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Published July 14, 2014 | Supplemental Material
Journal Article Open

Scope and Mechanism of Homogeneous Tantalum/Iridium Tandem Catalytic Alkane/Alkene Upgrading using Sacrificial Hydrogen Acceptors


An in-depth investigation of a dual homogeneous catalyst system for the coupling of alkanes and alkenes based on an early-/late-transition-metal pairing is reported. The system is composed of Cp*TaCl_2(alkene) for alkene dimerization and pincer-iridium hydrides for alkane/alkene transfer hydrogenation. Because there is no kinetically relevant interaction between the two catalysts, the tandem mechanism can be entirely described using the two independent catalytic cycles. The alkene dimerization mechanism is characterized by an entropically disfavored pre-equilibrium between Cp*TaCl_2(1-hexene) + 1-hexene and Cp*TaCl_2(metallacyclopentane) (ΔH° = −22(2) kcal/mol; ΔS° = −16(2) eu); thus, the overall rate of alkene dimerization is positive order in 1-hexene (exhibiting saturation kinetics), and increases only modestly with temperature. In contrast, the rate of 1-hexene/n-heptane transfer hydrogenation catalyzed by t-Bu[PCP]IrH_4 is inverse order in 1-hexene and increases substantially with temperature. Styrene has been investigated as an alternate sacrificial hydrogen acceptor. Styrene dimerization catalyzed by Cp*TaCl_2(alkene) is considerably slower than 1-hexene dimerization. The conversion of styrene/heptane mixtures by the Ta/Ir tandem system leads to three product types: styrene dimers, coupling of styrene and heptane, and heptene dimers (from heptane). Through careful control of reaction conditions, the production of heptene dimers can be favored, with up to 58% overall yield of heptane-derived products and cooperative TONs of up to 12 and 10 for Ta and Ir catalysts, respectively. There is only slight inhibition of Ir-catalyzed styrene/n-heptane transfer hydrogenation under the tandem catalysis conditions.

Additional Information

© 2014 American Chemical Society. Received: March 5, 2014. Publication Date (Web): June 17, 2014. The authors declare the following competing financial interest(s): a patent application has been filed based partially on this work. This work was funded by BP through the XC2 program. D.C.L. thanks the NSERC for a PDF.

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Supplemental Material - JKelly_NH4_NO3_CalNex2010_Supplement_R1.pdf


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