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

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.