Identifying the Imperative Role of Metal–Olefin Interactions in Catalytic C–O Reductive Elimination from Nickel(II)

We
present a series of experimental and computational mechanistic
investigations of an unusually facile example of Ni-catalyzed C–O
bond formation. Our method, originally reported in 2016, involves
the formation of cyclic enol ethers from vinyl iodides bearing pendant
alcohol groups. Our findings suggest that the observed reactivity
arises from the coordination of the olefin in the vinyl iodide starting
material and the enol ether product with Ni(0) intermediates. Density
functional theory calculations reveal a plausible catalytic mechanism
involving a Ni­(II)/Ni(0) redox cycle featuring two-electron C–I
oxidative addition and C–O reductive elimination steps. The
direct formation of a η2-enol ether Ni(0) complex
from a key Ni­(II) alkoxide intermediate dramatically alters the free
energy (ΔG) for the vinyl C–O reductive
elimination step relative to other examples of C–O reductive
elimination at Ni­(II). Furthermore, efficient σ-π mixing
in the course of vinyl C–O reductive elimination leads to lower
computed kinetic barriers (ΔG‡) relative to those of aryl C–O reductive elimination. The
conclusions drawn from these computational models are supported by
synthetic organometallic experiments, whereby a vinyl–Ni­(II)
iodide intermediate was isolated, characterized, and proved to yield
enol ether, following exposure to triethylamine. We conducted further
experiments and computations, which indicated that the two-electron
oxidative addition of vinyl iodides by Ni(0) depends on the formation
of an η2-vinyl iodide precomplex, based on the observation
of one-electron activation of the same vinyl iodide in the presence
of sterically encumbering ligands (e.g., tricyclohexylphosphine).