Nickel-Mediated Hydrogenolysis of C–O Bonds of Aryl Ethers: What Is the Source of the Hydrogen?

Mechanistic studies of the hydrogenolysis of aryl ethers
by nickel
were undertaken with (diphosphine)­aryl methyl ethers. A Ni(0) complex
containing Ni–arene interactions adjacent to the aryl–O
bond was isolated. Heating led to aryl–O bond activation and
generation of a nickel aryl methoxide complex. Formal β-H elimination
from this species produced a nickel aryl hydride which can undergo
reductive elimination in the presence of formaldehyde to generate
a carbon monoxide adduct of Ni(0). The reported complexes map out
a plausible mechanism of aryl ether hydrogenolysis catalyzed by nickel.
Investigations of a previously reported catalytic system using isotopically
labeled substrates are consistent with the mechanism proposed in the
stoichiometric system, involving β-H elimination from a nickel
alkoxide rather than cleavage of the Ni–O bond by H2.