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

Abstract

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 η 2-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 η 2-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).