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.本研究基于采用PBE/3ζ与M06-2X/cc-pVDZ量子化学方法、在密度泛函理论（DFT）级别下计算得到的可能反应路径的热力学与活化参数，提出了二氯二茂锆（Cp₂ZrCl₂）催化下三乙基铝（AlEt₃）介导α-烯烃环铝化反应的机理。本研究考察了可在核磁共振（NMR）时间尺度上观测到的、生成双金属Zr-Al与Zr-Zr配合物的反应步骤，上述配合物包括[Cp₂Zr(μ-Cl)CH₂CH₂AlEt₂]、[Cp₂Zr(μ-H)CH₂CH₂AlEt₂]以及[Cp₂Zr(Cl)CH₂CH₂Zr(Cl)Cp₂]。在所有可能的中间体中，锆杂环丙烷（zirconacyclopropane）被认为是对烯烃活性最高的催化活性物种，其生成过程经由二氯二茂锆与三乙基铝之间的两步配体交换，随后从二乙基二茂锆（Cp₂ZrEt₂）中消除乙烷。锆杂环丙烷与烯烃的反应生成锆杂环戊烷中间体，该中间体经二乙基氯化铝（ClAlEt₂）与三乙基铝的转金属化反应，通过2-取代二铝丁烷中间体，最终得到1-乙基-3-烷基铝杂环戊烷。