Concerted Reaction Path Research Articles

Switch From Pauli-Lowering to LUMO-Lowering Catalysis in Brønsted Acid-Catalyzed Aza-Diels-Alder Reactions.

Brønsted acid‐catalyzed inverse‐electron demand (IED) aza‐Diels‐Alder reactions between 2‐aza‐dienes and ethylene were studied using quantum chemical calculations. The computed activation energy systematically decreases as the basic sites of the diene progressively become protonated. Our activation strain and Kohn‐Sham molecular orbital analyses traced the origin of this enhanced reactivity to i) “Pauli‐lowering catalysis” for mono‐protonated 2‐aza‐dienes due to the induction of an asynchronous, but still concerted, reaction pathway that reduces the Pauli repulsion between the reactants; and ii) “LUMO‐lowering catalysis” for multi‐protonated 2‐aza‐dienes due to their highly stabilized LUMO(s) and more concerted synchronous reaction path that facilitates more efficient orbital overlaps in IED interactions. In all, we illustrate how the novel concept of “Pauli‐lowering catalysis” can be overruled by the traditional concept of “LUMO‐lowering catalysis” when the degree of LUMO stabilization is extreme as in the case of multi‐protonated 2‐aza‐dienes.

Flash Vacuum Pyrolysis (FVP) of cis‐N‐phenyl‐hexahydro‐2H‐benzo[d][1,3]oxazin‐2‐imine and Thiazin‐2‐imine Derivatives

AbstractCis‐N‐phenyl‐hexahydro‐2H‐benzo[d][1,3]oxazin‐2‐imines and cis‐N‐phenyl‐hexahydro‐2H‐benzo[d][1,3]thiazin‐2‐imines were prepared and subjected to Flash Vacuum Pyrolysis (FVP). NH− and NCH3− oxazines reacted at lower temperatures than the thio‐ analogs, showing lower thermal stability, while NCH2Ph− compounds presented the opposite behavior. Oxazines and thiazines yielded different products through homolytic fragmentations of the heterocycle moiety, the stability of which increased with the N− substitution. Based on the analysis of the reaction temperatures and the unraveling of the obtained products, it is proposed that any heterocyclic ring‐opening reactions (initiated by X3−C4, C8a−N1, or C2−X3 bond rupture) require less energy than the retro‐Diels–Alder process (RDA) that yields butadiene. This concerted reaction path is only achieved at temperatures higher than 470 °C. Kinetic measurements reveal negative entropies of activation, which suggest a partially concerted way of reaction or slightly asynchronous fragmentation processes, with higher energy of activation (Ea) values for the N‐substituted derivatives compared to the unsubstituted ones. Experimental results agree with calculations at the DFT level (B3LYP/6‐311+G(d,p)) and M06‐2X (6‐311+G(d,p)) from Gaussian09 software.

Hemiaminal route for the formation of interstellar glycine: a computational study.

Calculations related to two simple two-step paths (path-I: [Formula: see text] path-II: [Formula: see text]) for the formation of glycine have been discussed. Calculations show that at interstellar conditions these two paths are feasible only in hot cores, not in the cold interstellar clouds (cold core formation is possible only if CH2 = NH, H2O (excess) and CO of path-II, react in a concerted manner). For the laboratory synthesis of glycine, the possibility suggested is via path-I and the reaction being carried out as controlled temperature one-pot synthesis. This study can also be extended to other α-amino acids and possibly enantiomeric excess can be expected. We think this work will not only be able to enrich our future understanding about the formation of amino acids in interstellar medium but also be able to suggest alternative paths for laboratory synthesis of amino acids using either Strecker's or Miller's ingredients. Graphical abstract Using computational calculations, two different reaction paths which go through a hemiaminal (α-hydroxyamine) intermediate have been proposed. It has been proposed that the reaction [Formula: see text] is a thermodynamically favorable reaction path in the laboratory conditions, if carried out as a controlled temperature one-pot synthesis. On the hand, it has been argued that the reaction[Formula: see text] is a feasible reaction path in the interstellar conditions, if it proceeds not via the hemiaminal route, rather in a concerted reaction path.

