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

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posted on 17.12.2015 by Yimu Hu, Nuria Romero, Chiara Dinoi, Laure Vendier, Sonia Mallet-Ladeira, John E. McGrady, Abel Locati, Feliu Maseras, Michel Etienne
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.