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Keywords:

  • Rhodium;
  • Homogeneous ­catalysis;
  • Amination;
  • Redox chemistry;
  • Chelates

Abstract

We report two new analogues of the well-known C–H amination catalyst [Rh2(esp)2] (1) (esp = α,α,α′,α′-tetramethyl-1,3-benzenedipropanoate) that bear redox-active supporting ligands that are structurally similar to esp. The redox-active ligands are 2-[3-(1-carboxy-1-methylethoxy)phenoxy]-2-methylpropanoic acid (H2L1) and (3-methoxycarbonyl-2,5-di-tert-butylphenoxy)ethanoic acid (H2L2), which react with Rh2(OAc)4 to form the catalysts [Rh2(L1)2] (2) and [Rh2(L2)2] (3). Both 2 and 3 have been characterized by X-ray crystallography and cyclic voltammetry, inter alia. Compounds 2 and 3 are structurally similar to 1 but show more complex electrochemical features. Whereas 1 has a single reversible redox wave that corresponds to the Rh2II,II/Rh2II,III couple, 2 and 3 show multiple oxidations that are characteristic of ligand-centered oxidation. Catalysts 1, 2, and 3 perform well in a model intramolecular C–H amination reaction, and all three catalysts perform equally well during the first four hours of a model intermolecular reaction. After this point, 2 and 3 cease to function, whereas 1 continues to be active. These results support the hypothesis that intermolecular C–H amination utilizes two distinct mechanisms: (1) a nitrene interception/insertion mechanism that is fast but ceases to be operative after four hours, and (2) a one-electron mechanism that is more robust over extended time periods, but requires the catalyst to be able to undergo Rh2-centered oxidation.