From Bis(silylene) and Bis(germylene) Pincer-Type Nickel(II) Complexes to Isolable Intermediates of the Nickel-Catalyzed Sonogashira Cross-Coupling Reaction

• Published in: Journal of the American Chemical Society Vol. 135; no. 41; pp. 15617 - 15626
• Main Authors: Gallego, Daniel, Brück, Andreas, Irran, Elisabeth, Meier, Florian, Kaupp, Martin, Driess, Matthias, Hartwig, John F
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 16.10.2013; Amer Chemical Soc
• Subjects: alkynes; bromine; catalysts; Chemistry; Chemistry, Multidisciplinary; copper; ligands; nickel; nuclear magnetic resonance spectroscopy; Physical Sciences; Science & Technology; X-ray diffraction; COPPER(I); ACTIVATION; LIGANDS; AMMONIA; COORDINATION; SILVER(I); NONACTIVATED ALKYL-HALIDES; BOND; ELECTRON LOCALIZATION
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/ja408137t

The first [ECE]Ni(II) pincer complexes with E = SiII and E = GeII metallylene donor arms were synthesized via C–X (X = H, Br) oxidative addition, starting from the corresponding [EC(X)E] ligands. These novel complexes were fully characterized (NMR, MS, and XRD) and used as catalyst for Ni-catalyzed Sonogashira reactions. These catalysts allowed detailed information on the elementary steps of this catalytic reaction (transmetalation → oxidative addition → reductive elimination), resulting in the isolation and characterization of an unexpected intermediate in the transmetalation step. This complex, {[ECE]Ni acetylide → CuBr} contains both nickel and copper, with the copper bound to the alkyne π-system. Consistent with these unusual structural features, DFT calculations of the {[ECE]Ni acetylide → CuBr} intermediates revealed an unusual E–Cu–Ni three-center–two-electron bonding scheme. The results reveal a general reaction mechanism for the Ni-based Sonogashira coupling and broaden the application of metallylenes as strong σ-donor ligands for catalytic transformations.