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.