Deciphering the mechanism of the Ni-photocatalyzed C‒O cross-coupling reaction using a tridentate pyridinophane ligand

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 a new class of tridentate pyridinophane ligands R N3 that allow for detailed mechanistic studies of the photocatalytic C-O coupling reaction. The derived ( R N3)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)

Oxidative Addition of Aryl Halides to a Ni(I)-Bipyridine Complex

“…Indeed, while our studies were underway, the Mirica group reported that a tridentate pyridinophane ligand enabled direct observation of a Ni(I) → Ni(III) oxidative addition by EPR spectroscopy. 25 Treating 1 with pyridinophane-type aryl bromide 3 resulted in the Ni(III) complex 4, which was characterized by HRMS, EPR, and a low-resolution X-ray crystal structure that confirmed the atom connectivity (Figure 8A, S1, S2). A satisfactory, higher-quality crystal structure was obtained following exchange of the mixed halides with excess LiCl to afford 5.…”

“…Notably, spectroscopic evidence for a Ni(I) → Ni(III) oxidative addition has been obtained with other ligand scaffolds, but remains rare. 25,26 Access to a well-defined Ni(I) complex that reacts with aryl halides would enable thorough experimental mechanistic studies of the oxidative addition process. This would be particularly valuable as routine density functional theory calculations fail to accurately compute electronic structures and energies of Ni-bipyridine systems.…”