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August 4, 2003
Volume 81, Number 31
CENEAR 81 31 p. 6
ISSN 0009-2347


Catalyst precipitates at end of reaction, ready for another round of activity


A major problem in homogeneous catalysis is the difficulty in separating the catalyst from the product. Current approaches to circumventing this problem include two-phase reactions with fluorinated solvents and the use of supported catalysts--but both strategies have drawbacks. Fluorous biphasic reactions require special solvents, and catalysts that work well in solution generally lose activity when stuck to a solid support.

A catalyst that precipitates at the end of the reaction would offer a powerful solution. Now, chemists R. Morris Bullock and Vladimir K. Dioumaev at Brookhaven National Laboratory have demonstrated such a catalyst at work in the hydrosilylation of ketones [Nature, 424, 530 (2003)].

"This work represents a major breakthroug in terms of recovering homogeneous catalysts," comments John A. Gladysz, a chemistry professor at the University of Erlangen-Nuremberg, in Germany.

In hydrosilylation of a ketone, the reactants are miscible and make up a relatively polar reaction medium. The product is much less polar. As the reaction progresses, the solvent-free reaction medium becomes less and less polar. An ideal self-separating catalyst would be completely soluble as the reactants are consumed and product is formed but insoluble in the product. Striking the right balance of solubility properties is difficult, the researchers write.

A cationic tungsten complex with a weakly coordinating anion is the answer for hydrosilylation of aliphatic ketones. When reactants and catalyst are combined, a purple mixture forms, indicating coordination of the ketone to the metal. As the ketone is consumed, the purple color diminishes. Near the end of the reaction, an oily material forms before a precipitate finally settles.

The oily material may be the key, Bullock says. That material, he explains, is a liquid clathrate, a phase in which the catalyst is associated with molecules of product but is still partially soluble. "The reaction can still occur in that phase, and we think that is what allows the reaction to go to completion," he says. "To the best of our knowledge, no one has ever appreciated how useful these inclusion compounds can be to maintain liquidity of the otherwise solid catalyst," Dioumaev adds.

The precipitate is not the original catalyst but a mixture of what Bullock calls the catalyst's "resting states." They exist in equilibrium with the original catalyst and can themselves catalyze the hydrosilylation reaction, he says.

Bullock cautions that their results apply only to a special and limited case. "We hope the chemistry we explored can be made more general," he says. "A better understanding of liquid clathrates could lead to a broader application."


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