Acceleration Mechanisms of C-H Bond Functionalization Catalyzed by Electron-Deficient CpRh(III) Complexes

A rhodium(III) complex bearing a 1,3-bis(ethoxycarbonyl)-substituted or an unsubstituted cyclopentadienyl ligand (Cp-E or Cp) significantly accelerates a variety of oxidative C-H bond functionalization reactions. However, the driving force of the acceleration compared with a conventionally used Cp*Rh(III) complex has not been elucidated. Herein, we performed density functional theory (DFT) calculations of the rhodium(III)-catalyzed oxidative C-H bond olefination and annulation reactions using Cp*, Cp, and Cp-E ligands, which revealed that the (CpRh)-Rh-E(III) complex stabilizes transition states of not only a C-H bond activation step but also rate-determining reductive elimination and insertion steps by strong orbital interactions. For the sterically demanding substrates, the less sterically hindered CpRh(III) complex can stabilize the transition states of the reductive elimination step more than the (CpRh)-Rh-E(III) complex. Moreover, the whole reaction pathways were calculated to elucidate the mechanism and selectivity of the oxidative [4 + 2] and [2 + 2 + 2] annulation reactions under cationic and neutral conditions, respectively.

Automated summary

The research reveals that the (CpRh)-Rh-E(III) complex stabilizes key transition states in oxidative C-H bond functionalization reactions through strong orbital interactions, and the CpRh(III) complex is more stable in the reductive elimination step for sterically demanding substrates.