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.

Radical‐Type Reactivity and Catalysis by Single‐Electron Transfer to or from Redox‐Active Ligands

“…[42] It is postulated that no direct metal-CF 3 bond occurs because the reactivity differs from that of isolated species with this bond present. [43] Besides radical-radical coupling with TEMPO · ,c atalytic trifluoromethylation of alkynes, heteroaromatics andv inyl silyl ethers was reported as well. The complexes formed after electron transfer were spectroscopically investigated but not isolated.…”

A Copper(I) Complex with Two Unpaired Electrons, Synthesised by Oxidation of a Copper(II) Complex with Two Redox‐Active Ligands

“…reported trifluoromethylation of Cu II (SQ) 2 [28] and application of the resulting unstable complex as a source of CF 3 radicals [29] . Stoichiometric reactions with aryl boronic acids gave N‐arylation products [30] …”

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).…”