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Fluorous Synthesis

  Fluorous chemistry has emerged as one of the promising fields of green chemistry. The term “fluorous” is a coined word meaning having affinity with fluorocarbons. Highly fluorinated compounds, in another words fluorous compounds, are insoluble in general organic solvents or water. However they are highly soluble with fluorous solvents such as perfluoroalkane. Fluorous chemistry is the technique of making use of this property, and it is applied to many organic reactions. Although fluorous solvents are generally immiscible with common organic solvents and water, certain fluorous solvents are able to form homogeneous solutions with some organic solvents at elevated temperatures. The characteristic features of these fluorous solvents is that they have boiling points properties that are almost equal to those of the corresponding hydrocarbons, regardless of their molecular weight, and these solvents have a high solubility of many gases and incombustible property. Taking the advantage of these properties, Horváth et al. accomplished the hydroformylation of olefins using a fluorous rhodium catalyst in perfluoromethylcyclohexane and toluene in 1994.1) This was regarded as the origin of the fluorous chemistry. This reaction uses perfluoromethylcyclohexane and toluene as solvents, which exist as a biphasic system at room temperature. In this system the fluorous catalyst exists in the fluorous phase and the olefins in the organic phase. However, the two phases form a homogeneous solution when heated. The reaction then proceeds by introduction of carbon monoxide and hydrogen gases. When the reaction is complete and the system is cooled, the two phases reappears, where the resulting product is found in the organic phase and the fluorous catalyst is in the fluorous phase; thus, the catalyst and the product are easily separated. The biphasic system using a fluorous solvent and an organic solvent is called Fluorous Biphase System (FBS), and the multiple phase system is called Fluorous Multiphase System (FMS). The advantages of FBS and FMS are that the resulting product and the catalyst can be easily separated simply by separating the fluorous phase from the other phase after the reaction. After separation, the fluorous phase containing the fluorous catalyst can be reused.
  Curran et al. have introduced the use of fluorous substituents (fluorous tags) into non-fluorous substrates for the synthesis of isoxazoline.2) After the reaction, the fluorous product was separated by fractional extraction with dichloromethane, water, and perfluorohexane. Following this report, numerous applications of fluorous chemistry have been made in combinatorial chemistry3) and oligosaccharide syntheses4).

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