We use first principles quantum mechanics (density functional theory) to report a detailed reaction mechanism of the asymmetric Tsuji allylation involving prochiral nucleophiles and nonprochiral 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 Pd(0) 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,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.

Using tetradentate, dianionic ligands, several new rhodium complexes have been prepared. Some of these diamine-bis(phenolate) compounds, are active for C-H activation of benzene. These complexes are air and thermally stable. All four complexes were characterized by X-ray diffraction.

The mechanism of benzene C−H bond activation by [Ir(μ-acac-O,O,C3)(acac-O,O)(OAc)]2 (4) and [Ir(μ-acac-O,O,C3)(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 CD3COOD 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,C3)(acac-O,O)(acac-C3)]2 (3) (Matsumoto et al. J. Am. Chem. Soc. 2000, 122, 7414). Benzene H/D exchange in CF3COOD 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, (C6H5D). 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.

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 Mo3O9 cluster model) under conditions adjusted to describe both high and low partial pressures of NH3 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 ally] intermediate (Mo-NH-CH2-CH=CH2) to form Mo-NH=CH-CH=CH2). 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 (similar to 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 similar to 15 kcal/mol, Therefore, at higher NH3 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 NH3. At high NH3 pressures we find that the final hydrogen abstraction has a high barrier [Delta H double dagger(fourth-ab) = 31.6 kcal/mol compared to Delta H double dagger(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 Delta H double dagger(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 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 (Phi) 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) (Phi = 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 (10(11)-10(13) s(-1)) and significantly smaller for fac-Ir(pmb)(3) (10(9) 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 ((3)MC) character. Linear correlations are observed between the activation energy and the energy difference calculated for the emissive and (3)MC states. The energy level for the (3)MC 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).

Recently, a paper (“Mechanism and Kinetics of the Wacker Process: A Quantum Mechanical Approach” by S. Ali Beyramabadi, Hossein Eshtiagh-Hosseini, Mohammed R. Housaindokht, and Ali Morsali) appeared in Organometallics which concludes that anti-nucleophilic attack is the rate-determining step for the Wacker process (aerobic olefin oxidation via PdCl_2 and CuCl_2 in aqueous hydrochloric acid solution) under standard conditions.1 This paper also claims that these conclusions are consistent with experimental observations. In fact, as shown below, these conclusions dramatically contradict both experimental observations and other more complete calculations. Because of the importance of the Wacker process to the chemical industry and the many controversies and inconsistencies that confused the understanding of this system, we felt it important to bring to the attention of the Organometallics community that ethylene and olefin oxidation via PdCl_2 and CuCl_2 under standard conditions (low [Cl^-] and low [CuCl_2]) almost certainly proceeds via syn-nucleophilic attack.

This is the Final Report of the five-year project Using Ionic Liquids in Selective Hydrocarbon Conversion Processes (DE-FC36-04GO14276, July 1, 2004- June 30, 2009), in which we present our major accomplishments with detailed descriptions of our experimental and theoretical efforts. Upon the successful conduction of this project, we have followed our proposed breakdown work structure completing most of the technical tasks. Finally, we have developed and demonstrated several optimized homogenously catalytic methane conversion systems involving applications of novel ionic liquids, which present much more superior performance than the Catalytica system (the best-to-date system) in terms of three times higher reaction rates and longer catalysts lifetime and much stronger resistance to water deactivation. We have developed in-depth mechanistic understandings on the complicated chemistry involved in homogenously catalytic methane oxidation as well as developed the unique yet effective experimental protocols (reactors, analytical tools and screening methodologies) for achieving a highly efficient yet economically feasible and environmentally friendly catalytic methane conversion system. The most important findings have been published, patented as well as reported to DOE in this Final Report and our 20 Quarterly Reports.

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.

We report the Synthesis of the pincer-cyclometalated (NNCt-Bu)Ir(III) dihydroxo pyridyl complex 6, which catalyzes hydrogen-deuterium (H/D) exchange between water and benzene in the presence of base (TOF = similar to 6 x 10(-3) s(-1) at 190 degrees 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 H2O for D2O followed by the microscopic reverse of CH activation leads to deuterium incorporation into benzene. Synthesis of the mu-hydroxo phenyl dinuclear complex [(NNCt-Bu)lr(Ph)(mu-OH)](2) (9) also catalyzes H/D exchange with a turnover frequency (TOF = similar to 7 x 10(-3) S-1 at 190 degrees C) similar to that for 6.

We report that SeO2 catalyzes the facile oxy-functionalization of (CO)(5)Re(I)-Me delta- with IO4- 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 CH3Se(VI) species. Furthermore, (CO)(3)Re(I)(Bpy)-R (R = ethyl, n-propyl, and aryl) complexes show analogous transfer to SeO2 to generate the primary alcohols. This represents a new strategy for the oxy-functionalization of M-R delta- polarized bonds.

