Development of a practical non-noble metal catalyst for hydrogenation of N-heteroarenes

Due to their availability, price and biological relevance, the use of catalysts based on 3d transition metals is of substantial importance for the synthesis of industrial chemicals, but also for organic synthesis in general. Hence in recent years, especially in homogeneous catalysis, the use of such Earth-abundant, biocompatible metals has become a major area of interest. However, to achieve reactivity comparable to that of noble-metal catalysts, generally sophisticated ligands-typically expensive phosphorus derivatives-have to be used. Here, we report the chemoselective reduction of quinolines and related N-heterocycles by molecular hydrogen, using a simple Mn(i) complex [Mn(CO)(5)Br]. Under very mild reaction conditions this catalytic system is able to reduce a wide range of quinolines, affording high yields of the corresponding tetrahydroquinolines, a scaffold present in many bioactive compounds, including marketed pharmaceuticals. Mechanistic studies reveal the formation of the active catalyst and also show the important role of a concomitantly formed Mn(ii) species and HBr for the hydrogenation of the heterocyclic substrates. In order to use early, non-noble transition metals in homogeneous catalysis, complex ligands are typically needed, offsetting the benefits of inexpensive metals. Here the authors show that a simple manganese complex can be used in the hydrogenation of N-heteroarenes, without the need for additional ligands.