Showing papers on "Transition state published in 2014"

Unravelling the Mechanism of the Asymmetric Hydrogenation of Acetophenone by [RuX2(diphosphine)(1,2-diamine)] Catalysts

Abstract: The mechanism of catalytic hydrogenation of acetophenone by the chiral complex trans-[RuCl2{(S)-binap}{(S,S)-dpen}] and KO-t-C4H9 in propan-2-ol is revised on the basis of DFT computations carried out in dielectric continuum and the most recent experimental observations. The results of these collective studies suggest that neither a six-membered pericyclic transition state nor any multibond concerted transition states are involved. Instead, a hydride moiety is transferred in an outer-sphere manner to afford an ion-pair, and the corresponding transition state is both enantio- and rate-determining. Heterolytic dihydrogen cleavage proceeds neither by a (two-bond) concerted, four-membered transition state, nor by a (three-bond) concerted, six-membered transition state mediated by a solvent molecule. Instead, cleavage of the H-H bond is achieved via deprotonation of the η(2)-H2 ligand within a cationic Ru complex by the chiral conjugate base of (R)-1-phenylethanol. Thus, protonation of the generated (R)-1-phenylethoxide anion originates from the η(2)-H2 ligand of the cationic Ru complex and not from NH protons of a neutral Ru trans-dihydride complex, as initially suggested within the framework of a metal-ligand bifunctional mechanism. Detailed computational analysis reveals that the 16e(-) Ru amido complex [RuH{(S)-binap}{(S,S)-HN(CHPh)2NH2}] and the 18e(-) Ru alkoxo complex trans-[RuH{OCH(CH3)(R)}{(S)-binap}{(S,S)-dpen}] (R = CH3 or C6H5) are not intermediates within the catalytic cycle, but rather are off-loop species. The accelerative effect of KO-t-C4H9 is explained by the reversible formation of the potassium amidato complexes trans-[RuH2{(S)-binap}{(S,S)-N(K)H(CHPh)2NH2}] or trans-[RuH2{(S)-binap}{(S,S)-N(K)H(CHPh)2NH(K)}]. The three-dimensional (3D) cavity observed within these molecules results in a chiral pocket stabilized via several different noncovalent interactions, including neutral and ionic hydrogen bonding, cation-π interactions, and π-π stacking interactions. Cooperatively, these interactions modify the catalyst structure, in turn lowering the relative activation barrier of hydride transfer by ~1-2 kcal mol(-1) and the following H-H bond cleavage by ~10 kcal mol(-1), respectively. A combined computational study and analysis of recent experimental data of the reaction pool results in new mechanistic insight into the catalytic cycle for hydrogenation of acetophenone by Noyori's catalyst, in the presence or absence of KO-t-C4H9.

Kinetic, spectroscopic, and theoretical assessment of associative and dissociative methanol dehydration routes in zeolites.

Abstract: Mechanistic interpretations of rates and in situ IR spectra combined with density functionals that account for van der Waals interactions of intermediates and transition states within confining voids show that associative routes mediate the formation of dimethyl ether from methanol on zeolitic acids at the temperatures and pressures of practical dehydration catalysis. Methoxy-mediated dissociative routes become prevalent at higher temperatures and lower pressures, because they involve smaller transition states with higher enthalpy, but also higher entropy, than those in associative routes. These enthalpy–entropy trade-offs merely reflect the intervening role of temperature in activation free energies and the prevalence of more complex transition states at low temperatures and high pressures. This work provides a foundation for further inquiry into the contributions of H-bonded methanol and methoxy species in homologation and hydrocarbon synthesis reactions from methanol.

Implications of Transition State Confinement within Small Voids for Acid Catalysis

Abstract: The catalytic diversity of microporous aluminosilicates reflects their unique ability to confine transition states within intracrystalline voids of molecular dimensions and the number (but not the strength) of the protons that act as Bronsted acids. First-order rate constants for CH3OH conversion to dimethyl ether (DME) reflect the energy of transition states relative to those for gaseous and H-bonded CH3OH molecules; on zeolites, these constants depend exponentially on n-hexane physisorption energies for different void size and shape and proton location, indicating that van der Waals stabilization of transition states causes their different reactivity, without concomitant effects of void structure or proton location on acid strength. The dispersive contribution to adsorption enthalpies of DME, a proxy in shape and size for relevant transition states, was calculated using density functional theory and Lennard-Jones interactions on FAU, SFH, BEA, MOR, MTW, MFI, and MTT zeolites and averaged over all proton...

