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Dirhodium Catalysts That Bear Redox Noninnocent Chelating Dicarboxylate Ligands and Their Performance in Intra‐ and Intermolecular C–H Amination
Author(s) -
Kornecki Katherine P.,
Berry John F.
Publication year - 2012
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201100814
Subject(s) - chemistry , amination , redox , catalysis , intramolecular force , ligand (biochemistry) , reductive amination , rhodium , medicinal chemistry , cyclic voltammetry , intermolecular force , stereochemistry , electrochemistry , organic chemistry , molecule , biochemistry , receptor , electrode
We report two new analogues of the well‐known C–H amination catalyst [Rh 2 (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 (H 2 L1 ) and (3‐methoxycarbonyl‐2,5‐di‐ tert ‐butylphenoxy)ethanoic acid (H 2 L2 ), which react with Rh 2 (OAc) 4 to form the catalysts [Rh 2 ( L1 ) 2 ] ( 2 ) and [Rh 2 ( 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 Rh 2 II,II /Rh 2 II,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 Rh 2 ‐centered oxidation.