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Herbert C. Brown Award For Creative Research In Synthetic Methods

Recipients are honored for contributions of major significance to chemistry

by A. Maureen Rouhi
February 16, 2009 | A version of this story appeared in Volume 87, Issue 7

Credit: Courtesy of Scott Denmark
Credit: Courtesy of Scott Denmark

Sponsored by the Purdue Borane Research Fund and the Herbert C. Brown Award Endowment

Anyone who knows Scott E. Denmark well could probably conjecture that if he weren't a chemist, he would be a professional race car driver. A self-described adrenaline junkie, Denmark loves pushing the limits, not only of cars and motorcycles, but also of reactivity.

Denmark is the Reynold C. Fuson Professor of Chemistry at the University of Illinois, Urbana-Champaign (UIUC). His contributions are detailed in more than 300 publications, but his most seminal is laying the theoretical foundation and exploiting the manifestations of Lewis base activation of Lewis acids, a counterintuitive phenomenon.

When any acid associates with any base, one expects the interaction to neutralize the native acidity and basicity of the components. Yet with Lewis acids and bases, the electron redistribution makes the Lewis acid more electrophilic. "The phenomenon has been known," Denmark says, but "we are the first to harness it for chemical reactivity and catalysis."

The research has expanded the utility of weak Lewis acids, such as silicon tetrachloride. "It's a pathetic Lewis acid," Denmark says, "and cannot even open an epoxide." But activate it with a Lewis base, and it becomes able to do things strong Lewis acids can do.

Furthermore, in many Lewis acid reactions, the Lewis acid remains bound to the product and thus cannot be a catalyst. But in Denmark's system, the Lewis acid is kinetically competent only when paired with a base. Thus, the reaction can be catalytic by allowing the acid to bind to the reactant stoichiometrically and using catalytic amounts of the activating Lewis base. When product forms, the base breaks free to start another reaction cycle.

"The use of silicon tetrachloride is only proof of principle," Denmark says. The work has progressed to selenium, phosphorus, bromine, and sulfur and the interesting chemistry these elements do as electrophiles. "We can enhance their activity catalytically and, hopefully, asymmetrically by using chiral Lewis bases," he says.

This area of Denmark's research alone, says colleague John A. Katzenellenbogen, has yielded a "number of firsts: the first catalytic enantioselective aldol additions of aldehyde-derived enolates, the first catalytic enantioselective additions of ester enolates to ketones, the first catalytic enantioselective construction of quaternary stereogenic centers by asymmetric allylation, and the first catalytic enantioselective Passerini reaction."

Denmark, 55, got hooked on chemistry at an early age. As was common among the Sputnik generation, "my folks gave me a chemistry set," he recalls. At age eight, he ran experiments in a basement lab in his home and kept meticulous records in notebooks. The fact that he was causing colors to change or solids to appear captivated him.

And when he needed to remove vapors formed by halogen-generating experiments, Denmark took his mother's vacuum cleaner and attached it to a movable fume hood. As he generated chlorine, bromine, and iodine, "I could remove the vapors and nobody would smell them and know what I was doing" he recalls. "Until three days later, when the vapors destroyed the vacuum cleaner. My mother was pretty annoyed."

After receiving a bachelor's degree from Massachusetts Institute of Technology in 1975, Denmark worked with Albert Eschenmoser at the Swiss Federal Institute of Technology, Zurich, for his doctoral degree, which he received in 1980. That same year, he joined UIUC as an assistant professor of chemistry. Rising through the ranks, he reached his current position in 1991.

Denmark is receiving this award for "his insightful and scholarly analysis of reaction mechanisms" and "his creative approach and rigorous development of novel synthetic transformations of broad utility."

The award citation is predictable to those who know Denmark's chemical heroes: Gilbert N. Lewis and Sir Robert Robinson, two pillars of the current understanding of structure, reactivity, and mechanism in organic chemistry; and Eschenmoser, who, in Denmark's estimation, is unequaled "in his depth of intellect and ability to analyze problems of great significance."

Denmark will present the award address before the Division of Organic Chemistry.


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