A well-defined copper complex bearing iminosemiquinone ligands performs single electron reduction of an electrophilic CF3+ source into CF˙3 radicals. This redox behavior is enabled by the ligand ...which shuttles through two different redox states (iminosemiquinone and iminobenzoquinone) while the copper center is preserved as a Cu(ii). This system was used in the trifluoromethylation of silyl enol ethers, heteroaromatics and in the hydrotrifluoromethylation of alkynes. This is the first example of cooperative redox catalysis for the controlled generation of CF˙3 radicals.
Coupling reactions are staples in the synthetic world and their efficiency relies on well‐defined, mostly bis‐electronic, elementary catalytic steps. An area of great interest currently lies in the ...taming of radical species and their efficient introduction in catalytic cycles. Among these species bearing radical character, redox‐active ligands hold much potential and can be used to sustain synthetically relevant couplings by introducing ligand‐based electronic contribution. This minireview aims at presenting the current state of this promising field.
Ligand, camera, action! The advent of innovative (catalytic) pathways relying on ligand‐based redox events is reviewed in the context of coupling reactions. This cooperative approach between ligand and metal can offer attractive alternatives to the classic two‐electron catalytic cycles and foster new reactivities.
Nickel complexes have gained sustained attention as efficient catalysts in cross‐coupling reactions and co‐catalysts in dual systems due to their ability to react with radical species. Central to ...this reactivity is nickel's propensity to shuttle through several accessible redox states from Ni0 to NiIV. Here, we report the catalytic generation of trifluoromethyl radicals from a nickel complex bearing redox‐active iminosemiquinone ligands. This unprecedented reactivity is enabled through ligand‐based oxidation performing electron transfer to an electrophilic CF3+ source while the nickel oxidation state is preserved. Additionally, extension of this reactivity to a copper complex bearing a single redox equivalent is reported, thus providing a unified reactivity scheme. These results open new pathways in radical chemistry with redox‐active ligands.
ElectroNic: Electronic transfer from a nickel complex bearing redox‐active ligands to form radical species is studied. Combined UV/Vis and DFT experiments suggest that the redox‐active ligands in the complex sustain this reactivity, and have a greater influence than the metal‐based reactivity. This reactivity can be extended to a complex bearing a single “redox‐equivalent”.
Small‐molecule catalysts as mimics of biological systems illustrate the chemists’ attempts at emulating the tantalizing abilities displayed by nature's metalloenzymes. Among these innate behaviors, ...spin multistate reactivity is used by biological systems as it offers thermodynamic leverage towards challenging chemical reactivity but this concept is difficult to translate into the realm of synthetic organometallic catalysis. Here, we report a rare example of molecular spin catalysis involving multistate reactivity in a small‐molecule biomimetic copper catalyst applied to aziridination. This behavior is supported by spin state flexibility enabled by the redox‐active ligand.
Putting a (molecular) spin on copper catalysis: Metalloenzymes routinely perform multielectronic transformations using multistate reactivity and redox cofactors but this behavior is difficult to emulate in synthetic systems. We report a rare example of molecular spin catalysis in the context of copper‐catalyzed aziridination. The molecular spin fluxionality enabled by the redox‐active ligands is central to this behavior.
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.
CuIII in hiding: A stable copper(II) complex bearing fully oxidized iminobenzoquinone redox ligands reacts as a copper(III) species and performs high‐yielding C−N bond formation. 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.
The solvent‐ and ligand‐free 2+2+2 ruthenium‐promoted cycloaddition of α,ω‐diynes and alkynes provides a facile and efficient strategy for the synthesis of substituted benzene‐derived systems. The ...search for the optimal reaction conditions revealed the unprecedented catalytic activity of ruthenium trichloride for benzannulation reactions and this atom‐economical process allowed the synthesis of fused arenes including dihydrobenzofurans, isoindolines, indanes in good to high yields. This practical protocol also gave rise to the preparation of pentasubstituted aromatic derivatives and was applied to the one‐gram scale synthesis of a functionalized heterocycle.
A well-defined copper complex bearing iminosemiquinone ligands performs single electron reduction of an electrophilic CF sub(3) super(+) source into CF super(&z.rad; ) sub(3) radicals. This redox ...behavior is enabled by the ligand which shuttles through two different redox states (iminosemiquinone and iminobenzoquinone) while the copper center is preserved as a Cu(ii). This system was used in the trifluoromethylation of silyl enol ethers, heteroaromatics and in the hydrotrifluoromethylation of alkynes. This is the first example of cooperative redox catalysis for the controlled generation of CF super(&z.rad; ) sub(3) radicals.
The reaction of a copper complex bearing iminosemiquinone ligands with a CF sub(3) super(+) source provides an unprecedented Cu super(II)-CF sub(3) complex through ligand-based oxidation. Reactivity ...of this complex leads to nucleophilic trifluoromethylation of the ligand, suggesting an electronic interplay that results in a formal umpolungof the initial CF sub(3) super(+).
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.
CuIII spielt Verstecken: Ein beständiger Kupfer(II)‐Komplex mit vollständig oxidierten Iminobenzochinon‐Liganden reagiert wie eine Kupfer(III)‐Spezies und vermittelt C‐N‐Kupplungen mit hohen Ausbeuten. Mechanistische Studien führen dieses Verhalten auf einen Prozess zurück, welcher der reduktiven Eliminierung am Metallzentrum ähnlich ist, aber durch den redoxaktiven Ligand erleichtert wird.
The reaction of a copper complex bearing iminosemiquinone ligands with a CF3(+) source provides an unprecedented Cu(II)-CF3 complex through ligand-based oxidation. Reactivity of this complex leads to ...nucleophilic trifluoromethylation of the ligand, suggesting an electronic interplay that results in a formal umpolung of the initial CF3(+).
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