Pentamethylcyclopentadienyl ($Cp^*$)-based rhodium and iridium complexes have emerged as powerful tools for C−H functionalization reactions. While a diverse range of organic transformations by using $Cp^*Rh(III)$ and $Cp^*Ir(III)$ catalysts have been established, the development of $Cp^*Co(III)$ catalyst in the field of C−H functionalization is still in infancy. Along this line, described herein is the development of cobalt-catalyzed C−H bond functionalization reactions as follows.
First, $Cp^*Co(III)$-catalyzed C−H amidation of arenes with dioxazolone has been developed for the first time. Three types of substrates including heteroarenes, aromatic amides and anilides were readily amidated, thus exhibiting broad range of applicability of this cobalt catalytic system. A comparative study on the catalytic activity of Group 9 $Cp^*M(III)$ (M = Co, Rh, and Ir) complexes revealed the unique efficiency of the cobalt catalyst.
Second, the viability of synthesizing polyaminobenzene derivatives via $Cp^*Co(III)$-catalyzed iterative C−H amidation of anilides was investigated. This procedure needs sequential activation of C−H bonds of substrates by cascade chelation assistance of newly installed amide group. Since the multiple C−H functionalization with cobalt catalyst was not successful, ruthenium(II) catalytic system was utilized for iterative C−H amidation leading to the formation of polyamidobenzenes. Further mechanistic study including DFT calculation revealed that the intramolecular hydrogen bonding is important on the successful cascade amidation reaction. Although this iterative process could functionalize up to four C−H bonds at once, the fifth C−H bond did not activated because of the steric hindrance.
Third, cobalt catalyzed C−H amidation with azidoformates as an amidating reagent were developed. By utilizing LX-type bidentate ligand to the cobalt complex, the modified $Cp^*(LX)Co(III)$ complexes can catalyze the facile insertion of nitrogen atom into a C−H bond via outer-sphere mechanism. Both C($sp^2$)−H and C($sp^3$)−H bonds were easily amidated in intramolecular reaction leading to the formation of 5-membered cyclic amide. Also, intermolecular C−H amidation of n-hexane with azidoformate was also demonstrated.