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

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A mechanism of α-olefin cycloalumination by AlEt 3 in the presence of Cp 2 ZrCl 2 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, [Cp 2 Zr(μ-Cl)CH 2 CH 2 AlEt 2 ], [Cp 2 Zr(μ-H)CH 2 CH 2 AlEt 2 ], and [Cp 2 Zr(Cl)CH 2 CH 2 Zr(Cl)Cp 2 ], 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 Cp 2 ZrCl 2 and AlEt 3, and subsequent ethane elimination from the Cp 2 ZrEt 2 . The reaction of zirconacyclopropane with the alkene gives the zirconacyclopentane intermediate, in which transmetalation by ClAlEt 2 and AlEt 3 gives 1-ethyl-3-alkylalumolanes via 2-substituted dialuminobutane.