Experimental realization of catalytic CH_4 hydroxylation predicted for an iridium NNC pincer complex, demonstrating thermal, protic, and oxidant stability

A discrete, air, protic, and thermally stable (NNC)Ir(III) pincer complex was synthesized that catalytically activates the CH bond of methane in trifluoroacetic acid; functionalization using NaIO_4 and KIO_3 gives the oxy-ester

Unraveling the Wacker Oxidation Mechanisms

The mechanisms for the aqueous PdCl2 mediated olefin oxidation reaction (the Wacker process) have been studied with density functional theory, with special emphasis on determining competitive pathways that explain the product distribution's dependence on reaction conditions. Surprisingly, our results indicate that the previously suggested inner-sphere rate-determining step for this process is incompatible with the experimental observations. We describe three key steps, all with barriers between 22.7 and 23.3 kcal/mol. These results, together with literature experimental data, were used to construct a model that explains the observations in the Wacker process. We find that the rate-determining step under low [Cl^-] conditions is not hydroxypalladation as generally believed, but intermolecular isomerization after a lower-energy water-catalyzed internal nucleophilic attack. The pathway under high [Cl^-] leading to anti-addition aldehyde products is only accessible when CuCl_2 is available to selectively stabilize associative chloride exchange. The controversial switch in mechanisms is caused by both this selective stabilization from CuCl_2, and the prerequisite dissociation of Cl^- prior to internal attack. Finally, we suggest that the previously published rate expression for the Wacker process under high [Cl^-] is incomplete and should be replaced with a two-term expression, featuring one term first-order and one term second- (or higher) order in [CuCl_2]

Benzene C−H Bond Activation in Carboxylic Acids Catalyzed by O-Donor Iridium(III) Complexes: An Experimental and Density Functional Study

The mechanism of benzene C−H bond activation by [Ir(μ-acac-O,O,C^3)(acac-O,O)(OAc)]_2 (4) and [Ir(μ-acac-O,O,C^3)(acac-O,O)(TFA)]_2 (5) complexes (acac = acetylacetonato, OAc = acetate, and TFA = trifluoroacetate) was studied experimentally and theoretically. Hydrogen−deuterium (H/D) exchange between benzene and CD_(3)COOD solvent catalyzed by 4 (ΔH^‡ = 28.3 ± 1.1 kcal/mol, ΔS^‡ = 3.9 ± 3.0 cal K^(−1) mol^(−1)) results in a monotonic increase of all benzene isotopologues, suggesting that once benzene coordinates to the iridium center, there are multiple H/D exchange events prior to benzene dissociation. B3LYP density functional theory (DFT) calculations reveal that this benzene isotopologue pattern is due to a rate-determining step that involves acetate ligand dissociation and benzene coordination, which is then followed by heterolytic C−H bond cleavage to generate an iridium-phenyl intermediate. A synthesized iridium-phenyl intermediate was also shown to be competent for H/D exchange, giving similar rates to the proposed catalytic systems. This mechanism nicely explains why hydroarylation between benzene and alkenes is suppressed in the presence of acetic acid when catalyzed by [Ir(μ-acac-O,O,C^3)(acac-O,O)(acac-C^3)]_2 (3) (Matsumoto et al. J. Am. Chem. Soc. 2000, 122, 7414). Benzene H/D exchange in CF_(3)COOD solvent catalyzed by 5 (ΔH^‡ = 15.3 ± 3.5 kcal/mol, ΔS^‡ = −30.0 ± 5.1 cal K^(−1) mol^(−1)) results in significantly elevated H/D exchange rates and the formation of only a single benzene isotopologue, (C_(6)H_(5)D). DFT calculations show that this is due to a change in the rate-determining step. Now equilibrium between coordinated and uncoordinated benzene precedes a single rate-determining heterolytic C−H bond cleavage step

Oxy-functionalization of nucleophilic rhenium(I) metal carbon bonds catalyzed by selenium(IV)

We report that SeO_2 catalyzes the facile oxy-functionalization of (CO)_5Re(I)-Me^(δ−) with IO_4− to generate methanol. Mechanistic studies and DFT calculations reveal that catalysis involves methyl group transfer from Re to the electrophilic Se center followed by oxidation and subsequent reductive functionalization of the resulting CH_3Se(VI) species. Furthermore, (CO)_3Re(I)(Bpy)-R (R = ethyl, n-propyl, and aryl) complexes show analogous transfer to SeO_2 to generate the primary alcohols. This represents a new strategy for the oxy-functionalization of M−R^(δ−) polarized bonds

Heterolytic CH Activation and Catalysis by an O-Donor Iridium−Hydroxo Complex

A well-defined, O-donor ligated iridium hydroxide complex is reported that is competent for benzene CH activation and long-lived catalytic H/D exchange between benzene and water. An inverse dependence of the H/D exchange rate on added pyridine, a kinetic isotope effect (KIE) of 2.65 ± 0.56 for CH activation with 1,3,5-trideuteriobenzene, a KIE of 1.07 ± 0.24 with C_6H_6/C_6D_6, and DFT calculations are consistent with the CH activation proceeding via rate-determining benzene coordination followed by fast CH cleavage via a σ-bond-metathesis transition state

CH Activation with an O-Donor Iridium−Methoxo Complex

A thermally and air stable O-donor, iridium−methoxo complex is reported that undergoes stoichiometric, intermolecular C−H activation of benzene with co-generation of methanol and the iridium−phenyl complex

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(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