Zirconocene dichlorides as catalysts in alkene carbo- and cyclometalation by AlEt3: intermediate structures and dynamics

Abstract

A series of zirconocenes L₂ZrCl₂ (23 examples) were studied as catalysts in the reaction of alkenes with AlEt₃. The catalyst activity and reaction chemoselectivity were found to depend on the ligand structure in the complex and the nature of the solvent. The alkyl exchange in the triethylaluminum dimer was studied by NMR spectroscopy; a solvent effect on the alkyl exchange parameters was established. In the reaction between L₂ZrCl₂ and AlEt₃, the formation of intermediates L₂ZrEtCl, L₂Zr(μ-Cl)CH₂CH₂AlEt₂, L₂Zr(μ-H)CH₂CH₂AlEt₂, and L₂Zr(μ-Cl)CH₂CH(AlEt₂)₂ was shown; the ratio of the intermediates depends on the ligand structure in the initial complex and the solvent. The exchange in the ethanediyl bridge ZrCH₂CH₂Al, proceeding via zirconocenecyclopropane structures, was demonstrated for the first time for five-membered bimetallic complexes L₂Zr(μ-Cl)CH₂CH₂AlEt₂ with ansa-ligands (L₂ = Me₂SiCp₂, H₂CInd₂, Me₂CInd₂, Me₂SiInd₂, and C₂H₄Ind₂). Five-membered bimetallic complexes were shown to participate in the formation of cyclic organoaluminum products – 3-substituted alumolanes.