Rare-Earth Metallacycloheptatrienes: Synthesis, Structure, and Reactivity

As the key intermediates in metal-catalyzed [2 + 2 +
2] cycloaddition
reactions of three alkynes, main-group and transition-metal metallacycloheptatrienes
have been studied for decades, but rare-earth analogues have remained
illusive. Herein, we report the well-defined rare-earth metallacycloheptatrienes 2-Sc and 2-Lu obtained in high yields via the
reaction of Cp*Li (Cp* = pentamethylcyclopentadienyl) and RECl3 (RE = Sc, Lu) with TMEDA-coordinated 2,2′-dilithio-Z-stilbene 1. The single-crystal structure
of 2-Sc (RE = Sc) reveals it to be a separate ion pair,
and the scandacycloheptatriene ring adopts a nonplanar boat-like conformation.
The reactivity of compounds 2-Sc and 2-Lu has been investigated. When 2-Sc was treated with 2,6-dimethylphenyl
isocyanide, compound 3 was obtained via the successive
1,1-insertion and 1,2-insertion of the isocyanide. The structure of 3 was also fully characterized.