Multimetallic Zirconocene-Based Catalysis: Alkyne Dimerization and Cyclotrimerization Reactions

Abstract

Systematic exploration of zirconocene (Cp2ZrII) chemistry since the 1980s has offered a wide range of useful methods for organic synthesis. Many of these reactions are stoichiometric in zirconium, and even if several catalytic reactions are known, the development of catalytic zirconocene chemistry is of continuing interest. In this paper, the study of reactions catalytic in zirconium, i.e. alkyne dimerization and cyclotrimerization, is presented. These reactions are carried out by employing the recently introduced lanthanide-originated zirconocene equivalent. The use of 10 mol % of Cp2ZrCl2, together with La metal or mischmetall as reductant and AlCl3 as transmetalating agent, leads to an efficient formation of dienes from disubstituted alkynes. Under the same conditions, monosubstituted alkynes underwent cyclotrimerization reactions to afford benzene derivatives. This reaction also occurs efficiently in the absence of AlCl3. It was postulated that the catalytic cyclotrimerization did not proceed through a typical insertion of the alkyne into the zirconacyclopentadiene intermediate. This unprecedented reaction takes place in the presence of zirconocene and lanthanide species, possibly involving a bimetallic polarization process.