Cobalt: a less-expensive and more efficient catalyst than rhodium
By Tetsuo Satoh |
The research group of Shigeki Matsunaga, associate professor at the University of Tokyo (Japan; www.f.u-tokyo.ac.jp/~kanai), in collaboration with Ken Sakata at Hoshi University, has developed a new Co-based catalyst system that outperforms the existing rhodium-based catalyst system for the production of N-fused indol, an important precursor for pharmaceuticals. The catalyst combines Co(III) with acetate ions, which exhibits catalytic performance based on Lewis acidity intrinsic to cobalt. Both components are less expensive and more plentiful than rhodium. With the new Co-based catalyst system, Cp*Co(III) [Cp* = 1, 2, 3, 4, 5-pentamethylcyclopentadienil, C5(CH3)5 –], the researchers demonstrated a one-pot synthesis of N -fused indole (diagram) far superior to the Rh-based catalyst, Cp*Rh(III). The C2-selective indole alkenylation/annulation sequence proceeded smoothly with a catalytic amount (5 mol%) of [Cp*CoIII(C6H6)]-(PF6)2 complex and 20 mol% of KOAc in 1,2-dichloroethane (0.1 M) at 130°C in 20 h, giving pyrroloindolones in 58–89% yield in one pot.
The Cp*Co(III) catalysis was also shown to be suitable for simple alkenylation process of N-carbamoyl indoles, and a broad range of alkynes (including terminal alkynes)…