Combined Theoretical and Experimental Studies of Nickel-Catalyzed Cross-Coupling of Methoxyarenes with Arylboronic Esters via C–O Bond Cleavage

• Published in: Journal of the American Chemical Society Vol. 139; no. 30; pp. 10347 - 10358
• Main Authors: Schwarzer, Martin C, Konno, Ryosuke, Hojo, Takayuki, Ohtsuki, Akimichi, Nakamura, Keisuke, Yasutome, Ayaka, Takahashi, Hiroaki, Shimasaki, Toshiaki, Tobisu, Mamoru, Chatani, Naoto, Mori, Seiji
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 02.08.2017
• Subjects: activation energy; chemical bonding; cleavage (chemistry); cross-coupling reactions; density functional theory; esters; ligands; nickel; organic halogen compounds
• DOI: 10.1021/jacs.7b04279
• ISSN: 0002-7863; 1520-5126

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/PCy3-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/PCy3-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(PCy3)2/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.