On the
Nature of C(sp3)–C(sp2) Bond Formation
in Nickel-Catalyzed Tertiary Radical Cross-Couplings:
A Case Study of Ni/Photoredox Catalytic Cross-Coupling of Alkyl Radicals
and Aryl Halides
posted on 2020-03-31, 20:45authored byMingbin Yuan, Zhihui Song, Shorouk O. Badir, Gary A. Molander, Osvaldo Gutierrez
The merger of photoredox and nickel
catalysis has enabled the construction
of quaternary centers. However, the mechanism, role of the ligand,
and effect of the spin state for this transformation and related Ni-catalyzed
cross-couplings involving tertiary alkyl radicals in combination with
bipyridine and diketonate ligands remain unknown. Several mechanisms
have been proposed, all invoking a key Ni(III) species prior to undergoing
irreversible inner-sphere reductive elimination. In this work, we
have used open-shell dispersion-corrected DFT calculations, quasi-classical
dynamics calculations, and experiments to study in detail the mechanism
of carbon–carbon bond formation in Ni bipyridine- and diketonate-based
catalytic systems. These calculations revealed that access to high
spin states (e.g., triplet spin state tetrahedral Ni(II) species)
is critical for effective radical cross-coupling of tertiary alkyl
radicals. Further, these calculations revealed a disparate mechanism
for the C–C bond formation. Specifically, contrary to the neutral
Ni-bipyridyl system, diketonate ligands lead directly to the corresponding
tertiary radical cross-coupling products via an outer-sphere reductive
elimination step via triplet spin state from the Ni(III) intermediates.
Implications to related Ni-catalyzed radical cross-couplings and the
design of new transformations are discussed.