Faculty of Science and ‡Institute of Quantum Beam Science, Graduate School of Science and Engineering, Ibaraki University , Mito, Ibaraki 310-8512, Japan.
Center for Atomic and Molecular Technologies, Graduate School of Engineering and #Department of Applied Chemistry, Faculty of Engineering, Osaka University , Suita, Osaka 565-0871, Japan.
J Am Chem Soc. 2017 Aug 2;139(30):10347-10358. doi: 10.1021/jacs.7b04279. Epub 2017 Jul 21.
Nickel(0)-catalyzed cross-coupling of methoxyarenes through C-O bond activation has been the subject of considerable research because of their favorable features compared with those of the cross-coupling of aryl halides, such as atom economy and efficiency. In 2008, we have reported nickel/PCy-catalyzed cross-coupling of methoxyarenes with arylboronic esters in which the addition of a stoichiometric base such as CsF is essential for the reaction to proceed. Recently, we have also found that the scope of the substrate in the Suzuki-Miyaura-type cross-coupling of methoxyarenes can be greatly expanded by using 1,3-dicyclohexylimidazol-2-ylidene (ICy) as the ligand. Interestingly, a stoichiometric amount of external base is not required for the nickel/ICy-catalyzed cross-coupling. For the mechanism and origin of the effect of the external base to be elucidated, density functional theory calculations are conducted. In the nickel/PCy-catalyzed reactions, the activation energy for the oxidative addition of the C(aryl)-OMe bond is too high to occur under the catalytic conditions. However, the oxidative addition process becomes energetically feasible when CsF and an arylboronic ester interact with a Ni(PCy)/methoxyarene fragment to form a quaternary complex. In the nickel/ICy-catalyzed reactions, the oxidative addition of the C(aryl)-OMe bond can proceed more easily without the aid of CsF because the nickel-ligand bonds are stronger and therefore stabilize the transition state. The subsequent transmetalation from an Ar-Ni-OMe intermediate is determined to proceed through a pathway with lower energies than those required for β-hydrogen elimination. The overall driving force of the reaction is the reductive elimination to form the carbon-carbon bond.
镍催化的通过 C-O 键活化的甲氧基芳烃的交叉偶联反应由于其与芳基卤化物的交叉偶联相比具有有利的特点，例如原子经济性和效率，因此受到了相当多的研究。在 2008 年，我们报道了镍/PCy 催化的甲氧基芳烃与芳基硼酸酯的交叉偶联反应，其中添加化学计量的碱（如 CsF）对于反应的进行是必不可少的。最近，我们还发现通过使用 1,3-二环己基咪唑-2-亚基（ICy）作为配体，可以大大扩展甲氧基芳烃的 Suzuki-Miyaura 型交叉偶联反应的底物范围。有趣的是，对于镍/ICy 催化的交叉偶联反应，不需要化学计量的外部碱。为了阐明外部碱的作用的机制和起源，进行了密度泛函理论计算。在镍/PCy 催化的反应中，C(aryl)-OMe 键的氧化加成的活化能太高，无法在催化条件下发生。然而，当 CsF 和芳基硼酸酯与 Ni(PCy)/甲氧基芳烃片段相互作用形成四元复合物时，氧化加成过程在能量上变得可行。在镍/ICy 催化的反应中，C(aryl)-OMe 键的氧化加成可以更轻易地进行，而不需要 CsF 的帮助，因为镍-配体键更强，因此稳定了过渡态。随后从 Ar-Ni-OMe 中间体进行的转金属化被确定通过能量比β-氢消除所需的能量更低的途径进行。反应的总体驱动力是还原消除以形成碳-碳键。
