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

The mechanism of the hydroarylation reaction between unactivated olefins (ethylene, propylene, and styrene) and benzene catalyzed by [(R)Ir(mu-acac-O, O, C-3)-(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. C-13-labelling studies with (CH3CH2)-C-13-Ir-Py showed that reversible beta-hydride elimination is facile, but unproductive, giving exclusively saturated alkylarene products. The migration of the C-13-label from the alpha to beta-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 beta-hydride elimination (k(CH):k(beta)) of similar to 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.