Jonas Oxgaard’s research while affiliated with California Institute of Technology and other places

We use first principles quantum mechanics (density functional theory) to report a detailed reaction mechanism of the asymmetric Tsuji allylation involving prochiral nucleophiles and non-prochiral allyl fragments, which is consistent with experimental findings. The observed enantioselectivity is best explained with an inner-sphere mechanism involving the formation of a 5-coordinate Pd species that undergoes a ligand rearrangement, which is selective with regard to the prochiral faces of the intermediate enolate. Subsequent reductive elimination generates the product and a Pd0 complex. The reductive elimination occurs via an unconventional seven-centered transition state that contrasts dramatically with the standard three-centered C-C reductive elimination mechanism. Although limitations in the present theory prevent the conclusive identification of the enantioselective step, we note that three different computational schemes using different levels of theory all find that inner-sphere pathways are lower in energy than outer-sphere pathways. This result qualitatively contrasts with established allylation reaction mechanisms involving prochiral nucleophiles and prochiral allyl fragments. Energetic profiles of all reaction pathways are presented in detail.

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.

First reported in 2000,(1) the family of trans-(kappa(2)-acac-O,O)(2)Ir(R)(L) (acac = acetylacetonato, R = hydrocarbyl, L = dative ligand) O-donor complexes have been shown to be capable of activating C-H bonds and catalyze the selective, anti-Markovnikov hydroarylation of unactivated olefins with arenes.(2) These complexes are relatively simple to synthesize and remarkably, likely due to unique properties imparted by O-donor ligands, solutions are thermally stable to air and basic as well as acidic media. Mechanistic studies show that these trans bis-acac-O,O Ir(III) complexes are catalyst precursors and that the active catalysts are generated by loss of L, followed by rate determining trans to cis isomerization to generate coordinatively unsaturated, five-coordinate pseudo square pyramidal complexes with the four O's of the two acacO,O ligands in a meridional geometry that places the R group cis to an open site. 3 In an effort to design more active catalysts with similar reactivity and stability we sought to explore related metal complexes with tetradentate O-donor ligands that were already locked into meridional geometry. Recently, tetradentate diaminebis(phenols) {NN'O(2)} that enforce a meridional geometry with Ti and Zr have emerged as alternative ligands in developing new catalysts for polymerization of alpha-olefins(4) and as sulfoxidation catalysts with V.(5) In order to test whether replacing the bis-acac-O, O motifs with these ligand could lead to more efficient C-H activation and olefin arylation chemistry we synthesized several new rhodium complexes, Rh(NN'O(2))(R)(L). In this contribution we report the synthesis, structure, and reactivity of these new complexes towards arenes.

In order to understand the mechanism for selective ammoxidation of propene to acrylonitrile by bismuth molybdates, we report quantum mechanical studies (using the B3LYP flavor of density functional theory) for the various steps involved in converting the allyl-activated intermediate to acrylonitrile over molybdenum oxide (using a Mo_3O_9 cluster model) under conditions adjusted to describe both high and low partial pressures of NH_3 in the feed. We find that the rate-determining step in converting of allyl to acrylonitrile at all feed partial pressures is the second hydrogen abstraction from the nitrogen-bound allyl intermediate (Mo−NH−CH_2−CH═CH_2) to form Mo−NH═CH−CH═CH_2). We find that imido groups (Mo═NH) have two roles: (1) a direct effect on H abstraction barriers, H abstraction by an imido moiety is (~8 kcal/mol) more favorable than abstraction by an oxo moiety (Mo═O), and (2) an indirect effect, the presence of spectator imido groups decreases the H abstraction barriers by an additional ~15 kcal/mol. Therefore, at higher NH_3 pressures (which increases the number of Mo═NH groups), the second H abstraction barrier decreases significantly, in agreement with experimental observations that propene conversion is higher at higher partial pressures of NH_3. At high NH_3 pressures we find that the final hydrogen abstraction has a high barrier [ΔH‡_(fourth-ab) = 31.6 kcal/mol compared to ΔH‡_(second-ab) = 16.4 kcal/mol] due to formation of low Mo oxidation states in the final state. However, we find that reoxidizing the surface prior to the last hydrogen abstraction leads to a significant reduction of this barrier to ΔH‡_(fourth-ab) = 15.9 kcal/mol, so that this step is no longer rate determining. Therefore, we conclude that reoxidation during the reaction is necessary for facile conversion of allyl to acrylonitrile.

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.

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We report the synthesis of the pincer-cyclometalated (NNC^(t-Bu))Ir(III) dihydroxo pyridyl complex 6, which catalyzes hydrogen−deuterium (H/D) exchange between water and benzene in the presence of base (TOF = ~6 × 10^(−3) s^(−1) at 190 °C). Experimental and density functional theory (B3LYP) studies suggest that H/D exchange occurs through loss of pyridine followed by benzene coordination and C−H bond activation by a heterolytic substitution mechanism to give a phenyl aquo complex, which may dimerize. Exchange of H_2O for D_2O followed by the microscopic reverse of CH activation leads to deuterium incorporation into benzene. Synthesis of the μ-hydroxo phenyl dinuclear complex [(NNC^(t-Bu))Ir(Ph)(μ-OH)]_2 (9) also catalyzes H/D exchange with a turnover frequency (TOF = ~7 × 10^(−3) s^(−1) at 190 °C) similar to that for 6.

The photophysical properties for a series of facial (fac) cyclometalated Ir(III) complexes (fac-Ir(C∧N)3 (C∧N = 2-phenylpyridyl (ppy), 2-(4,6-difluorophenyl)pyridyl (F2ppy), 1-phenylpyrazolyl (ppz), 1-(2,4-difluorophenyl)pyrazolyl) (F2ppz), and 1-(2-(9,9′- dimethylfluorenyl))pyrazolyl (flz)), fac-Ir(C∧N)2(C∧N′) (C∧N = ppz or F2ppz and C∧N′ = ppy or F2ppy), and fac-Ir(C∧C′)3 (C∧C′ = 1-phenyl-3- methylbenzimidazolyl (pmb)) have been studied in dilute 2-methyltetrahydrofuran (2-MeTHF) solution in a temperature range of 77-378 K. Photoluminescent quantum yields (Φ) for the 10 compounds at room temperature vary between near zero and unity, whereas all emit with high efficiency at low temperature (77 K). The quantum yield for fac-Ir(ppy)3 (Φ = 0.97) is temperature- independent. For the other complexes, the temperature-dependent data indicates that the luminescent efficiency is primarily determined by thermal deactivation to a nonradiative state. Activation energies and rate constants for both radiative and nonradiative processes were obtained using a Boltzmann analysis of the temperature-dependent luminescent decay data. Activation energies to the nonradiative state are found to range between 1600 and 4800 cm-1. The pre-exponential factors for deactivation are large for complexes with C∧N ligands (1011-1013 s-1) and significantly smaller for fac-Ir(pmb)3 (109 s-1). The kinetic parameters for decay and results from density functional theory (DFT) calculations of the triplet state are consistent with a nonradiative process involving Ir-N (Ir-C for fac-Ir(pmb)3) bond rupture leading to a five-coordinate species that has triplet metal-centered (3MC) character. Linear correlations are observed between the activation energy and the energy difference calculated for the emissive and 3MC states. The energy level for the 3MC state is estimated to lie between 21 700 and 24 000 cm-1 for the fac-Ir(C∧N)3 complexes and at 28 000 cm-1 for fac-Ir(pmb)3.

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 NaIO4 and KIO3 gives the oxy-ester.