Abstract
Photoredox nickel catalysis has emerged as a powerful strategy for cross-coupling reactions. Although the involvement of paramagnetic Ni(I)/Ni(III) species as active intermediates in the catalytic cycle has been proposed, a thorough spectroscopic investigation of these species is lacking. Herein, we report the tridentate pyridinophane ligands RN3 that allow for detailed mechanistic studies of the photocatalytic C–O coupling reaction. The derived (RN3)Ni complexes are active catalysts under mild conditions and without an additional photocatalyst. We also provide direct evidence for the key steps involving paramagnetic Ni species in the proposed catalytic cycle: the oxidative addition of an aryl halide to a Ni(I) species, the ligand exchange/transmetalation at a Ni(III) center, and the C–O reductive elimination from a Ni(III) species. Overall, the present work suggests the RN3 ligands are a practical platform for mechanistic studies of Ni-catalyzed reactions and for the development of new catalytic applications.
Highlights
Photoredox nickel catalysis has emerged as a powerful strategy for cross-coupling reactions
We show that the utilization of the tridentate RN3 ligands allows for a comprehensive examination of the Ni intermediates proposed in the individual steps of the cross-coupling catalytic cycle: oxidative addition, transmetalation/ligand exchange, and reductive elimination, all these steps involving paramagnetic NiI or NiIII species
In 1958, Overberger et al reported the reduction of N-nitrosodibenzylamines by Na2S2O4 to generate the hydrocarbon product with evolution of N229. This reaction has received little attention since and only one report by Takemura et al in 1988 exploited the N-nitroso reduction reaction to obtain [2.2]cyclophane derivatives[30]. We speculated that this N-extrusion reaction would provide an efficient synthetic pathway for tridentate pyridinophane ligands
Summary
Photoredox nickel catalysis has emerged as a powerful strategy for cross-coupling reactions. We report the tridentate pyridinophane ligands RN3 that allow for detailed mechanistic studies of the photocatalytic C–O coupling reaction. A bidentate ligand structure is not suitable for stabilizing high-valent Ni species, hampering the investigation of such Ni intermediates[22] Considering these aspects, we sought to identify an optimal ligand framework positioned in between bipyridine and RN4 ligands in terms of denticity, molecular structure, and functionality, and the pyridinophane tridentate RN3 ligands were targeted (Fig. 1a). We show that the utilization of the tridentate RN3 ligands allows for a comprehensive examination of the Ni intermediates proposed in the individual steps of the cross-coupling catalytic cycle: oxidative addition, transmetalation/ligand exchange, and reductive elimination, all these steps involving paramagnetic NiI or NiIII species
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