On the Mechanism of Bifunctional Squaramide‐Catalyzed Organocatalytic Michael Addition: A Protonated Catalyst as an Oxyanion Hole

Abstract: A joint experimental-theoretical study of a bifunctional squaramide-amine-catalyzed Michael addition reaction between 1,3-dioxo nucleophiles and nitrostyrene has been undertaken to gain insight into the nature of bifunctional organocatalytic activation For this highly stereoselective reaction, three previously proposed mechanistic scenarios for the critical C-C bond-formation step were examined Accordingly, the formation of the major stereoisomeric products is most plausible by one of the bifunctional pathways that involve electrophile activation by the protonated amine group of the catalyst However, some of the minor product isomers are also accessible through alternative reaction routes Structural analysis of transition states points to the structural invariance of certain fragments of the transition state, such as the protonated catalyst and the anionic fragment of approaching reactants Our topological analysis provides deeper insight and a more general understanding of bifunctional noncovalent organocatalysis © 2014 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim

Reactivity Descriptor in Solid Acid Catalysis: Predicting Turnover Frequencies for Propene Methylation in Zeotypes.

Abstract: Recent work has reported the discovery of metal surface catalysts by employing a descriptor-based approach, establishing a correlation between a few well-defined properties of a material and its catalytic activity. This theoretical work aims for a similar approach in solid acid catalysis, focusing on the reaction between propene and methanol catalyzed by Bronsted acidic zeotype catalysts. Experimentally, the ammonia heat of adsorption is often used as a measure of the strength of acid sites. Using periodic DFT calculations, we show that this measure can be used to establish scaling relations for the energy of intermediates and transition states, effectively describing the reactivity of the acid site. This allows us to use microkinetic modeling to predict a quantitative relation between the ammonia heat of adsorption and the rate of propene methylation from first principles. We propose that this is the first step toward descriptor-based design of solid acid catalysts.

Metal-catalyzed C-C bond cleavage in alkanes: effects of methyl substitution on transition-state structures and stability.

Abstract: Methyl substituents at C–C bonds influence hydrogenolysis rates and selectivities of acyclic and cyclic C2–C8 alkanes on Ir, Rh, Ru, and Pt catalysts. C–C cleavage transition states form via equilibrated dehydrogenation steps that replace several C–H bonds with C-metal bonds, desorb H atoms (H*) from saturated surfaces, and form λ H2(g) molecules. Activation enthalpies (ΔH⧧) and entropies (ΔS⧧) and λ values for 3C–xC cleavage are larger than for 2C–2C or 2C–1C bonds, irrespective of the composition of metal clusters or the cyclic/acyclic structure of the reactants. 3C–xC bonds cleave through α,β,γ- or α,β,γ,δ-bound transition states, as indicated by the agreement between measured activation entropies and those estimated for such structures using statistical mechanics. In contrast, less substituted C–C bonds involve α,β-bound species with each C atom bound to several surface atoms. These α,β configurations weaken C–C bonds through back-donation to antibonding orbitals, but such configurations cannot form w...

Mechanism of Cu/Pd-catalyzed decarboxylative cross-couplings: a DFT investigation.

Abstract: The reaction mechanism of decarboxylative cross-couplings of benzoates with aryl halides to give biaryls, which is cooperatively catalyzed by copper/palladium systems, was investigated with DFT methods. The geometries and energies of all starting materials, products, intermediates, and transition states of the catalytic cycle were calculated for the two model reactions of potassium 2- and 4-fluorobenzoate with bromobenzene in the presence of a catalyst system consisting of copper(I)/1,10-phenanthroline and the anionic monophosphine palladium complex [Pd(PMe3)Br]−. Several neutral and anionic pathways were compared, and a reasonable catalytic cycle was identified. The key finding is that the transmetalation has a comparably high barrier as the decarboxylation, which was previously believed to be solely rate-determining. The electronic activation energy of the transmetalation is rather reasonable, but the free energy loss in the initial Cu/Pd adduct formation is high. These results suggested that research a...

