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

摘要

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 eta(2)-alkene intermediate. The X-ray-characterized [Cp2Zr(c-C3H5)(2)] eliminates cyclopropane by a beta-H abstraction reaction to generate the transient eta(2)-cyclopropene [Cp2Zr(eta(2)-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(eta(2)-alkene) bond of A follows the rate-determining beta-H abstraction reaction. DFT computations also suggest that an a-CC agostic rotamer of [Cp2Zr(c-C3H5)(2)] assists the beta-H abstraction of cyclopropane. The nature of the a-CC agostic interaction is discussed in the light of an NBO analysis.