Investigations on the Reactivity of Bis(bromomagnesio)trimethylsilylmethane − Molecular Structure of a Tebbe-Type Spiro-Organomagnesium Compound

The reactivity of the 1,1-di-Grignard reagent Me3SiCH(MgBr)2 (8) towards transition metal dihalides has been investigated. While most transformations were sluggish and gave no well-defined products, the reaction of 8 with [Cp2ZrCl2] in a 1:1 ratio gave, in low yield, the novel spiro-organomagnesium compound [Cp2ZrCHSiMe3(μ-Br)]2Mg (12), which is stable at room temperature and does not disproportionate on addition of dioxane. An X-ray crystal structure determination has shown that, in the solid state, 12 is present as the C2 symmetric stereoisomer 12a; it is sterically very crowded due to the trimethylsilyl substituents. Partial metalla-alkene character is indicated by the short zirconium−carbon bonds, and agostic zirconium−hydrogen interactions by the short zirconium−hydrogen distances (Zr···H−C) and small 1JC,H coupling constants. In benzene solution, equilibrium between 12a and its C1 symmetric stereoisomer 12b was detected by 1H and 13C NMR spectroscopy. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)