| RESEARCH |
One of the first groups to demonstrate "fluorous biphasic catalysis without fluorous solvents" is that of chemistry professor John A. Gladysz of the University of Erlangen-Nuremberg, in Germany. Gladysz, an early collaborator with Horváth, focused on designing fluorinated catalysts that themselves have a temperature-dependent miscibility--that is, solubility--in ordinary organic solvents. Gladysz reviewed the work of graduate student Marc Wende on fluorinated phosphine catalysts such as P[(CH2)2(CF2)7CF3]3, and he added some nuances about controlling the solubility of catalysts by varying the length of the fluorinated ponytails and hydrocarbon spacer groups. The catalysts have been tested by carrying out a series of additions of alcohols to methyl propiolate in octane [J. Am. Chem. Soc., 125, 5861 (2003)]. The phosphine catalysts are insoluble in octane at room temperature, but upon heating to 65 °C, they become soluble and the reaction proceeds, Gladysz explained. After cooling, the catalysts precipitate out of the reaction mixture and are recovered by decanting the liquid. The catalysts can be recycled multiple times with yields consistently above 80%, he noted. In a further refinement, Gladysz and coworkers have shown that the same reactions can be made even greener by not using any solvent. Raising the temperature of a mixture of the neat reactants and solid catalyst above the catalyst's melting point of 47 °C yields the addition product. The catalyst simply precipitates from the liquid product at room temperature and is recyclable. Product yields are consistently above 95%. "The message is that we can eliminate the fluorous solvent but retain the biphasic recoverability of the phosphine catalysts," Gladysz told C&EN. "Homogeneous catalysts that can be recovered by simple precipitation are going to have a big future." |
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