β‑H Abstraction/1,3‑CH Bond Addition as a Mechanism for the Activation of CH Bonds at Early Transition Metal Centers
2015-12-17T06:32:06Z (GMT) by
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.