| RESEARCH |
For the past few years, metal-containing bisoxazoline catalysts have been a focus of Karl Anker Jørgensen, professor of chemistry at Aarhus University and head of the Danish National Research Foundation's Center for Catalysis. He has studied them in asymmetric C-C bond formation via Friedel-Crafts, aldol, and Mannich reactions, as well as in the amination and halogenation of ß-ketoesters and ß-ketophosphonates. More recently, he has been using organocatalysts to explore many of these same reactions. "Organocatalysis offers opportunities to do things that you normally cannot do in metal-mediated catalysis," Jørgensen says. Organocatalysts are relatively easy to handle and require mild reaction conditions, he adds. And they avoid metal-related impurities in the product or waste stream. "With organocatalysis you can easily approach addition to carbonyl compounds, such as aldehydes and ketones," he continues. Jørgensen's lab has reported the catalytic asymmetric epoxidation of ,ß-unsaturated aldehydes using chiral amines and hydrogen peroxide with greater than 95% ee (J. Am. Chem. Soc. 2005, 127, 6964). "It's a very simple organocatalyzed reaction that was not possible with Lewis acids," he adds. Jørgensen and coworkers have used cinchona alkaloid derivatives as catalysts for stereoselective nucleophilic aromatic substitutions, asymmetric Mannich reactions, and enantioselective allylic aminations. They have also used chiral amines in various other enantioselective reactions, including a-sulfenylation, -chlorination, -fluorination, and, most recently, -bromination of aldehydes and ketones. (Chem. Commun., published online Aug. 30, dx.doi.org/10.1039/b509366j). |
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