Big Role For Tiny Impurity

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 major 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.

Buchwald alerted Bolm that coupling outcomes varied depending on the purity and supplier of the iron source, iron(III) chloride, Buchwald tells C&EN. Furthermore, he informed Bolm that his team obtained similar coupling results without iron, instead using parts-per-million levels of copper. Bolm's collaborator, Per-Ola Norrby of the University of Gothenburg, in Sweden, also notified Bolm about copper's parts-per-million effect. 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). Two couplings worked without any iron source at all, provided copper was present.

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