Mechanism of Cp₂ZrCl₂‑Catalyzed Olefin Cycloalumination with AlEt₃: Quantum Chemical Approach

A mechanism of α-olefin cycloalumination by AlEt₃ in the presence of Cp₂ZrCl₂ catalyst was proposed, based on theoretical estimation of the thermodynamic and activation parameters of the possible reaction pathways calculated at the DFT level using the PBE/3ζ and M06-2X/cc-pVDZ quantum chemical methods. The reaction steps to bimetallic Zr,Al- and Zr,Zr-complexes, [Cp₂Zr­(μ-Cl)­CH₂CH₂­AlEt₂], [Cp₂Zr­(μ-H)­CH₂CH₂­AlEt₂], and [Cp₂Zr­(Cl)­CH₂CH₂­Zr­(Cl)­Cp₂], observable on the NMR time scale were considered. Among the possible intermediates, zirconacyclopropane was proposed as the catalyst species most active toward the alkenes, whose formation goes via two-step ligand exchange between Cp₂ZrCl₂ and AlEt₃, and subsequent ethane elimination from the Cp₂ZrEt₂. The reaction of zirconacyclopropane with the alkene gives the zirconacyclopentane intermediate, in which transmetalation by ClAlEt₂ and AlEt₃ gives 1-ethyl-3-alkyl­alumolanes via 2-substituted dialuminobutane.