Elucidating ligand effects in rhodium(III)-catalyzed arene-alkene coupling reactions

Rhodium(III)-catalyzed CH couplings of arenes with alkenes are among the most powerful methods for CC bond formation. For these transformations, subtle manipulation of ancillary ligands can lead to dramatic changes in reactivity and selectivity. However, detailed mechanistic studies concerning the ligand effects are rare. In this study, we investigated the origin of ligand-controlled product-selectivity in rhodium(III)-catalyzed CH couplings of arenes with alkenes, using a series of well-defined [(CpRhIII)-Rh-X] complexes that feature electronically or sterically distinct Cp-X (Cp (eta(5)-C5H5), Cp-CF3 (eta(5)-C5Me4CF3) and Cp* (eta(5)-C5Me5)) ligands. A combination of experimental and theoretical investigations showed that (i) rhodium hydride species containing the electron rich Cp* ligand can undergo reinsertion of the alkene, thereby allowing rhodium-walking, (ii) rhodium hydride species involving the electron-deficient Cp or Cp-CF3 ligands prefer reductive elimination rather than alkene insertion. These findings offer valuable insights on future rational catalyst design for selective arene-alkene cross coupling reactions.