Understanding zeolite-catalyzed benzene methylation reactions by methanol and dimethyl ether at operating conditions from first principle microkinetic modeling and experiments

In methanol-to-hydrocarbon chemistry, methanol and dimethyl ether (DME) can act as methylating agents. Therefore, we focus on the different reactivity of methanol and DME towards benzene methylation in H-ZSM-5 at operating conditions by combining first principles microkinetic modeling and experiments. Methylation reactions are known to follow either a concerted reaction path or a stepwise mechanism going through a framework-bound methoxide. By constructing a DFT based microkinetic model including the concerted and stepwise reactions, product formation rates can be calculated at conditions that closely mimic the experimentally applied conditions. Trends in measured rates are relatively well reproduced by our DFT based microkinetic model. We find that benzene methylation with DME is faster than with methanol but the difference decreases with increasing temperature. At low temperatures, the concerted mechanism dominates, however at higher temperatures and low pressures the mechanism shifts to the stepwise pathway. This transition occurs at lower temperatures for methanol than for DME, resulting in smaller reactivity differences between methanol and DME at high temperature. Our theory-experiment approach shows that the widely assumed rate law with zeroth and first order in oxygenate and hydrocarbon partial pressure is not generally applicable and depends on the applied temperature, pressure and feed composition.

Photophysical properties and excited state proton transfer in 1,8-Dihydroxydibenzo[a,h]phenazine: A theoretical study

The photophysical properties as well as proton transfer process from normal form (Enol) to tautomer form (Keto) in 1,8-Dihydroxydibenzo[a,h]phenazine [DHBP] have been theoretically investigated by TDDFT method. The potential energy profiles of the reaction paths for concerted proton transfer and stepwise proton transfer via single-proton transferred intermediate (INT) are examined, and the results show that proton transfer occurs in locally excited state following the stepwise mechanism at the lowest excitation energy in the gas phase. The concerted reaction path is found to be unstable with respect to anti-symmetric mode statically. The steady state emission band detected in Piechowska et al. experiment was assigned to INT, while emissions from Enol and Keto are less likely to be observed. The low intensity of the fluorescence at INT can be explained by the inter-system crossing between the lowest singlet state S1 with the triplet states T1 and T2 instead of the internal conversion from S1 to S0 introduced by torsion at INT structure at current computational level. This study provides new insight into the quenching mechanism in π-extended rigid molecule DHBP.

Phillip Britt wins Storch Award

Phillip F. Britt, chemical sciences division director at Oak Ridge National Laboratory, is the 2016 recipient of the Henry H. Storch Award in Fuel Science, sponsored by the ACS Division of Energy & Fuels and the Elsevier journal Fuel. The award, consisting of $5,000 and a plaque, is given annually to an individual who has made outstanding contributions to research in the field of fuel science. Britt was honored for his elucidation of new fundamental insights into the complex mix of free-radical, ionic, and concerted reaction paths of oxygen functional groups in organic energy resources. This research led to the development of improved methods for converting fossil and biomass resources into fuels and chemicals.

Double-proton transfer mechanism in 1,8-dihydroxydibenzo[a,c]phenazine: a TDDFT and ab initio study

The double-proton transfer mechanism in the newly synthesized molecule 1,8-dihydroxydibenzo[a,c]phenazine containing two hydrogen bonds is investigated by means of the time-dependent density functional theory and resolution-of-identity second-order approximate coupled-cluster methods. Both reaction paths of the concerted and the stepwise mechanisms are examined, and the results show that the double-proton transfer is likely to occur in the excited state following the stepwise mechanism in the gas phase at the lowest excitation energy. Based on the calculated potential energy scheme, the single-proton transfer and the double-proton transfer are likely to coexist, with the former is expected to occupy in a significant number. The concerted reaction path of C 2 symmetry is found to be less preferred compared with the stepwise reaction path, and also could be dynamically open due to the small activation energy.