Reactivity for the Diels–Alder Reaction of Cumulenes: A Distortion-Interaction Analysis along the Reaction Pathway

Abstract: Cumulenes, including allene, ketenimine, and ketene, can be employed as dienophiles in Diels–Alder type reactions. The activation energies of a Diels–Alder reaction between cyclopentadiene and either the C═C bond or the other C═X (X = C, N, or O) bond in cumulenes have been calculated by G3B3, CBS-QB3, M06-2X, and B3LYP methods. The reactivity trend for the C═C bond in cumulenes is allene > ketenimine > ketene and that of the C═X bond in cumulenes is ketene > allene > ketenimine. Application of distortion-interaction analysis only at transition states does not give a satisfactory explanation for these reactivities. By employing distortion-interaction analysis along reaction pathways, we found that the reactivity of the C═C and C═X bond in cumulenes is controlled by both of its distortion and interaction energies. The lowest distortion energy of allene leads to its highest reactivity; the higher interaction energy results in higher activation energy of ketene than that of ketenimine. Compared with the reac...

Reaction Mechanism for the Dual Gold-Catalyzed Synthesis of Dibenzopentalene: A DFT Study

Abstract: A wide range of gold-catalyzed reactions based on a dual activation mechanism has recently been reported in the literature. Herein, we present a computational investigation of the mechanism for the formation of dibenzopentalenes from 1-ethynyl-2-(phenylethynyl)benzene. Transition states have been found, which substantiate the dual activation mechanism previously published and furthermore point towards a continuous presence of two gold moieties throughout the mechanistic cycle, an observation of high importance for all reactions in the field of dual activation. The initial activation of the diyne has been shown to proceed via an intermolecular transfer of a cationic gold catalyst from the thermodynamically preferred geminal-σ,π-acetylide complex to the active non-geminal analogue. Furthermore, the regioselectivity of a 5-endo versus a 6-endo cyclization has been addressed, and the 5-endo cyclization was found to be most favorable both thermodynamically and with regard to the activation barrier.

Diels-Alder reactions of allene with benzene and butadiene: concerted, stepwise, and ambimodal transition states.

Abstract: Multiconfigurational complete active space methods (CASSCF and CASPT2) have been used to investigate the (4 + 2) cycloadditions of allene with butadiene and with benzene. Both concerted and stepwise radical pathways were examined to determine the mechanism of the Diels–Alder reactions with an allene dienophile. Reaction with butadiene occurs via a single ambimodal transition state that can lead to either the concerted or stepwise trajectories along the potential energy surface, while reaction with benzene involves two separate transition states and favors the concerted mechanism relative to the stepwise mechanism via a diradical intermediate.

Brønsted–Evans–Polanyi and Transition State Scaling Relations of Furan Derivatives on Pd(111) and Their Relation to Those of Small Molecules

Abstract: Bronsted–Evans–Polanyi (BEP) and transition state scaling (TSS) linear free energy relations are extended to C–H, O–H, C–O and C–C bond breaking reactions occurring on the ring and the functional groups of furan (hydrofuran, dihydrofuran, trihydrofuran, and tetrahydrofuran) and furfural derivatives (e.g., furfural, furfuryl alcohol, methyl furan, etc.) on Pd(111). The relations perform statistically as well as those for small molecules reported previously. Hydrogenation/dehydrogenation reactions have smaller deviations compared to C–C and C–O bond breaking ones. This is in line with the degree of structural change during reaction and agrees with observations in previous works. We conclude that BEP relations developed for small molecules are not statistically different from those developed for furanics. A universal BEP relation is not statistically different from most of the BEP relations developed for furanics, with the exception of C–O, O–H and C–H scission reactions at the functional group, for which on...

Mechanism of Trifluoromethylation of Aryl Halides with CuCF3 and the Ortho Effect

Abstract: A combined experimental (radical clock, kinetic, Hammett) and computational (DFT, MM) study of the trifluoromethylation reaction of aryl halides with CuCF3 reveals a nonradical mechanism involving Ar–X oxidative addition to the Cu(I) center as the rate determining step. The reaction is second order, first order in each reactant with ΔG⧧ ≈ 24 kcal/mol for PhI (computed ΔG⧧ = 21.9 kcal/mol). An abrupt change in the gradient on the Hammett plot of log(kR/kH) versus σp for 11 p-RC6H4I substrates produces two correlations (ρ = +0.69 and +1.83), which is temptingly suggestive of two different reaction pathways. Only one mechanism is operational, however, as advocated by a single linear correlation with σp– (ρ = +0.91), analysis of the experimental ρ values, close similarity of the transition states varying in R and displaying clear signs of −M interactions, and excellent reproduction of the plot by DFT. The long-known yet previously uncomprehended ortho effect has been quantified, for the first time, using the ...

