Catalyzing the Synthesis - Pharmaceutical Technology

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Catalyzing the Synthesis
Researchers develop various catalytic approaches for improving yield, purity, stereoselectivity, and process conditions.

Pharmaceutical Technology
pp. 44-50

Evaluating steric and electronic effects in asymmetric catalysis

Copper-catalyzed click chemistry for biomolecular labeling
In designing a catalyst system, understanding the steric and electronic effects is important, but it is particularly important in asymmetric synthesis. Researchers at the University of Utah recently reported on a systematic approach to optimize the steric and electronic factors in designing a catalyst for the selective addition of alkyne derivatives to ketones (4). The researchers applied a three-dimensional free-energy relationship that correlated steric and electronic effects to design and optimize a class of ligands (i.e., quinoline proline ligands) for the enantioselective Nozaki–Hiyama–Kishi propargylation of ketones. The data from the study suggested that steric–electronic correlations provided an efficient optimization of a catalytic system and a synergistic relationship between steric and electronic effects in reactions. The researchers assert that the approach is particularly useful for optimizing reactions with limited detailed mechanistic and structural understanding.

Revealing the catalytic site in oxidation catalysis

Oxidation catalysis plays an important role in the chemical, fine-chemical, and pharmaceutical industries. Approximately 80% of all compounds in the chemical and pharmaceutical industries require at least one catalytic step in their synthesis, according to some estimates. Hydrocarbon compounds used to make commodity or fine chemicals often require an oxidation step, which is mediated by a transition metal compound (5). Advances in oxidation catalysis, therefore, are of broad interest to process chemists.

Researchers at the University of Virginia recently provided a study that details a new type of catalytic site where oxidation catalysis occurs. Using a titanium dioxide substrate holding nanometer-size gold particles, the researchers identified a site that serves as the catalyst at the perimeter of the gold and titanium dioxide substrate, according to an Aug. 4, 2011, University of Virginia press release describing the research.

"This site is special because it involves the bonding of the oxygen molecule to a gold atom and to an adjacent titanium atom in the support," said John Yates, professor of chemistry at the University of Virginia and co-author of a recent research article on the study (6). "Neither the gold nor the titanium dioxide exhibits this catalytic activity when studied alone," he said.

Using spectroscopic measurements, and computational-chemistry aproaches, the researchers were able to follow the specific molecular transformations and determine where they occurred on the catalyst. The researchers observed that significant catalytic activity occurred on unique sites formed at the perimeter region between the gold particles and their titania support, according to the release.

Classifying it as a dual catalytic site because two dissimilar atoms were involved, the researchers observed that an oxygen molecule bound chemically to both a gold atom at the edge of the gold cluster and a nearby titanium atom on the titania support and reacted with an adsorbed carbon monoxide molecule to form carbon dioxide. They used spectroscopy to follow the consumption of carbon monoxide at the dual site. "This particular site is specific for causing the activation of the oxygen molecule to produce an oxidation reaction on the surface of the catalyst," said Yates in the university press release. "It's a new class of reactive site not identified before."

Moreover, the study has broader implications for catalysis research. "We have both experimental tools, such as spectrometers, and theoretical tools, such as computational chemistry, which now allow us to study catalysis at the atomic level," said Yates. "We can focus in and find that sweet spot more efficiently than ever. What we've found with this discovery could be broadly useful for designing catalysts for all catalytic reactions.


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