Mechanistic Investigations of the Hydrogenolysis of Diaryl Ethers Catalyzed by Nickel Complexes of N-Heterocyclic Carbene Ligands.
Mechanistic Investigations of the Hydrogenolysis of Diaryl Ethers Catalyzed by Nickel Complexes of N-Heterocyclic Carbene Ligands.
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DOI：
10.1021/jacs.7b10537
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发表时间：
2017-11
影响因子：
15
通讯作者：
N. Saper;J. Hartwig
中科院分区：
文献类型：
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作者：
N. Saper;J. Hartwig
最近对木质素增值的兴趣导致了涉及强芳香C-O键裂解的反应。然而，这些反应的实验机理研究很少发表。本文报道了Ni(COD)2 （COD = 1,5-环二烯）和n -杂环碳烯（NHC）结合催化二芳醚氢解的机理分析。催化实验表明，NaOt-Bu是催化反应所必需的，但动力学分析表明，该碱不参与C-O键的限速裂解。催化剂的静息态为NHC-Ni（η - 6-芳烃）络合物。用二芳基醚取代配位溶剂可以分离二芳基醚结合的Ni配合物。限制速率的C-O键裂解产生氧化加成的三坐标产物，其金属环版本已被独立制备。化学计量学研究表明，芳烃和苯酚与H2反应后释放产物。发现NaOt-Bu能使苯酚产物去质子化，并防止无活性NiI二聚体的形成。
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.