Novel [2 + 1] Concerted Reaction Path for Disilacyclobutenes with Acetylene

Thermal reactions of benzodisilacyclobutene (1) and disilacyclobutene (2) with acetylene were investigated theoretically. The reactions are thought to proceed via the conventional Diels–Alder reaction of disilabutadiene, the conrotatory ring-opening product of disilacyclobutene, with acetylene. However, this mechanism is incompatible with the observed similar reactivities of 1 and 2 with acetylene and the retention of stereochemistry during the reaction. In our previous paper, we proposed an alternative [2 + 1] cycloaddition pathway that involved the direct addition of acetylene to the Si–Si σ–bond of 1 without ring opening. In this study, we extensively investigated the reaction pathways for both 1 and 2 on a theoretical basis. We found that charge transfer (CT) played a key role in the [2 + 1] cycloaddition pathway. On the basis of natural bond orbital (NBO) analysis, the interaction of the Si–Si σ-bond orbital (donor) with the π* orbital (acceptor) of the acetylene was attributed mainly to the CT proce...

Proton Transfers along Hydrogen Bonds in the Tautomerization of Purine

Tautomerization of purine in the water cluster was investigated by the use of DFT calculations. The correlation between the reaction paths and the number of water molecules (n) was examined. For n = 3 and n = 4, concerted reaction paths were obtained. However, for n = 5, a stepwise path including an ion pair intermediate was found with small activation energies. The n = 4 + 3 and n = 4 + 3 + 9 models were calculated to give further small activation energies, where n = 4 constitutes the reaction center and +3 and +3 + 9 denote the number of catalytic water molecules. The combination of the in-plane deprotonation at the N9 site and the out-of-plane protonation at the N7 site makes the n = 4 model probable. Three protonated n = 4 + 3 + 9 routes, a, b, and c, composed of purineH(+)(H(2)O)(4+3+9) were investigated. The n = 4 + 3 moiety is also included in the three routes, and the route c (with the N1 protonation) was found to be most favorable. The purine tautomerization was found to involve the Zundel cation in the ion pair intermediate.

Electron reorganization in allowed and forbidden pericyclic reactions: multicenter bond indices as a measure of aromaticity and/or anti‐aromaticity in transition states of pericyclic electrocyclizations

AbstractThe multicenter bond indices (MCI), recently proposed as quantitative measures of cyclic delocalization in aromatic systems, have been applied to characterize the differences in the nature of the electron reorganization in a series of allowed and forbidden electrocyclic reactions of linear neutral polyenes of general formula CnHn+2 and related charged systems of formula CnHn+2(+) and CnHn+2(−) for n ranging from 4 to 7. The proposed methodology, which is based on the monitoring of the variation of the extent of cyclic delocalization along the concerted reaction paths, is shown to be completely consistent with the empirical Evans/Dewar classification anticipating aromatic transition states for allowed and anti‐aromatic transition states for forbidden electrocyclic reactions. Although the study reports the results of the analysis of electron reorganization just for the above‐mentioned particular class of electrocyclic reactions, the proposed approach is completely general and its conclusions remain valid for any pericyclic reaction. Copyright © 2009 John Wiley & Sons, Ltd.

Electrostatic Potential Topography for Exploring Electronic Reorganizations in 1,3 Dipolar Cycloadditions

Topographical analysis of the molecular electrostatic potential (MESP) along a reaction path is employed for bringing out sequential electronic reorganizations for 1,3-dipolar cycloadditions of ethyne to fulminic acid as well as diazomethane. A simple and consistent set of rules for portraying electronic mechanisms of chemical reactions using the MESP topography is applied for this purpose. The MESP topography at each point on the concerted reaction path is associated with a classical electronic structure yielding a clear picture of the electronic reorganization along the reaction path.

Density Functional Studies on the Thermal Aryl Migration in β-Dicarbonyl Ylides and Related Compounds

The potential energy surfaces for the unimolecular rearrangement reactions of beta-dicarbonyl ylides and beta-carbonylimidoyl ylides have been studied using the density functional method. All of the stationary points were determined at the B3LYP/LANL2DZdp level of theory. Four kinds of beta-dicarbonyl ylide species containing fluorine, chlorine, bromine, and iodine have been chosen in this work as model reactants. Also, five beta-carbonylimidoyl ylide molecules bearing nitrogen, phosphorus, arsenic, stibium, and bismuth have been used in the present study. In the latter reactions, two different reaction pathways have been proposed: (1) a 1,2-aryl shift to the pnicogen element and (2) A 1,2-aryl shift to the oxygen atom. That is, path 1 is reactant-->TS-1-->Pro-1 and path 2 is reactant-->TS-2-->Pro-2. Our theoretical findings strongly suggest that all intramolecular aryl migration reactions proceed via a one-step (concerted) reaction path. For the beta-dicarbonyl ylide species, the smaller the atomic number of the halogen atom, the lower the barrier height, the larger the reaction enthalpy, and, in turn, the easier it is to undergo the intramolecular aryl migration under thermal conditions. Alternatively, the heavier the pnicogen element in the beta-carbonylimidoyl ylides, the smaller the barrier height, and the larger the migration reaction enthalpy, even under thermal conditions. The results obtained allow a number of predictions to be made.

