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Synthesis

The Power Of Two

Organic Synthesis: Dual catalysis creates stereochemical complexity

by Bethany Halford
June 3, 2013 | APPEARED IN VOLUME 91, ISSUE 22

STEREOSELECTIVE
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Depending on which chiral catalysts they choose, chemists can make any one of four possible stereoisomers.
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Depending on which chiral catalysts they choose, chemists can make any one of four possible stereoisomers.

Chemists, seeking quick ways to make complex structures, are often on the lookout for a single reaction that can selectively create molecules with multiple stereocenters.

Now, by doubling up on their use of chiral catalysts, chemists in Switzerland have come up with an elegant way to do this (Science 2013, DOI: 10.1126/science.1237068). This dual catalysis strategy provides synthetic chemists with a new route for making complex molecules, such as pharmaceuticals and agrochemicals.

In recent years, researchers have adopted the concept of dual catalysis (also known as synergistic catalysis) to accomplish such stereoselective syntheses. Dual catalysis involves using two catalysts to activate two starting materials, resulting in the generation of one or two stereocenters. One limitation of previous attempts, though, is that the catalysts worked sequentially, with one catalyst overcoming or complementing the actions of the other. Now, chemistry professor Erick M. Carreira, along with coworkers Simon Krautwald, David Sarlah, and Michael A. Schafroth of the Swiss Federal Institute of Technology, Zurich, has developed a version of dual catalysis in which each chiral catalyst controls the configuration of different stereocenters independently.

The reaction they use to demonstrate the strategy couples an allylic alcohol with an aldehyde. An iridium catalyst with a chiral ligand activates the allylic alcohol while at the same time a cinchona-alkaloid-derived primary amine catalyst activates the aldehyde. Depending upon which chiral version of each of the two catalysts is used, the researchers can selectively access any one of four possible stereoisomers of the γ,δ-unsaturated aldehyde product.

“This catalysis strategy is very general and should in principle be applicable to many other reactions and processes,” Krautwald tells C&EN. “It should therefore allow the development of a new generation of stereo­selective organic reactions that make the synthesis of many organic molecules simpler and faster.”

“This work is an elegant example of the value of utilizing two discrete catalysts in place of a single catalyst with respect to complete modular control of the diastereochemical outcome,” comments David W. C. MacMillan, a synthetic organic chemist at Princeton University who studies dual catalysis. “This is a beautiful example of synergistic catalysis, and it really brings home the capacity to build complex molecules from simple substrates.”

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