Mechanism of Olefin Asymmetric Hydrogenation Catalyzed by Iridium Phosphino-Oxazoline: A Pair Natural Orbital Coupled Cluster Study

Abstract: Since the development of chiral phosphino-oxazoline iridium catalysts, which hydrogenate unfunctionalized alkenes enantioselectively, the asymmetric hydrogenation of prochiral olefins has become important in the production of chiral compounds. For the last 10 years, details of the mechanism, including formal oxidation state assignment of the metal center and the nature of intermediates and transition states have been debated. Various contributions have been given from a theoretical point of view, but due to the size of the structures, these have been forced to rely on density functional theory (DFT) methods. In our investigation of the catalytic cycle, we employ both DFT and a correlated ab initio method, namely, the newly implemented domain-based local pair natural orbital coupled-cluster theory with single and double excitations and the inclusion of perturbative triples correction (DLPNO-CCSD(T)). Our results show that the most likely active paths involve the formation of an intermediate Ir(V) species. Furthermore, we have been able to predict the absolute configuration of the major products, and where comparison to experiment is possible, the results of our calculations agree with the enantiomeric excess obtained from hydrogenating five prochiral substrates. This work also shows that it is now possible to study catalytic reactions with untruncated models (having up to 88 atoms) at the CCSD(T) level of theory.

The fast C(3P) + CH3OH reaction as an efficient loss process for gas-phase interstellar methanol

Abstract: Rate constants for the C(3P) + CH3OH reaction have been measured in a continuous supersonic flow reactor over the range 50 K ≤ T ≤ 296 K. C(3P) was created by the in situ pulsed laser photolysis of CBr4, a multiphoton process which also produced some C(1D), allowing us to investigate simultaneously the low temperature kinetics of the C(1D) + CH3OH reaction. C(1D) atoms were followed by an indirect chemiluminescent tracer method in the presence of excess CH3OH. C(3P) atoms were detected by the same chemiluminescence technique and also by direct vacuum ultra-violet laser induced fluorescence (VUV LIF). Secondary measurements of product H(2S) atom formation have been undertaken allowing absolute H atom yields to be obtained by comparison with a suitable reference reaction. In parallel, statistical calculations have been performed based on ab initio calculations of the complexes, adducts and transition states (TSs) relevant to the title reaction. By comparison with the experimental H atom yields, the preferred reaction pathways could be determined, placing important constraints on the statistical calculations. The experimental and theoretical work are in excellent agreement, predicting a negative temperature dependence of the rate constant increasing from 2.2 × 10−11 cm3 molecule−1 s−1 at 296 K to 20.0 × 10−11 cm3 molecule−1 s−1 at 50 K. CH3 and HCO are found to be the major products under our experimental conditions. As this reaction is not considered in current astrochemical networks, its influence on interstellar methanol abundances is tested using a dense interstellar cloud model.

Theoretical study on the tautomerization of 1,5-diaminotetrazole (DAT).

Abstract: The tautomerization pathways and kinetics of 1,5-diaminotetrazole (DAT) have been investigated by means of second-order Moller-Plesset perturbation theory (MP2) and coupled-cluster theory, with single and double excitations including perturbative corrections for triple excitations (CCSD(T)). Five possible tautomers, namely 4-hydro-1-amino-5-imino-tetrazole (a), 2,5-diamino-tetrazole (b), 1,5-diamino-tetrazole (c), 2-hydro-1-imino-5-amino-tetrazole (d), and 2,4-dihydro-1,5-diimino-tetrazole (e) were identified. The structures of the reactants, transition states, and products along with the tautomerism pathways were optimized by the MP2 method using the 6-311G** basis set, and the energies were refined using CCSD(T)/6-311G**. The minimum-energy path (MEP) information for DAT was obtained at the CCSD(T)/6-311G**//MP2/6-311G** level of theory. Therein, reaction 2 (c → b) is an amino-shift reaction, while reaction 1 (c → a), reaction 3 (c → d), reaction 4 (a → e), and reaction 5 (d → e) are reactions of hydrogen-shift tautomerization. The calculated results show that 2,5-diaminotetrazole (b) with the minimum energy (taking c as a standard) among five tautomers, is the energetically preferred tautomer of DAT in the gas phase. In addition, the energy barrier of reaction 2 is 71.65 kcal · mol−1 in the gas phase, while reaction 1 takes place more easily with an activation barrier of 61.53 kcal · mol−1 also as compared to 63.71 kcal · mol−1 in reaction 3. Moreover, the tautomerization of reaction 4 requires the largest energy barrier of 83.29 kcal · mol−1, which is obviously bigger than reaction 5 with a value of 73.78 kcal · mol−1. Thus, the hydrogen-shift of c to a is the easiest transformation, while the tautomerization of a to e is the hardest one. Again, the rate constants of tautomerization have been obtained by TST, TST/Eckart, CVT, CVT/SCT, and CVT/ZCT methods in the range 200-2500 K, and analysis indicated that variational effects are small over the whole temperature range, while tunneling effects are significant in the lower temperature range.

