N–O Bond as External Oxidant in Group 9 Cp*M(III)-Catalyzed Oxidative C–H Coupling Reactions

Group 9 Cp*M(III) (M = Co, Rh, Ir) complexes have been extensively investigated as catalysts in a variety of C–H activation reactions. Typically, late metal-based silver or copper salt was used (while needed) as oxidant in these catalytic systems. Herein, we report our discovery of a potentially general type of N–O bond-containing oxidants, which allowed the mild and efficient syntheses of isocoumarins, isoquinolines, isoquinolinone, and styrenes via C–H activation catalyzed by group 9 Cp*M(III) complexes. By using Cp*Rh(III)-catalyzed isocoumarin synthesis as a model reaction, experimental and theoretical mechanistic studies were conducted. The results concluded that the Rh(III)–Rh(I)–Rh(III) rather than the Rh(III)–Rh(V)–Rh(III) pathway is more likely involved in the mechanism, and both the C–H activation and oxidation of the Cp*Rh(I) species were involved in the turnover-limiting step.