Quantum chemical model study of the acyl migration in 2′(3′)‐formylnucleosides

AbstractThis work reports a density functional study of the mechanism of acyl migration between vicinal OH groups in monoformylated cis‐tetrahydrofuran‐3,4‐diol as a model system for the acyl migration in amino‐acylated tRNA. In addition, migration toward the ionized hydroxyl group was modeled, which simulates the process in the presence of basic reagents. The polarized continuum model (PCM) for four solvents evaluated the solvent effect on the reaction energetics. The computational results suggest that the stepwise mechanism via an orthoester intermediate is preferred by 45–48 kJ/mol over the concerted mechanism where migration of the formyl group, and the proton from the vicinal OH group occurs simultaneously via a four‐atom transition state. The conformational constrains of the THF ring could increase the energy of the transition states by ∼24 and 37 kJ/mol for the stepwise and concerted reaction paths, respectively. The calculated lowest activation energy of the reaction both in vacuum and in solvents is rather high, at 188 and 172–182 kJ/mol, respectively. These results suggest that in real systems, the process is most likely catalyzed by preliminary deprotonation of the vicinal OH group. In this case, the formyl group migration proceeds spontaneously, and the intermediate formed is 15–30 kJ/mol more stable than the initial anion of the monoformylated diol. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Reaction Paths of the Water-Assisted Neutral Hydrolysis of Ethyl Acetate

Density functional theory calculations were conducted on the title reactions with explicit inclusion of a variety of water molecules. Concerted reaction paths were examined first in the reaction model, ester(H2O)n --> MeCOOH(H2O)(n-1)EtOH, with n = 1-4. Their Gibbs activation energies are much larger than the experimental value, and the concerted paths are unfavorable. Various stepwise paths were investigated, and the ester(H2O)4 reactant gives a likely stepwise path. The n = 4 based reaction models, n = 4 + 5 and n = 4 + 12, were found to have similar proton-relay shapes with good hydrogen-bond directionality. The distinction of either the concerted or the stepwise path is described by the position of only one proton in the "junction" water molecule.

Quantum Dynamics Study of the Excited‐State Double‐Proton Transfer in 2,2′‐Bipyridyl‐3,3′‐diol

Density functional theory and quantum dynamics simulations have been used to study the double-proton transfer reaction in 2,2'-bipyridyl-3,3'-diol in the first singlet excited electronic state. This process is experimentally known to be branched: It consists of a fast, concerted reaction mechanism (tau approximately 100 fs) and a stepwise reaction mechanism [with a fast initial step (tau approximately 100 fs) and a slower final step (tau approximately 10 ps)]. Quantum dynamics simulations on a two-dimensional model reveal that the concerted reaction occurs despite the nonexistence of a concerted reaction path, but they fail to explain the relative slowness of the stepwise mechanism. A qualitative simulation using a three-dimensional model suggests that internal vibrational relaxation (IVR) might be the reason why the second stage of the stepwise mechanism is so slow.

A New Trans-to-Cis Photoisomerization Mechanism of Azobenzene on the S1(n,π*) Surface

