths of the technique heavily fluorous compounds tend to be highly insoluble in organic solvents while organic compounds (especially polar ones) tend to be highly... 

The technique now called fluorous biphasic catalysis was apparently first described in the Ph.D. thesis of M. Vogt in 1991 however, these studies did not become known to the community until sometime later. W. Keim, M. Vogt, P. Wasserscheid, B. Driessen-Holscher, Perfluorinated polyethers for the immobilization of homogeneous nickel catalysts , J. Mol. Catal A Chem. 1999,139,171. 

FA techniques, see Flowing afterglow techniques /-block metallaboranes future research, 3, 257 overview, 3, 133-174 /-block metallacarboranes characteristics, 3, 246 overview, 3, 175-264 FBS, see Fluorous biphasic system [5,5,5,5]-Fenestranes, via carbonylative carbocyclization,... 

Cornils, B. (1997) Fluorous biphase systems - the new phase-separation and immobilization technique. Angew. Chem. Int. Ed. Engl, 36, 2057. 

Another biphasic technique uses a fluorinated substance (fluorous phase), which is immiscible with water and most non-fluorinated solvents at room temperature, but which can form a single phase at higher temperatures. Products can thus be separated at room temperature and the soluble catalyst recycled. One such catalyst is made by adding two perfluorinated chains to dba, a classic palladium(0) ligand. 

This new experimental technique, using fluorous solvents or fluorous biphasic systems (FBS) with fluorous biphase catalysis (FBC), was developed by Vogt and Kaim [884] and by Horvath and Rabai [885] in 1991 and 1994, respectively. Since then, this method has found many applications in synthetic organic chemistry and has already been reviewed repeatedly [886-893]. Incidentally, temperature-dependent two-phase one-phase transitions are not limited to combinations of fluorous solvents with organic solvents. For example, certain mixtures of water and l-cyclohexylpyrrolidin-2-one form one phase at ambient temperature and a two-phase system at higher temperatures >ca. 50 °C), also allowing interesting separation possibilities. 

The direct condensation of carboxylic acids with aliphatic alcohols has been investigated by the use of fluorous ammonium salts as metal-free catalysts (Equation 4.24). Esterification reactions were carried out under mild fluorous biphasic conditions, in the presence of 1 mol% of fluorous ammonium triflate and without any water removal techniques. In the case of primary and secondary aliphatic alcohols, good to excellent ester yields were obtained. The fluorous ammonium triflate salt was easily recovered by simple phase separation and reused at least three times without any significant loss of activity [45],... 

Some further studies still deal with the Friedel-Crafts acylation in fluorous fluids. These fluids all have very unusual properties such as high density and high stability, low solvent strength and extremely low solubility in water and organic compounds, and, finally, nonflammability. These properties allow their easy handling and reuse. Friedel-Crafts acylation of electron-rich aromatic substrates can be very efficiently performed in a fluorous biphasic system (FBS), which represents a benign technique for phase separation, and catalyst immobilization and recycling. 

Fiuorous catalysis is now regarded by some specialists as a well-established area providing complementary approaches to aqueous (see Chapter 2) or ionic-biphase (see Chapter 5) catalysis. So far, no economical application of fluorous-biphase catalysis is known (except for highly specialized lab uses) but fluorous-biphasic separation techniques without catalytic reactions involved may be the trailblazers of later catalytic uses. 

Fluorous biphasic catalysis has emerged since the late 1990s as an attractive alternative to traditional catalysis methods [1], Fluorous techniques take advantage of the temperature-dependent miscibility of organic and perfluorocarbon solvents to provide easier isolation of products and recovery of a fluorinated catalyst. The large-scale use of fluorous solvents, however, has drawbacks cost, and concern over environmental persistence. 

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