Prediction of experimentally unavailable product branching ratios for biofuel combustion: The role of anharmonicity in the reaction of isobutanol with OH

Abstract: Isobutanol is a prototype biofuel, and sorting out the mechanism of its combustion is an important objective where theoretical modeling can provide information that is unavailable and not easily obtained by experiment. In the present work the rate constants and branching ratios for the hydrogen abstraction reactions from isobutanol by hydroxyl radical have been calculated using multi-path variational transition-state theory with small-curvature tunneling. We use hybrid degeneracy-corrected vibrational perturbation theory to show that it is critical to consider the anharmonicity difference of high-frequency modes between reactants and transition states. To obtain accurate rate constants, we must apply different scaling factors to the calculated harmonic vibrational frequencies at the reactants and at the transition states. The factors determining the reaction rate constants have been analyzed in detail, including variational effects, tunneling contributions, the effect of multiple reaction paths on transmi...

Isotope Effects, Dynamic Matching, and Solvent Dynamics in a Wittig Reaction. Betaines as Bypassed Intermediates

Abstract: The mechanism of the Wittig reaction of anisaldehyde with a stabilized ylide was studied by a combination of 13C kinetic isotope effects, conventional calculations, and molecular dynamics calculations in a cluster of 53 THF molecules. The isotope effects support a cycloaddition mechanism involving two sequential transition states associated with separate C–C and P–O bond formations. However, the betaine structure in between the two transition states is bypassed as an equilibrated intermediate in most trajectories. The role of the dynamics of solvent equilibration in the nature of mechanistic intermediates is discussed.

Theoretical chemical kinetic study of the H-atom abstraction reactions from aldehydes and acids by Ḣ atoms and ȮH, HȮ2, and ĊH3 radicals.

Abstract: We have performed a systematic, theoretical chemical kinetic investigation of H atom abstraction by Ḣ atoms and ȮH, HȮ2, and ĊH3 radicals from aldehydes (methanal, ethanal, propanal, and isobutanal) and acids (methanoic acid, ethanoic acid, propanoic acid, and isobutanoic acid). The geometry optimizations and frequencies of all of the species in the reaction mechanisms of the title reactions were calculated using the MP2 method and the 6-311G(d,p) basis set. The one-dimensional hindered rotor treatment for reactants and transition states and the intrinsic reaction coordinate calculations were also determined at the MP2/6-311G(d,p) level of theory. For the reactions of methanal and methanoic acid with Ḣ atoms and ȮH, HȮ2, and ĊH3 radicals, the calculated relative electronic energies were obtained with the CCSD(T)/cc-pVXZ (where X = D, T, and Q) method and were extrapolated to the complete basis set limit. The electronic energies obtained with the CCSD(T)/cc-pVTZ method were benchmarked against the CCSD(T)/CBS energies and were found to be within 1 kcal mol(-1) of one another. Thus, the energies calculated using the less expensive CCSD(T)/cc-pVTZ method were used in all of the reaction mechanisms and in calculating our high-pressure limit rate constants for the title reactions. Rate constants were calculated using conventional transition state theory with an asymmetric Eckart tunneling correction, as implemented in Variflex. Herein, we report the individual and average rate constants, on a per H atom basis, and total rate constants in the temperature range 500-2000 K. We have compared some of our rate constant results to available experimental and theoretical data, and our results are generally in good agreement.

