Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 139, Issue 30, Pages 10347-10358Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jacs.7b04279
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Funding
- MEXT, Japan [16H01022, 16H01001]
- ACT-C from JST, Japan [JPMJCR12ZF, JPMJCR12YN]
- Grants-in-Aid for Scientific Research [16H01022, 16H01001] Funding Source: KAKEN
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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 elucidated, density functional theory calculations are conducted. In the nickel/PCy3-catalyzed reactions, the activation energy for the oxidative addition of the Caryl)-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 Caryl)-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 beta-hydrogen elimination. The overall driving force of the reaction is the reductive elimination to form the carbon carbon bond.
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