The rotation-inversion controversy for the photoisomerization mechanism of azobenzene has been addressed on the basis of the CASSCF calculations for the excited-state relaxed surface scan along the CNN bending (inversion), the CNNC torsion (rotation), and the concerted CNN bending (concerted inversion) reaction pathways. According to our calculated results, the inversion channel involves a substantial energy barrier and a large S0−S1 energy gap so that it is a highly unfavorable channel to be considered for an efficient electronic relaxation. The rotation channel is essentially barrierless with a conical intersection close to the midpoint of the pathway to reasonably account for the subpicoseond to picosecond relaxation times observed in recent ultrafast experiments. Along the concerted inversion reaction path, a sloped conical intersection (S0/S1 CI_inv) was found to have the geometry close to a linear CNNC configuration. When the photoexcitation occurs in the S2 state, the concerted inversion channel may be open and the S0/S1 CI_inv would become energetically accessible to produce more trans-S0 isomers. A new mechanism is proposed to rationalize the early experimental results for the following two cases: (1) the trans-to-cis photoisomerization quantum yields on S2 excitation are much lower than the yields on S1 excitation; (2) the yields from both the S2 and the S1 excitations become essentially equal when the rotation channel is blocked by chemical modification or in an inclusion complex.

On the mechanism of catalytic enantioselective hetero-Diels–Alder reactions of carbonyl compounds catalyzed by chiral aluminum complexes—a concerted, step-wise or Mukaiyama–aldol pathway

The mechanism for the hetero-Diels–Alder reaction of benzaldehyde with Danishefsky's diene catalyzed by various types of achiral and chiral aluminum complexes has been studied from a theoretical point of view using semi-empirical and ab initio calculations. The uncatalyzed reaction proceeds as a concerted reaction with an unsymmetrical transition state. The catalytic reaction has been studied using first (MeO) 2AlMe, followed by ( S)-BINOL-AlMe as the catalysts, and the transition states and intermediates have been calculated for different reaction paths. The catalyst activates benzaldehyde making the carbon atom in the carbonyl functionality more electrophilic. Attempts to calculate a concerted reaction path failed. However, a two-step process, the first step being a nucleophilic attack of the activated diene to the carbonyl carbon atom, with a transition-state energy of up to 13 kcal mol −1, depending on the catalyst and calculation method used, was found to take place leading to an aldol-like local energy-minimum intermediate. The second step, the ring-closure, which has a significantly lower transition-state energy leads to the hetero-Diels–Alder adduct. The mechanistic aspects of the catalytic hetero-Diels–Alder reaction is discussed on the basis of the calculations.

Approaching a possible stepwise/concerted mechanistic crossover point in the cation radical cycloadditions of cis- and trans-anethole

The Diels–Alder cycloadditions of the cis- and trans-anethole cation radicals to cyclopenta-1,3-diene have carefully been examined and are found to produce sharply different stereochemical results. The cis-anethole cation radical adds via a distinctly stepwise mechanism, yielding comparable amounts of all four diastereoisomeric Diels–Alder adducts. In contrast, the trans-anethole cation radical yields only trans adducts, with the endo isomer predominating. None of the cis adducts could be detected, and the percentage of cis adducts formed, if any, is significantly less than 0.1%. This result stands in contrast to the additions of aryl cis- and trans-propenyl ethers to the same diene, in which both geometric alkene isomers yield all four adducts in comparable amounts. Consequently, if trans-anethole also reacts by a stepwise mechanism, the rate of cyclization in the intermediate distonic cation radical must be of the order of 1000 times the rate of bond rotation. Alternatively, although it is less likely, the trans cation radical could react via a concerted reaction path.

Abstraction of the Hydrogen Atom of Methane by Iron−Oxo Species: The Concerted Reaction Path Is Energetically More Favorable

Two kinds of H-atom abstractions from methane by iron−oxo complexes with different charges are discussed from density functional theory calculations. A concerted H-atom abstraction via a four-centered transition state is shown to be energetically more favorable than a direct H-atom abstraction via a transition state with a linear C−H−O array. Iron(IV)−oxo complexes appear to be the most effective for the cleavage of the C−H bond of alkanes in the concerted mechanism, which is rationalized from qualitative orbital interaction analyses. The results of this paper support the establishment of the two-step concerted mechanism that we have proposed for alkane hydroxylations by iron−oxo species. The proposed reaction mechanism may have relevance to our understanding of some catalytic and enzymatic processes concerning alkane hydroxylations if coordinatively unsaturated transition-metal oxides are responsible for such important chemical reactions.

Electron reorganization in chemical reactions: Pair population analysis along concerted reaction path of allowed and forbidden pericyclic reactions

Electron reorganization in chemical reactions: Pair population analysis along concerted reaction path of allowed and forbidden pericyclic reactions