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FLUOROUS METHOD GROWS FRIENDLIER
Fluorocarbon solvent can be omitted from biphasic catalysis reactions
STEVE RITTER
Fluorous biphasic catalysis has emerged during the past five years as an environmentally attractive alternative to traditional catalysis methods. 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 perfluorocarbon solvents, however, has drawbacks: cost and concern over environmental persistence.
Two research groups now report that the perfluorocarbon solvent can be skipped by designing fluorinated catalysts that themselves have a temperature-dependent phase miscibility--that is, solubility--in ordinary organic solvents.
The fluorous biphasic technique involves dissolving a catalyst with long fluorinated alkyl chains in a perfluorocarbon. The reactants are added to an organic solvent that is immiscible with the perfluorocarbon at room temperature, forming a second phase. On heating, the two phases mix and the reaction occurs; on cooling, the fluorinated and organic layers separate. The organic phase can be removed and the product isolated, while the fluorinated catalyst-solvent phase can be reused.
In one of the reports that now demonstrate that the perfluorocarbon solvent isn't essential, graduate student Marc Wende, postdoc Ralf Meier, and professor John A. Gladysz of the Institute for Organic Chemistry at Friedrich-Alexander University in Erlangen, Germany, describe the temperature-dependent solubility of the solid phosphine catalyst P[CH2CH2(CF2)7CF3]3 in octane [J. Am. Chem. Soc., 123, 11490 (2001)]. Gladysz' group has been working on developing fluorinated catalysts that are amenable to biphasic recovery. The temperature dependence of the phosphine suggested that a liquid-solid catalyst-recycling method might be possible.
The method was tested by carrying out a series of additions of alcohols to methyl propiolate in octane. The catalyst is insoluble in octane at room temperature, but upon heating it becomes soluble and the reaction proceeds. After cooling, the catalyst precipitates out of the reaction mixture and is recovered by decanting the liquid. The catalyst was recycled four times with yields consistently above 80%.
In an additional refinement, Gladysz and coworkers show that the same reaction can be made even greener by not using a solvent at all. 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 solid catalyst can be recovered at room temperature and is recyclable with yields consistently above 95%.
In a separate report, involving the fluorous synthesis of amides, associate professor Kazuaki Ishihara, graduate student Shoichi Kondo, and professor Hisashi Yamamoto of the department of biotechnology at Nagoya University in Japan describe a related process that uses a fluorinated boronic acid catalyst [Synlett, 2001, 1371]. The catalyst is insoluble in organic solvents at room temperature but becomes soluble under reflux.
Postreaction, the catalyst can be recovered by precipitation at room temperature. Yamamoto and coworkers demonstrated the condensation of cyclohexanecarboxylic acid and benzylamine in o-xylene 10 times with the same catalyst to yield the corresponding amide in 96% cumulative yield.
"Researchers in this field have simply overlooked a very general and exploitable property of the so-called heavy fluorous substances," Gladysz says. The one-phase and solventless fluorous-catalyzed reactions, which have been extended to metal-containing catalysts by Gladysz' group, show considerable potential for commercial applications, he adds.
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