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Noninnocent Ligand in Rhodium(III)-Complex-Catalyzed C–H Bond Amination with Tosyl Azide

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journal contribution
posted on 2018-05-18, 07:18 authored by Daiki Fujita, Hideki Sugimoto, Yuma Morimoto, Shinobu Itoh
Six-coordinate rhodium­(III) complexes coordinated by a planar trianionic ligand (L3–) are synthesized. One of the axial positions is occupied by chloride (Cl), bromide (Br), or iodide (I), and another axial position is coordinated by a solvent molecule such as acetonitrile (AN), water (H2O), tetrahydrofuran (THF), or pyridine (PY) to complete an octahedral rhodium­(III) center; [RhIII(L3–)­(X)­(Y)] (1X/Y; X = Cl, Br, or I, Y = AN, H2O, THF, or PY). Coordination of the AN, H2O, and THF ligands to the metal center is rather weak, so that these solvent molecules are easily replaced by PY to give [RhIII(L3–)­(Cl)­(PY)]. In the electrochemical measurements, all complexes have two reversible redox couples based on the ligand-centered oxidation L3– to L•2– and to L, as reflected by the very similar redox potentials regardless of the different axial ligands. The rhodium­(III) complexes catalyze C–H bond amination of xanthene with tosyl azide (TsN3). Because the yields of the aminated product are nearly the same among the complexes, replacement of the axial solvent ligands with TsN3 readily occurs to give a nitrene-radical-bound rhodium­(III) complex, [RhIII(L•2–)­(NTs)­(X)], as an active oxidant, which is generated by one-electron transfer from the trianionic L3– to the nitrene nitrogen atom. Generation of such a diradical intermediate was substantiated by the direct reaction of 1Cl/AN with TsN3 in the absence of the substrate (xanthene). In this case, a rhodium­(III) iminosemiquinone complex, 2, was generated by the intramolecular reaction between the nitrene-radical moiety and the radical moiety of the ligand L•2–.