ja7b10537_si_005.cif (2.3 MB)
Mechanistic Investigations of the Hydrogenolysis of Diaryl Ethers Catalyzed by Nickel Complexes of N‑Heterocyclic Carbene Ligands
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posted on 2017-11-08, 00:00 authored by Noam I. Saper, John F. HartwigRecent interest in the valorization
of lignin has led to reactions involving the cleavage of strong aromatic
C−O bonds. However, few experimental mechanistic studies of
these reactions have been published. We report detailed mechanistic
analysis of the hydrogenolysis of diaryl ethers catalyzed by the combination
of Ni(COD)2 (COD = 1,5-cyclooctadiene) and an N-heterocyclic carbene (NHC). Experiments on the catalytic reaction
indicated that NaOt-Bu was necessary for catalysis,
but kinetic analysis showed that the base is not involved in the rate-limiting
C−O bond cleavage. The resting state of the catalyst is an
NHC-Ni(η6-arene) complex. Substitution of the coordinated
solvent with diaryl ether allowed isolation of a diaryl ether-bound
Ni complex. Rate-limiting C−O bond cleavage occurs to generate
a three-coordinate product of oxidative addition, a metallacyclic
version of which has been prepared independently. Stoichiometric studies
show that arene and phenol products are released following reaction
with H2. NaOt-Bu was found to deprotonate
the phenol product and to prevent formation of inactive NiI dimers.