Ene-ene-yne reactions: Activation strain analysis and the role of aromaticity

Abstract: The trend in reactivity of the thermal cycloisomerization reactions of 1,3-hexadien-5-ynes, A=B-C=D-E≡F, were explored and analyzed by using density functional theory at the M06-2X/def2-TZVPP level These reactions proceed through formally aromatic transition states to form a bent-allene intermediate with relatively high activation barriers Activation-strain analyses show that the major factor controlling this Hopf cyclization is the geometrical strain energy associated with the rotation of the terminal [A] group This rotation is necessary for achieving a favorable HOMO-LUMO overlap with the yne-moiety [F] associated with the formation of the new A-F single bond In addition, the relationship between the aromaticity of the corresponding cyclic transition states (all six-membered rings) and the computed activation barriers were analyzed The calculations also indicate that the aromatization of the bent-allene structures takes place through two consecutive 1,2-hydrogen shifts, the second one exhibiting negligible energy barriers Twisted! The barrier of Hopf cyclizations is primarily controlled by the activation strain (see figure, red) associated with twisting the terminal double bond, needed to achieve optimal HOMO-LUMO overlap and single-bond formation between ene and yne terminus (green) Substitution of a heteroatom, for example, NH, for the terminal CH

Unique Reaction Path in Heterogeneous Catalysis: The Concerted Semi-Hydrogenation of Propyne to Propene on CeO2

Abstract: Despite its ubiquity in homogeneous and enzymatic catalysis, concerted mechanisms have been overlooked for heterogeneously catalyzed reactions. The elusive nature of transition states leaves Density Functional Theory, DFT, as the only robust tool for their identification and characterization. By means of this method, we show that a concerted path takes part in the recently discovered semihydrogenation of propyne on CeO2, for which an excellent activity and selectivity have been reported. The high surface H coverage imposed by the experimental hydrogenation conditions induces site isolation and drives the reaction through a six-membered ring transition state. This unprecedented pathway accounts for many of the experimental observations, such as the unique syn-stereoselectivity, the excellent alkene selectivities, or the high temperature and large H2/alkyne ratios required.

Theoretical Study of Reaction Mechanism and Stereoselectivity of Arylmalonate Decarboxylase

Abstract: The reaction mechanism of arylmalonate decarboxylase is investigated using density functional theory calculations. This enzyme catalyzes the asymmetric decarboxylation of prochiral disubstituted malonic acids to yield the corresponding enantiopure carboxylic acids. The quantum chemical cluster approach is employed, and two different models of the active site are designed: a small one to study the mechanism and characterize the stationary points and a large one to study the enantioselectivity. The reactions of both α-methyl-α-phenylmalonate and α-methyl-α-vinylmalonate are considered, and different substrate binding modes are assessed. The calculations overall give strong support to the suggested mechanism in which decarboxylation of the substrate first takes place, followed by a stereoselective protonation by a cysteine residue. The enediolate intermediate and the transition states are stabilized by a number of hydrogen bonds that make up the dioxyanion hole, resulting in feasible energy barriers. It is f...

Linear free-energy relationship and rate study on a silylation-based kinetic resolution: mechanistic insights.

Abstract: The substituent effect of different p-substituted triphenylsilyl chlorides on silylation-based kinetic resolutions was explored. Electron-donating groups slow down the reaction rate and improve the selectivity, while electron-withdrawing groups increase the reaction rate and decrease the selectivity. Linear free-energy relationships were found correlating both selectivity factors and initial rates to the σpara Hammett parameters. A weak correlation of selectivity factors to Charton values was also observed when just alkyl substituents were employed but was nonexistent when substituents with more electronic effects were incorporated. The rate data suggest that a significant redistribution of charge occurs in the transition state, with an overall decrease in positive charge. The linear free-energy relationship derived from selectivity factors is best understood by the Hammond postulate. Early and late transition states describe the amount of substrate participation in the transition state and therefore the ...

Ionic and covalent stabilization of intermediates and transition states in catalysis by solid acids.

Abstract: Reactivity descriptors describe catalyst properties that determine the stability of kinetically relevant transition states and adsorbed intermediates. Theoretical descriptors, such as deprotonation energies (DPE), rigorously account for Bronsted acid strength for catalytic solids with known structure. Here, mechanistic interpretations of methanol dehydration turnover rates are used to assess how charge reorganization (covalency) and electrostatic interactions determine DPE and how such interactions are recovered when intermediates and transition states interact with the conjugate anion in W and Mo polyoxometalate (POM) clusters and gaseous mineral acids. Turnover rates are lower and kinetically relevant species are less stable on Mo than W POM clusters with similar acid strength, and such species are more stable on mineral acids than that predicted from W-POM DPE–reactivity trends, indicating that DPE and acid strength are essential but incomplete reactivity descriptors. Born–Haber thermochemical cycles i...

