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.