2019 Volume 77 Issue 4 Pages 330-340
Transition metal-catalyzed C-H activation/functionalization reactions are attractive and promising methods in modern organic synthesis. Trivalent rhodium with a pentamethylcyclopentadienyl ligand, Cp*RhIII, is one of the most successful catalysts for directing group-assisted C-H functionalization reactions, exhibiting high reactivity, robustness, and good functional group compatibility. Cobalt is analogous to rhodium in the periodic table, but less expensive and more available than rhodium. In this article, we describe our studies on Cp*CoIII-catalyzed directing group-assisted C-H functionalization reactions. The cobalt catalysts not only enable the same transformations as the rhodium catalysts, but also exhibit unique reactivity and selectivity in several reactions due to the lower electronegativity, smaller size, and harder nature of cobalt compared with those of rhodium.
Catalytic asymmetric C-H functionalization using Cp*MIII (M=Co, Rh, Ir) catalysts is highly challenging because additional chiral ligands cannot participate in an enantio-determining step. Although well-designed chiral Cpx ligands were developed and used for catalytic asymmetric reactions, the synthesis of such chiral Cpx ligands and complexes is sometimes an obstacle to tuning the catalyst structure for achieving high enantioselectivity. As a different approach for asymmetric reactions, we demonstrated that chiral sulfonates and chiral carboxylic acids hybridized with achiral rhodium and cobalt complexes enable enantioselective C-H functionalization.