C-H Bond Activation of Benzene by Unsaturated η~2-Cyclopropene and η~2-Benzyne Complexes of Niobium

摘要

We report the synthesis of a niobium cyclopropyl complex, Tp~(Me2)NbMe(c-C_3H_5)(MeCCMe), and show that thermal loss of methane from this compound generates an intermediate that is capable of activating both aliphatic and aromatic C-H bonds. Isotopic labeling, trapping studies, a detailed kinetic analysis, and density functional theory all suggest that the active intermediate is an η~2-cyclopropene complex formed via β-hydrogen abstraction rather than an isomeric cyclopropylidene species. C-H activation chemistry of this type represents a rather unusual reactivity pattern for η~2-alkene complexes but is favored in this case by the strain in the C_3 ring which prevents the decomposition of the key intermediate via loss of cyclopropene.