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The Quest for the Ideal Base: Rational Design of a Nickel Precatalyst Enables Mild, Homogeneous C–N Cross-Coupling

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posted on 19.02.2020, 18:58 by Richard Y. Liu, Joseph M. Dennis, Stephen L. Buchwald
Palladium-catalyzed amination reactions using soluble organic bases have provided a solution to the many issues associated with heterogeneous reaction conditions. Still, homogeneous C–N cross-coupling approaches cannot yet employ bases as weak and economical as trialkyl­amines. Furthermore, organic base-mediated methods have not been developed for Ni­(0/II) catalysis, despite some advantages of such systems over those employing Pd-based catalysts. We designed a new air-stable and easily prepared Ni­(II) precatalyst bearing an electron-deficient bidentate phosphine ligand that enables the cross-coupling of aryl triflates with aryl amines using triethyl­amine (TEA) as base. The method is tolerant of sterically congested coupling partners, as well as those bearing base- and nucleophile-sensitive functional groups. With the aid of density functional theory (DFT) calculations, we determined that the electron-deficient auxiliary ligands decrease both the pKa of the Ni-bound amine and the barrier to reductive elimination from the resultant Ni­(II)–amido complex. Moreover, we determined that the preclusion of Lewis acid–base complexation between the Ni catalyst and the base, due to steric factors, is important for avoiding catalyst inhibition.

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