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

Abstract

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(<i>c</i>-C₃H₅)₂] eliminates cyclo­propane by a β‑H abstraction reaction to generate the transient η²-cyclo­propene [Cp₂Zr­(η²-<i>c</i>-C₃H₄)] intermediate <b>A</b>. <b>A</b> rapidly cleaves the CH bond of furan and thiophene to give the furyl and thienyl complexes [Cp₂Zr­(<i>c</i>-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 <b>A</b> follows the rate-determining β‑H abstraction reaction. DFT computations also suggest that an α‑CC agostic rotamer of [Cp₂Zr­(<i>c</i>-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