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 sigma-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.