Credit: © 2009 Wiley

Over the past decade, organocatalysis has become an important tool in asymmetric synthesis, complementing transition metal and enzyme-catalysed reactions. However, there are still many chiral compounds that are inaccessible in their enatiomerically pure form using these methods. For instance, reactions that involve photochemical activation still pose a problem.

Now, Thorsten Bach and co-workers at Munich Technical University have developed1 a chiral organocatalyst capable of harvesting light and using the energy in a highly enantioselective [2+2]-cycloaddition. The researchers prepared a chiral xanthone catalyst that can form a hydrogen-bonded complex with a substrate molecule. On irradiation with UV light, the xanthone moiety is selectively activated and triplet energy transfer occurs to the bound molecule. This controlled activation of the substrate results in the formation of cycloadducts in good yields and with high enantiomeric excess, even with low catalyst loadings.

This catalyst fulfils many of the requirements for an effective sensitizer for enantioselective photoreactions. The xanthone absorbs light at different wavelengths than the substrate molecule, allowing selective activation by the catalyst. The catalyst–substrate complex forms rapidly, so intramolecular triplet energy transfer is faster than the intermolecular process. This allows differentiation of the enantiotopic faces of the molecule.