Big Role For Tiny Impurity

RESULTS MAY VARY Yields of this coupling differed depending on the iron catalyst. Copper turns out to play a crucial role in the reaction.

Small amounts of copper or other non-iron metals may have a dramatic effect on certain reactions previously believed to be catalyzed solely by iron, according to a new report. The finding serves as a reminder that impurities in a reaction sometimes can play unexpectedly significant roles.

In efforts to use nontoxic and inexpensive metals for catalysis, organic chemist Carsten Bolm of RWTH Aachen University, in Germany, developed a series of cross-coupling reactions catalyzed by iron salts (C&EN, July 28, 2008, page 53). But chemists at Aachen and in Stephen L. Buchwald's group at Massachusetts Institute of Technology independently noted parallels between the iron chemistry and chemistry carried out with copper catalysts developed in Buchwald's group and elsewhere.

Buchwald alerted Bolm that outcomes of some of the coupling reactions varied widely depending on the purity and supplier of the iron source, iron(III) chloride, Buchwald tells C&EN. Furthermore, he informed Bolm that the MIT chemists had obtained similar coupling results without iron, instead using parts-per-million levels of copper. Bolm tells C&EN that although he was skeptical about the strong effect of copper initially, he opted to collaborate with Buchwald's group.

"We decided that a collaborative approach was superior to a confrontational approach to work out what was happening," Buchwald emphasizes.

The teams examined a variety of FeCl₃ sources in four different coupling reactions and confirmed that parts-per-million levels of copper impurities play a key role (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200902237). In two cases, the couplings worked without any iron source at all, provided copper was present.

It's not yet clear how broadly the new findings impact iron-mediated reactions. Historically, metal contaminants have been important in other processes, such as the Nozaki-Hiyama-Kishi reaction, a nickel- and chromium-catalyzed bond-forming reaction.

"While a setback for the area of iron-catalyzed cross-coupling, the broader field of base metal catalysis still has many demonstrated successes and presents exciting challenges and opportunities for the future," comments Paul J. Chirik, who studies iron catalysis at Cornell University.

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