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AlCl3-Catalyzed Synthesis of Zirconacyclopentadienes from Alkynes, Cp2ZrCl2 and Mg
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1  Institute of Petrochemistry and Catalysis of Russian Academy of Sciences


One-step synthesis of cyclopentadienylzirconacyclopentadienes via the reaction of alkynes with Cp2ZrCl2 - Mg system is well established reaction. However, this procedure has not received wide synthetic application and is currently superseded by Negishi and Takahashi reagents. According to the literature the preparation of zirconacyclopentadienes from 3-hexyne needs the use of stoichiometric amounts of HgCl2 that makes this methodology unattractive for widespread use. Thus the development of a new effective method for the homo-coupling of alkynes which excludes the use of both pyrophoric organometallic compounds and salts of toxic heavy metals could contribute to a wider use of the Cp2ZrCl2 - Mg system for the preparation of diene derivatives and cyclization of enynes. We found that 10 mol % of AlCl3 significantly accelerates the cyclozirconation of alkyl-substituted alkynes giving after iodinolysis the corresponding iodine-containing homo-coupling products in a quantitative yield. Thus, the reaction of 5-decyne with one equivalent each of Cp2ZrCl2 and Mg in THF in the presence of 10 mol % of AlCl3 proceeds with complete conversion of alkyne into zirconacyclopentadiene at room temperature in less than 10 minutes. Similarly, catalytic amounts of AlCl3 accelerate the reaction with aryl- and silyl-substituted alkynes. At the same time, the reaction of 5-decyne with Cp2ZrCl2 and Mg in THF at room temperature without AlCl3 does not proceed even overnight. The use of catalytic amounts of Me3SiCl or InCl3 instead of AlCl3 also did not lead to the formation of the dimerization product after 5 hours at room temperature.

Keywords: Alkynes; Cyclometallation; Zirconocene; Zirconacyclopentadienes