C−N Bond Formation from a Masked High‐Valent Copper Complex Stabilized by Redox Non‐Innocent Ligands

Abstract

The reactivity of a stable copper(II) complex bearing fully oxidized iminobenzoquinone redox ligands towards nucleophiles is described. In sharp contrast with its genuine low‐valent counterpart bearing reduced ligands, this complex performs high‐yielding C−N bond formations. Mechanistic studies suggest that this behavior could stem from a mechanism akin to reductive elimination occurring at the metal center but facilitated by the ligand: it is proposed that a masked high oxidation state of the metal can be stabilized as a lower copper(II) oxidation state by the redox ligands without forfeiting its ability to behave as a high‐valent copper(III) center. These observations are substantiated by a combination of advanced EPR spectroscopy techniques with DFT studies. This work sheds light on the potential of redox ligands as promoters of unusual reactivities at metal centers and illustrates the concept of masked high‐valent metallic species.

High-Performance Group Transfer Catalysis by Copper Complex with Redox-Active Ligand in an Entatic State

“…The doublet ground spin state of 5 has been also predicted by DFT calculations (Fig. 34). The UV--vis spectrum of 1 displays around 1200 nm a LLCT band associated with the oxidation of a SQ moiety into BQ, whereas no LLCT can be identified for fully oxidized 5 (Fig.…”

“…3c and d). 34 Regarding the aziridination reaction (Fig. 3c), the first important observation is the absence of spectral differences between the beginning and the end of the reaction (2h), indicating that the catalytic reaction is extremely fast.…”

“…29 This structural study was the first report interfacing the two concepts of entasis and redox--active ligands, and was focused on the electronic behavior of the entatic pair. Using redox--active copper complex Cu(SQ) 2 (SQ : iminosemiquinone) originally developed as a Galactose Oxidase (GAO) model, 30,31 we have shown that ligand--based redox activity influences the overall reactivity of the complex towards electron transfer, 32,33 C-N bond formation 34 and nitrogen group transfer 35 while preserving a Cu(II) oxidation state. Here we report a novel acyclic tetradentate redox--active ligand exhibiting C 2 symmetry with overall three possible rotations around single bonds (degrees of freedom).…”

“…Figure 3. Operando 1 H NMR and EPR studies for aziridination (a, c) and cyclopropanation (b, d) of 4-chlorostyrene with complex 1.The group transfer reactions were studied by continuous wave EPR spectroscopy and operando mode, as our previous work has shown that these techniques can monitor the redox state changes of the complexes and provide structural and kinetic insights into the intermediate species (Fig.3c and d) 34. Regarding the aziridination reaction (Fig.3c), the first important observation is the absence of spectral differences between the beginning and the end of the reaction (2h), indicating that the catalytic reaction is extremely fast.…”

Copper-Promoted Functionalization of Organic Molecules: from Biologically Relevant Cu/O2 Model Systems to Organometallic Transformations

“…In a second contribution, the authors reported that the bis-iminobenzoquinone Cu II −CF 3 complex reacted with arylboronic acids to generate the C−N coupling product derived from the oxidation of the ligand scaffold (Figure 80, ii). 306 It was proposed that intramolecular CF 3 − transfer produced a copper(III) complex bearing the oxidized ligand scaffold and an iminosemiquinone radical species (species C in Figure 80). It was also implied that addition of PhB(OH) 2 could generate a copper(II) species with the phenyl bound to the copper in a trans position relative to the CF 3 group (species D in Figure 80).…”