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Issue 1, 2011
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Mechanism of efficient anti-Markovnikov olefin hydroarylation catalyzed by homogeneous Ir(III) complexes

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Abstract

The mechanism of the hydroarylation reaction between unactivated olefins (ethylene, propylene, and styrene) and benzene catalyzed by [(R)Ir(μ-acac-O,O,C3)-(acac-O,O)2]2 and [R-Ir(acac-O,O)2(L)] (R = acetylacetonato, CH3, CH2CH3, Ph, or CH2CH2Ph, and L = H2O 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. 13C-labelling studies with CH313CH2-Ir-Py showed that reversible β-hydride elimination is facile, but unproductive, giving exclusively saturated alkylarene products. The migration of the 13C-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-d5-Ir-Py). Kinetic analysis under steady state conditions gave a ratio of the rate constants for CH activation and β-hydride elimination (kCH: kβ) 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 cisisomerization 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.

Graphical abstract: Mechanism of efficient anti-Markovnikov olefin hydroarylation catalyzed by homogeneous Ir(iii) complexes

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Publication details

The article was received on 15 Jul 2010, accepted on 28 Oct 2010 and first published on 24 Nov 2010


Article type: Paper
DOI: 10.1039/C0GC00330A
Green Chem., 2011,13, 69-81

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    Mechanism of efficient anti-Markovnikov olefin hydroarylation catalyzed by homogeneous Ir(III) complexes

    G. Bhalla, S. M. Bischof, S. K. Ganesh, X. Y. Liu, C. J. Jones, A. Borzenko, W. J. Tenn, III, D. H. Ess, B. G. Hashiguchi, K. S. Lokare, C. H. Leung, J. Oxgaard, W. A. Goddard, III and R. A. Periana, Green Chem., 2011, 13, 69
    DOI: 10.1039/C0GC00330A

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