We have developed a new ReaxFF reactive force field to describe accurately reactions of hydrocarbons with vanadium oxide catalysts. The ReaxFF force field parameters have been fit to a large quantum mechanics (QM) training set containing over 700 structures and energetics related to bond dissociations, angle and dihedral distortions, and reactions between hydrocarbons and vanadium oxide clusters. In addition, the training set contains charge distributions for small vanadium oxide Clusters and the stabilities of condensed-phase systems. We find that ReaxFF reproduces accurately the QM training set tor structures and energetics of small clusters. Most important is that ReaxFF describes accurately the energetics for various oxidation states of the condensed phases, including V2O5, VO2 and V2O3 in addition to metallic V (V-0). To demonstrate the capability of the ReaxFF force field for describing catalytic processes involving vanadium oxides, we performed molecular dynamics (MD) simulation for reactions of a gas of methanol exposed to the (001) surface of V2O5, We find that formaldehyde is the major product, in agreement with experiment. These studies find that water desorption from surface V-III sites is facilitated by interlayer bonding.

The mechanism for the acid-mediated substitution of a phenolic hydroxyl group with a sulfur nucleophile has been investigated by a combination of experimental and theoretical methods. We conclude that the mechanism is distinctively different in nonpolar solvents (i.e., toluene) compared with polar solvents. The cationic mechanism, proposed for the reaction in polar solvents, is not feasible and the reaction instead proceeds through a multistep mechanism in which the acid (pTsOH) mediates the proton shuffling. From DFT calculations, we found a rate-determining transition state with protonation of the hydroxyl group to generate free water and a tight ion pair between a cationic protonated naphthalene species and a tosylate anion. Kinetic experiments support this mechanism and show that, at moderate concentrations, the reaction is first order with respect to 2-naphthol, n-propanethiol, and p-toluenesulfonic acid (pTsOH). Experimentally determined activation parameters are similar to the calculated values (Delta H exp not equal =105+/-9, Delta H calcd not equal =118 kJ mol(-1); Delta G exp not equal =112+/-18, Delta G calcd not equal =142 kJ mol(-1)).

Fließender Übergang: Methanol entsteht glatt bei der hydroxidvermittelten Übertragung einer nucleophilen Methylgruppe von Methyltrioxorhenium auf Osmiumtetroxid. Der Prozess läuft in basischer wässriger Lösung über einen cyclischen (2+3)-Übergangszustand ab und wurde 1H-NMR-spektroskopisch untersucht. Mögliche Reaktionswege wurden mit Dichtefunktionalmethoden verglichen. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2008/z802575_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Chelate-assisted and internal electrophilic substitution type transition states were studied using a DFT-based energy decomposition method. Interaction energies for benzene and methane C-H bond activation by (acac-O,O)(2)Ir(X) complexes (X = CH3COO and OH) were evaluated using the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA). A ratio of similar to 1.5:1 for forward to reverse charge-transfer between (acac-O,O)(2)Ir(X) and benzene or methane transition state fragments confirms "ambiphilic" bonding, the result of an interplay between the electrophilic iridium center and the internal base component. This analysis also revealed that polarization effects account for a significant amount of transition state stabilization. The energy penalty to deform reactants into their transition state geometry, distortion energy, was also used to understand the large activation energy difference between six-membered and four-membered acetate-assisted transition states and help explain why these complexes do not activate the methane C-H bond.

General Considerations: All air and water sensitive procedures were carried out either in a Vacuum Atmosphere glove box under argon, or using standard Schlenk techniques under argon. Labeled solvents, D2O and CD3CN were purchased from Cambridge Isotopes and used without further purification. CD3CN was dried over CaH2 and distilled for NMR used for compound characterizations. 2,2’-bipyridine (Lancaster Synthesis, Inc. 99+%) was sublimed at 100-120°C and stored under argon prior to use. CO5ReBr and rhenium carbonyl ,Re2(CO)10, were purchased from Strem. THF (Mallinckrodt) was refluxed over Na/benzophenone and distilled under argon before use in synthesis or for column chromatography. Basic alumina (EMD) was dried for 2 days at 100 o C in a vacuum oven, and then kept in the glove box antechamber under vacuum overnight before use. 48% aqueous HBr, GOLD LABEL SeO2 (99.9+%), methaneseleninic acid (CH3Se(O)OH, 95%), CH3MgI (Grignard, 3.0M in diethyl ether), CH3I (95.5% stabilized with Cu) and 4,4 ́-Dimethoxy-2,2 ́-bipyridine were all purchased from Aldrich, degassed and stored under argon. 13 CH3I was purchased from Cambridge Isotopes, degassed, and stored under argon. CH3I was vacuum transferred and stored over molecular sieves. GC/MS analysis was performed on a Shimadzu GC-MS QP5000 (ver. 2) equipped with cross-linked methyl silicone gum capillary column (DB5). The retention times of the products were confirmed by comparison to authentic samples. NMR spectra were obtained on a Varian Mercury-400 MHz and a Bruker 400MHz spectrometer at room temperature. All chemical shifts are reported in units of ppm and referenced to the residual protonated solvent. Standard C,H,N elemental analysis was performed by Desert Analytics Laboratory in Tucson, AZ. Fast atom bombardment (FAB) high resolution mass spectra were collected by the University of California, Riverside Mass Spectrometry facility. DIP-CI high resolution mass spectra were collected by the University of Florida, Gainesville, Florida Mass Spectrometry facility. IR Spectra were recorded on a Perkin-Elmer Spectrum One FTIR Spectrometer.