Bhalla, Gaurav and Bischof, Steven M. and Ganesh, Somesh K. and Liu, Xiang Yang and Jones, C. J. and Borzenko, Andrey and Tenn, William J., III and Ess, Daniel H. and Hashiguchi, Brian G. and Lokare, Kapil S. and Leung, Chin Hin and Oxgaard, Jonas and Goddard, William A., III and Periana, Roy A. (2011) Mechanism of efficient anti-Markovnikov olefin hydroarylation catalyzed by homogeneous Ir(III) complexes. Green Chemistry, 13 (1). pp. 69-81. ISSN 1463-9262 http://resolver.caltech.edu/CaltechAUTHORS:20110222-083820318
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The mechanism of the hydroarylation reaction between unactivated olefins (ethylene, propylene, and styrene) and benzene catalyzed by [(R)Ir(μ-acac-O,O,C^3)-(acac-O,O)_2]_2 and [R-Ir(acac-O,O)_2(L)] (R = acetylacetonato, CH_3, CH_2CH_3, Ph, or CH_2CH_2Ph, and L = H_2O or pyridine) Ir(III) complexes was studied by experimental methods. The system is selective for generating the anti-Markovnikov product of linear alkylarenes (61 : 39 for benzene + propylene and 98 : 2 for benzene + styrene). The reaction mechanism was found to follow a rate law with first-order dependence on benzene and catalyst, but a non-linear dependence on olefin. ^(13)C-labelling studies with CH_3^(13)CH_2-Ir-Py showed that reversible β-hydride elimination is facile, but unproductive, giving exclusively saturated alkylarene products. The migration of the ^(13)C-label from the α to β-positions was found to be slower than the C–H activation of benzene (and thus formation of ethane and Ph-d_5-Ir-Py). Kinetic analysis under steady state conditions gave a ratio of the rate constants for CH activation and β-hydride elimination (k_(CH): k_β) of 0.5. The comparable magnitude of these rates suggests a common rate determining transition state/intermediate, which has been shown previously with B3LYP density functional theory (DFT) calculations. Overall, the mechanism of hydroarylation proceeds through a series of pre-equilibrium dissociative steps involving rupture of the dinuclear species or the loss of L from Ph-Ir-L to the solvento, 16-electron species, Ph-Ir(acac-O,O)_2-Sol (where Sol refers to coordinated solvent). This species then undergoes trans to cis isomerization of the acetylacetonato ligand to yield the pseudo octahedral species cis-Ph-Ir-Sol, which is followed by olefin insertion (the regioselective and rate determining step), and then activation of the C–H bond of an incoming benzene to generate the product and regenerate the catalyst.
|Additional Information:||© 2011 Royal Society of Chemistry. Received 15th July 2010, Accepted 28th October 2010. We gratefully acknowledge financial support of this research by the Chevron Corporation, the University of Southern California, The Scripps Research Institute, and the Center for Catalytic Hydrocarbon Functionalization, a DOE Energy Frontier Research Center (DOE DE-SC000-1298) for financial support. We also thank Dr William Schinski for helpful discussions during the preparation of this manuscript.|
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|Deposited By:||Tony Diaz|
|Deposited On:||08 Mar 2011 23:44|
|Last Modified:||26 Dec 2012 12:57|
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