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FLUOROUS BIPHASIC TRANSESTERIFICATION
Catalyst-solvent system mediates quantitative 1:1 ester-alcohol reactions
A new fluorinated catalyst-solvent system that overcomes some of the traditional limitations of transesterification reactions--thus boosting its potential as a green industrial process--has been developed by a research team at Okayama University of Science in Japan.
Professor Junzo Otera, assistant professor Akihiro Orita, and coworkers in the department of applied chemistry report the 1:1 reaction of esters with alcohols using a fluorinated distannoxane catalyst in perfluorohexanes. The method leads to essentially 100% conversion and yield of the desired esters [Angew. Chem. Int. Ed., 40, 3670 (2001)].
Generally, transesterification involves the acid- or base-catalyzed reaction of an ester and an alcohol that leads to double displacement of substituent groups. They are equilibrium-dependent reactions, meaning a large excess of the ester or alcohol is needed to drive the reaction, with one of the reactants usually acting as the solvent.
Otera and coworkers were looking for the "ultimate practical transesterification" that could satisfy some of the tenets of green chemistry. The goals included having a 1:1 ratio of ester and alcohol reactants (atom economy), a neutral catalyst that is readily recoverable, and 100% conversion and yield.
Earlier work by Otera's group on tetraalkyldistannoxanes led to the idea that such a catalyst with polyfluoroalkyl groups, when dissolved in a perfluorocarbon solvent, could lead to a useful fluorous biphasic reaction system. In this system, the catalyst and solvent form one layer, while the ester and alcohol reactants form a separate organic layer. On heating, the two layers are sufficiently miscible for the reaction to occur; on cooling, the organic and fluorinated layers separate.
The system was tested by carrying out a series of millimolar-scale reactions of methyl and ethyl esters with various high-boiling alcohols, including some acid-sensitive alcohols and protected alcohols. One representative reaction was ethyl 3-phenylpropionate with 1-octanol at 150 ºC for 16 hours that gave greater than 99% yield of octyl 3-phenylpropionate. The pure ester was recovered by evaporation of the ethanol coproduct.
The catalyst can be recovered from the perfluorocarbon solvent, the researchers note, but it is more practical to reuse the solution directly in subsequent reactions. The viability of the fluorous phase was thus demonstrated by running a set of 14 different esterifications consecutively with the same catalyst-solvent sample, each of which proceeded in near 100% conversion and yield. In a future paper, the Okayama group plans to explore how the 1:1 reactions proceed so efficiently.
PRACTICAL PROCESS Fluorous biphasic transesterification with nearly 100% yield is recyclable up to 20 times without significant loss of catalytic activity.
ADAPTED FROM ANGEW. CHEM. INT. ED.
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