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Reaction of a Zirconocene–Carboryne Complex with Pyridines: Ligand C–H Activation

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journal contribution
posted on 14.11.2011, 00:00 authored by Shikuo Ren, Zuowei Xie
Reactions of Cp2Zr(μ-Cl)(μ-C2B10H10)Li(OEt2)2 (1) with various N-heterocycles derived from pyridine were studied. Treatment of 1 with pyridine, 2-bromopyridine, 2,4-lutidine, quinoline, and 2-(1-hexynyl)pyridine generated α-C–H activation (σ-bond metathesis) products Cp2Zr(η2-C,N-C5H4N)(σ-C2B10H11) (2), Cp2Zr[η2-C,N-(6-Br-C5H3N)](σ-C2B10H11) (3), Cp2Zr[η2-C,N-(4,6-Me2-C5H2N)](σ-C2B10H11) (4), Cp2Zr(η2-C,N-C9H6N)(σ-C2B10H11) (5), and Cp2Zr{η2-C,N-[6-(nBuCC)-C5H3N]}(σ-C2B10H11) (7), respectively. On the other hand, reaction of 1 with acridine gave the addition product 1,2-[Cp2Zr(10,9-C13H9N)]-1,2-C2B10H10 (6) in 85% isolated yield. Complex 1 reacted with 3-(1-hexynyl)pyridine to afford α-C–H activation species Cp2Zr{η2-C,N-[5-(nBuCC)C5H3N]}(σ-C2B10H11) (8a) and Cp2Zr{η2-C,N-[3-(nBuCC)C5H3N]}(σ-C2B10H11) (8b) in a molar ratio of 42:58, as determined by the 1H NMR spectrum. In the presence of CuI, however, the CC insertion products zirconacyclopentenes 1,2-[Cp2ZrC(2-C5H4N)CR]-1,2-C2B10H10 [R = Bun (9), Ph (10)] were obtained in 74–77% yields. It is suggested that the coordination of pyridine to the Zr atom is crucial for α-C–H activation (σ-bond metathesis). The presence of CuI can alter the reaction path by preventing the coordination of pyridine to the Zr atom, which blocks the α-C–H activation path, leading to the alkyne insertion reaction. All complexes were characterized by 1H, 13C, and 11B NMR spectra as well as elemental analyses. Their structures were further confirmed by single-crystal X-ray analyses.