Understanding the mechanisms of unusually fast H-H, C-H, and C-C bond reductive eliminations from gold(III) complexes.

Abstract: Carbon-carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co-workers have demonstrated extremely rapid CC reductive elimination from cis-[AuPPh3 (4-F-C6 H4 )2 Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy-atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis-[Au(PPh3 )2 (4-F-C6 H4 )2 ](+) was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for HH, CH, and CC bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp(3) carbon atoms. Metal-carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis-[AuPPh3 (H)CH3 ](+) predict that at -52 °C, about 82 % of the reaction occurs by hydrogen-atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.

Roaming dynamics in ion-molecule reactions: Phase space reaction pathways and geometrical interpretation

Abstract: A model Hamiltonian for the reaction CH 4+→ CH 3+ + H, parametrized to exhibit either early or late inner transition states, is employed to investigate the dynamical characteristics of the roaming mechanism. Tight/loose transition states and conventional/roaming reaction pathways are identified in terms of time-invariant objects in phase space. These are dividing surfaces associated with normally hyperbolic invariant manifolds (NHIMs). For systems with two degrees of freedom NHIMS are unstable periodic orbits which, in conjunction with their stable and unstable manifolds, unambiguously define the (locally) non-recrossing dividing surfaces assumed in statistical theories of reaction rates. By constructing periodic orbit continuation/bifurcation diagrams for two values of the potential function parameter corresponding to late and early transition states, respectively, and using the total energy as another parameter, we dynamically assign different regions of phase space to reactants and products as well as ...

Electrostatic control of regioselectivity via ion pairing in a Au(I)-catalyzed rearrangement

Abstract: The rearrangement of 3-substituted aryl alkynyl sulfoxides catalyzed by cationic Au(I) complexes was studied with different counterions in solvents spanning a range of dielectric constants (e). Pulsed-gradient diffusion NMR experiments demonstrated strong ion pairing in low-e solvents. The regioselectivity of the reaction was insensitive to e when ion pairing was weak but increased monotonically as e was decreased in the regime of strong ion pairing. DFT calculations of putative product-determining transition states indicated that the product resulting from the more polar transition state is favored due to electrostatic stabilization in the presence of strong ion pairing.

Following Molecules through Reactive Networks: Surface Catalyzed Decomposition of Methanol on Pd(111), Pt(111), and Ni(111)

Abstract: We present a model of the surface kinetics of the dehydrogenation reaction of methanol on the Pd(111), Pt(111), and Ni(111) metal surfaces. The mechanism consists of 10 reversible dehydrogenation reactions that lead to the final products of CO and H2. The rate coefficients for each step are calculated using ab initio transition state theory that employs a new approach to obtain the symmetry factors. The potential energies and frequencies of the reagents and transition states are computed using plane wave DFT with the PW91 exchange correlation functional. The mechanism is investigated for low coverages using a global sensitivity analysis that monitors the response of a target function of the kinetics to the value of the rate coefficients. On Pd(111) and Ni(111), the reaction COH → CO + H is found to be rate limiting, and overall rates are highly dependent upon the decomposition time of the COH intermediate. Reactions at branches in the reaction network are also particularly important in the kinetics. A sto...

Asymmetric Michael Addition Using Bifunctional Bicyclic Guanidine Organocatalyst: A Theoretical Perspective

Abstract: To illustrate the general principle of asymmetric organocatalysis of chiral bicyclic guanidine, a density functional theory study was carried out to examine the catalytic mechanism, activation mode, origin of stereoselectivity of a [5,5]-bicyclic guanidine-catalyzed Michael addition of dimethyl malonate to 2-cyclopenten-1-one. Two types of bifunctional activation modes were examined: Bronsted acid and Bronsted-Lewis acid. The calculated enantioselectivity (ee), based on eight C–C bond forming transition states and their pre-transition state complexes, is in excellent accord with experimental result. The ternary pre-transition state complexes are stable species, which strongly influence the stereoselectivity. Similar to enzyme catalysis, the bicyclic guanidinium catalyst plays an essential recognition role in assembling the substrates together via hydrogen bonds, multiple C–H···O interactions (as oxyanion hole), donor–acceptor, and electrostatic interactions.