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Synthesis

High-Valent Gold Catalyst Shines

Organometallics: Team makes stable version of hard-to-come-by gold(III) catalyst

by Bethany Halford
January 22, 2015 | APPEARED IN VOLUME 93, ISSUE 4

More of gold’s charms have been revealed, as chemists report a rare stable gold catalyst in the +3 oxidation state. The high-valent transition-metal catalyst offers molecule makers a shiny new tool that does chemistry distinct from that of gold catalysts in the more common +1 oxidation state.

For more than a decade, gold(I) catalysts have proven to be increasingly useful in the assembly of complex molecules. Gold(III) catalysts, on the other hand, have been harder to come by because it usually takes harsh reagents to prepare them and they tend not to be stable.

F. Dean Toste, Chung-Yeh Wu, Takahiro Horibe, and Christian Borch Jacobsen, chemists at the University of California, Berkeley, have now made a stable gold(III) catalyst by getting a gold(I) catalyst to insert into a carbon-carbon bond—a process known as oxidative addition (Nature 2015, DOI: 10.1038/nature14104).

Toste tells C&EN that the team got the idea while studying the opposite reaction, the reductive elimination of gold(III) to gold(I). Because gold prefers to be in the +1 oxidation state, that reaction is thermodynamically favored. But the Berkeley team reckoned that by adding some strain to the system, they could tip the scales in favor of the +3 oxidation state. Gold(III) turns out to behave differently from gold(I) in several different types of reactions, including Mukaiyama-Michael additions and Diels-Alder reactions. “You’ve changed the electronic nature of the metal, and therefore it’s going to have different reactivity,” Toste explains.

“This work is particularly impressive because it puts together several well-known principles of gold chemistry and employs this knowledge in combination with brilliant new ideas to come up with novel and highly versatile synthetic protocols,” comments Hubert Schmidbaur, an expert in gold chemistry at Germany’s Technical University of Munich.

Steven P. Nolan, an expert in organometallic catalysis at the University of St. Andrews, in Scotland, adds, “This contribution will provide access to up-to-now only proposed entities that will enable a vast landscape of reactivity to be developed.”

Gold catalysts in different oxidation states behave differently in this Mukaiyama-Michael addition.
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