Mechanistic Investigations of the Hydrogenolysis of Diaryl Ethers Catalyzed by Nickel Complexes of N‑Heterocyclic Carbene Ligands

Recent 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 Ni^I dimers.