C(sp3)–H Oxidative Addition at Tantalocene Hydrides

Although late transition metals are well-known to activate alkane C–H bonds through oxidative addition, this mechanistic step is atypical for early transition metals. Instead, prior examples of intermolecular C(sp3)–H activation at early transition metals tend to proceed through σ-bond metathesis or 1,2-addition mechanisms. Recent theoretical work suggested that tantalocenes may be capable of activating aliphatic C–H bonds by oxidative addition. Herein, we demonstrate that monoalkyl-substituted tantalocenes RCp2TaH3 undergo H/D exchange on the alkyl substituent “R” in the presence of C6D6, indicating that intramolecular C(sp3)–H activation takes place. Moreover, Cp2TaH3 was found to catalyze H/D exchange between H2 and octane-d18 and methylcyclohexane-d14, an indication of the involvement of an intermolecular C(sp3)–H activation step. Density functional theory calculations support C(sp3)–H oxidative addition at transient Ta(III), a mechanistic step that has not been previously seen for intermolecular activation of alkanes at tantalum.