Photocatalyst Produces Chiral Excess

Photochemical reactions arerarely used for asymmetric synthesis because they are difficult to steer toward a specific enantiomeric product. That may change, however, with a chiral photocatalyst that promotes photoinduced electron-transfer reactions with considerable turnover and high enantioselectivity (Nature 2005, 436, 1139). The catalyst, reported by Thorsten Bach and colleagues at the Munich University of Technology and the University of Mnster, facilitates the intramolecular addition of an a-amino radical to an enone, with enantiomeric excesses as high as 70%, at a catalyst loading of 30 mole %. The catalyst—a benzophenone with a rigid, chiral wing—initiates electron transfer upon photoexcitation and shields one face of the substrate from attack by the ensuing radical species. Two hydrogen bonds hold the substrate and catalyst in close proximity and in a specific orientation that leaves one face of the achiral substrate (shown in red) more open to radical addition.