Half-Cleavage of a Zirconacyclopentadiene

1. Half-Cleavage of a Zirconacyclopentadiene Matthias Zeller a aYoungstown State University, Department of Chemistry, 1 University Plaza, Youngstown, Ohio 44555-3663, USA, e-mail: mzeller@cc.ysu.edu . Introduction: Zirconacyclopentadienes which can be easily prepared from two alkynes are useful intermediates for organic synthesis. Various metallacyclopentadienes containing main group elements such as Ge and Sn have been prepared by the direct reaction of zirconacyclopentadienes with metal halides such as GeCl4 and SnCl4. However, the direct reaction with Silicon halides does not give silacyclopentadienes derivatives. In order to synthesize these from the zirconacyclopentadienes, diiododienes are prepared by the iodination of the zirconacyclopentadienes and then the reaction of their lithiated dienes with silylhalides provides the silacyclopentadienes. To achieve full conversion to the diiodinated diienes either a large excess of iodine and long reaction times are used or the reactions are catalyzed with copper(I) chloride. 1,2 With no catalysis or only two equivalents of I2 the monoiodides are isolated as the major product after workup. 1 Vinylzirconocenes, the intermediates of this reaction, are itself highly valuable starting materials for the selective CC bond formation as e.g. nickel catalyzed cyclizations and multicomponent couplings 3 or in the synthesis of -silyl substituted ,-unsubstituted ketones.4While this compounds are generally synthesized via hydrozirconation or carbozirconation, a direct isolation of the half-cleaved zirconacyclopentadienes would offer an elegant and straightforward synthetic pathway. Results: When the sterically strained bicyclic zirconacyclopentadiene 1 is reacted with two equivalents of iodine in the presence of copper(I) chloride, the diiodinated product 3 forms in the range of 10 hours. However, when reacted only for two hours, the monoiodinated product 2 can be isolated as the only product by aqueous workup with Na2S2O3 and extraction with ether as large yellow crystals. As a solid 2 is indefinitely stable in air. Structure of 2 in the solid state. The rather unexpected stability of 1 can be partly traced back to the sterically protecting cyclohexyl backbone. Nevertheless, it can be expected, that this class of compounds are stable enough to be either isolated or to be used as reactive intermediates. They are easily synthesized in a one pot synthesis from readily available starting alkynes, and, with having two functional groups of opposite polarity, this class of compounds offers new attractive synthetic pathways for organic synthesis. • C. Xi, S. Huo, T. H. Afifi, R. Hara, T. Takahashi, Tett. Lett.1997, 38(23), 4099-4102. 2) S. Yamaguchi, R.-Z. Jin, K. Tamao, J. Organometal. Chem.1998, 559, 73-80. 3) Y. Ni, K. K. D. Amarasinghe, J. Montgomery, Org. Lett.2002, 4(10), 1743-1745. 4) A. Sun, X. Huang, Synthesis2000, 6, 775-777.