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Transition-Metal-Promoted or -Catalyzed Exocyclic Alkyne Insertion via Zirconacyclopentene with Carborane Auxiliary: Formation of Symmetric or Unsymmetric Benzocarboranes

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posted on 22.02.2016, 05:57 by Shikuo Ren, Zaozao Qiu, Zuowei Xie
Reactions of Cp2Zr­(μ-Cl)­(μ-C2B10H10)­Li­(OEt2)2 with alkynes R1CCR2 gave as insertion products zirconacyclopentenes incorporating a carboranyl unit, 1,2-[Cp2ZrC­(R1)C­(R2)]-1,2-C2B10H10 (1). Treatment of 1 with another type of alkyne R3CCR4 in the presence of stoichiometric amounts of NiCl2 and FeCl3 or a catalytic amount of NiCl2 afforded symmetric or unsymmetric benzocarboranes. The regioselectivity was dominated by the polarity of the corresponding alkynes. These reactions could also be carried out in one pot, leading to the equivalent of a three-component [2 + 2 + 2] cycloaddition of carboryne and two different alkynes promoted by transition metals. A reaction mechanism was proposed after the isolation and structural characterization of the key intermediate nickelacycle. These results show that nickel complexes are more reactive than the iron ones toward the insertion of alkynes but that the latter do not initiate the trimerization of alkynes, making the insertion of activated alkynes possible. This work also demonstrates that a catalytic amount of nickel works as well as a stoichiometric amount of nickel in the presence of excess FeCl3 for the reactions. Such a catalytic reaction may shed some light on the development of zirconocene-based catalytic reactions.