β‑H Abstraction/1,3‑CH Bond Addition as a Mechanism for the Activation of CH Bonds at Early Transition Metal Centers

This
article describes the generalization of an overlooked mechanism
for CH bond activation at early transition metal centers, namely 1,3‑CH
bond addition at an η2-alkene intermediate. The X-ray-characterized
[Cp2Zr­(c-C3H5)2] eliminates cyclo­propane by a β‑H abstraction
reaction to generate the transient η2-cyclo­propene
[Cp2Zr­(η2-c-C3H4)] intermediate A. A rapidly
cleaves the CH bond of furan and thiophene to give the furyl and thienyl
complexes [Cp2Zr­(c-C3H5)­(2-C4H3X)] (X = O, S), respectively.
Benzene is less cleanly activated. Mechanistic investigations including
kinetic studies, isotope labeling, and DFT computation of the reaction
profile all confirm that rapid stereo­specific 1,3‑CH
bond addition across the Zr­(η2-alkene) bond of A follows the rate-determining β‑H abstraction
reaction. DFT computations also suggest that an α‑CC
agostic rotamer of [Cp2Zr­(c-C3H5)2] assists the β‑H abstraction
of cyclo­propane. The nature of the α‑CC agostic
interaction is discussed in the light of an NBO analysis.

本文阐述了在早期过渡金属中心对碳氢键（CH）活化过程中被忽视的机制之泛化，即1,3-CH键在η2-烯烃中间体上的加成反应。由X射线表征的[Cp2Zr(c-C3H5)2]通过β-氢消除反应消除环丙烷，从而生成瞬态的η2-环丙烯中间体A[Cp2Zr(η2-c-C3H4)]。中间体A迅速断裂呋喃和噻吩的CH键，分别生成 furyl和thienyl复合物[Cp2Zr(c-C3H5)-(2-C4H3X)]（X = O, S）。苯的活化过程较为复杂。机制研究包括动力学研究、同位素标记以及反应路径的DFT计算，均证实了中间体A在Zr(η2-烯烃)键上发生的快速立体专一性1,3-CH键加成反应遵循速率决定性的β-氢消除反应。DFT计算还表明，[Cp2Zr(c-C3H5)2]的α-CC agostic构象有助于环丙烷的β-氢消除。在NBO分析的基础上，对α-CC agostic相互作用的本质进行了讨论。