A Highly Enantioselective Br?nsted Acid Catalyzed Cascade Reaction: Organocatalytic Transfer Hydrogenation of Quinolines and their Application in the Synthesis of Alkaloids

摘要

The enantioselective hydrogenation of olefins, ketones, and imines still represents an important topic in organic synthesis and catalysis. Although many highly enantioselective processes based on chiral Rh, Ru, and Ir complexes have been reported,[1] most of these catalysts failed to give satisfactory results in the asymmetric hydrogenation of aromatic and heteroaromatic compounds. Examples of efficient catalysts for these conversions are rare.[2] This is also true for the partial reduction of readily available quinoline derivatives,[3] the most convenient route to 1,2,3,4-tetrahydroquinolines, which are of great synthetic importance in the preparation of pharmaceuticals and agrochemicals, as well as in material sciences.[4] Furthermore, many natural products, particularly alkaloids, consist of this structural key element. We wondered whether it would be possible to extend our recently developed biomimetic, enantioselective Br?nsted acid catalyzed transfer hydrogenation of imines[5] [Eq. (1)] to the enantioselective hydrogenation of quinolines. This would not only represent the first example of a metal-free reduction of heteroaromatic compounds but would additionally give straightforward access to optically pure tetrahydroquinoline derivatives.