Mechanistic Investigations of the Hydrogenolysis of Diaryl Ethers Catalyzed by Nickel Complexes of <i>N</i>‑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 NiI dimers.近年来，学界对木质素增值利用的研究兴趣持续升温，催生了一系列涉及强芳香族C-O键断裂的转化反应。然而，目前针对此类反应的实验机理研究却鲜有报道。本文报道了由双(1,5-环辛二烯)合镍[Ni(COD)₂，COD=1,5-环辛二烯]与氮杂环卡宾（N-heterocyclic carbene, NHC）组合催化的二芳基醚氢解反应的详细机理分析。催化反应实验表明，叔丁醇钠（NaOt-Bu）对催化过程不可或缺，但动力学分析显示，该碱并未参与限速的C-O键断裂步骤。催化剂的催化循环静止态为NHC-Ni(η⁶-芳烃，η⁶-arene)配合物。将配位溶剂替换为二芳基醚后，可分离得到与二芳基醚配位的镍配合物。限速的C-O键断裂步骤会生成三配位的氧化加成产物，其金属环型衍生物已被独立合成。化学计量研究表明，芳烃与苯酚产物可在与氢气发生反应后释放。研究发现，叔丁醇钠可使苯酚产物脱质子，并有效防止失活的一价镍二聚体形成。