Concerted Path Research Articles

Kinetics and Mechanism of the Thermal Isomerization of Cyclopropane to Propene: A Comprehensive Theoretical Study.

The kinetics of the homogeneous gas-phase thermal isomerization of cyclopropane to propene has been studied theoretically to clarify existing discrepancies regarding the interpretation of its mechanism. High-level ab initio and density functional theory calculations were used to determine the branching ratios of the biradical and carbene reaction channels over wide temperature and pressure ranges. For this, relevant molecular and thermochemical properties of the proposed intermediates and related transition states were computed and compared with literature values. The Arrhenius equation, derived between 400 and 1400 K in the high-pressure limit at the CCSD(T)/6-311++G(3df,3pd)//CCSD/6-311++G(d,p) level of theory, is given by log10(koverall,∞/s-1) = (15.60 ± 0.06) - (65.70 ± 0.17) kcal mol-1 (2.303 RT)-1. This expression is in very good agreement with the available experimental data. According to these results, the biradical pathway is the predominant mechanism, while the carbene pathway contributes 1-2% at higher temperatures. The G4//B3LYP/6-311++G(3df,3pd) and G4//M06-L/6-311++G(3df,3pd) levels showed comparable Arrhenius parameters. Low-pressure limit rate coefficients and falloff curves were also estimated to evaluate the effect of pressure on the reaction. Additionally, the possibility of a concerted path is considered, but calculations showed unstable wave functions, suggesting that this mechanism would not be plausible.

Syn vs. Anti? What Controls Addition Stereochemistry in Chlorolactonization.

The stereochemistry of the uncatalyzed chlorolactonization of 4-phenylpent-4-enoic acid at room temperature was examined to probe the reaction's intrinsic diastereoselectivities as a function of chlorenium ion donor, solvent polarity, and reactant concentration ranges. Kinetic studies using Variable Time Normalization Analysis (VTNA) revealed differing reaction orders for the syn and anti alkene addition processes. Aided and illustrated by quantum chemical modeling, this detailed mechanistic analysis of the substrate's intrinsic chlorolactonization reactions points to concerted AdE3-type paths for both syn and anti additions. By illuminating the factors selecting for syn- vs anti-addition paths, the results provide key reference points for future studies of stereocontrol in halofunctionalization reactions.

Concise Total Syntheses of (-)-Crinipellins A and B Enabled by a Controlled Cargill Rearrangement.

Herein, we report concise total syntheses of diterpene natural products (-)-crinipellins A and B with a tetraquinane skeleton, three adjacent all-carbon quaternary centers, and multiple oxygenated and labile functional groups. Our synthesis features a convergent Kozikowski β-alkylation to unite two readily available building blocks with all the required carbon atoms, an intramolecular photochemical [2 + 2] cycloaddition to install three challenging and adjacent all-carbon quaternary centers and a 5-6-4-5 tetracyclic skeleton, and a controlled Cargill rearrangement to rearrange the 5-6-4-5 tetracyclic skeleton to the desired tetraquinane skeleton. These strategically enabling transformations allowed us to complete total syntheses of (-)-crinipellins A and B in 12 and 13 steps, respectively. The results of quantum chemical computations revealed that the Bronsted acid-catalyzed Cargill rearrangements likely involve stepwise paths to products and the AlR3-catalyzed Cargill rearrangements likely involve a concerted path with asynchronous alkyl shifting events to form the desired product.

The photochemical trans → cis and thermal cis → trans isomerization pathways of azobenzo-13-crown ether: A computational study on a strained cyclic azobenzene system.

The isomerization of azobenzo-13-crown ether can be expected to be hindered due to the polyoxyethylene linkage connecting the 2,2'-positions of azobenzene. The mixed reference spin-flip time-dependent density functional theory results reveal that the planar and rotational minima of the first photo-excited singlet state (S1) of the trans-isomer pass through a barrier (2.5-5.0 kcal/mol) as it goes toward the torsional conical intersection (S0/S1) geometry (<CNNC ≈ 98°), which is responsible for the cis isomer formation. The second excited singlet state (S2) of the trans form has a nearly planar minimum along the N-N stretching mode, which approaches a sloped S2/S1 intersection geometry. This excited state has a rotational minimum (<CNNC ≈ 99°) as well. Both these minima have a characteristic S2-S1 energy gap of 9 kcal/mol and may undergo internal conversion. A comparison of this system with an analogous 2,2'-dimethoxy-substituted azobenzene system reveals less strain in the rotational path of the latter on the S1 surface, indicating the possibility of its better trans → cis yield than the azocrown. The completely planar S2 geometry of the dimethoxy system has easy access to the linear concerted inversion path, which seems to be the reason behind its reported slightly lower π-π*(S2) yield than n-π*(S1). The thermal cis → trans isomerization path of the azobenzo-13-crown passes through a transition state (frequency 453i cm-1), which corresponds to Gibbs free energy of activation value of 26 kcal/mol in the gas-phase and isooctane. Our study also confirms that its trans isomer strongly binds Li+ among the alkali metal ions, and this observation may open up possibilities for practical applications of this azobenzo-crown.

Unexplored Isomerization Pathways of Azobis(benzo-15-crown-5): Computational Studies on a Butterfly Crown Ether.

Computational studies on trans → cis and cis → trans isomerizations of photoresponsive azobis(benzo-15-crown-5) have been reported in this work. The photoexcited ππ* state (S2) of the trans isomer relaxes through the planar S2 minimum and the planar S2/S1 conical intersection (both situated around 9 kcal/mol below the vertically excited S2 state) arising along the N═N stretching coordinate. The nπ* state (S1) of this isomer has both planar and rotated (clockwise and anticlockwise) minima, which may lead to a torsional conical intersection (S0/S1) geometry having a <CNNC dihedral angle value close to 90°. This rotational isomerization path is found to have an energy barrier. It has been noticed that an N-N-out-of-plane motion coupled with the torsion can bring down the barrier and may facilitate the isomerization process. On the other hand, the vertically excited S1 state of the cis-isomer undergoes a barrierless path to reach a torsional conical intersection (S0/S1) geometry (<CNNC = 88°), responsible for the trans-isomer formation. The cis-S2/S1 CI (conical intersection) torsional geometry (<CNNC = 45°) is situated around 16 kcal/mol above the vertically excited ππ* state of the cis-isomer and not reachable on S2 photoexcitation. The two optimized torsional S2 minima of this system are close to the S1 surface and seem to be involved in the S2-S1 internal conversion. A less efficient concerted inversion photoisomerization path through a linear geometry has also been identified. The thermal cis → trans isomerization has been found to undergo an inversion motion, which forms a less stable trans isomer. Possible differences between the isomerization channels of this azobis(benzo-15-crown-5) and the unsubstituted azobenzene system have also been highlighted in the work.

Mechanism insight into ethylene oligomerization on zeolite K-LTA surface: A DFT and kMC study

The regeneration gas used in the K-LTA zeolite dryer of the MTO plant is planning to be changed from high purity nitrogen to the top gas of the demethanizer tower with high ethylene content, which is helpful to energy saving and emission reduction. However, there is a risk of ethylene oligomerization and carbon deposition when the regeneration gas passes through the K-LTA zeolite. The reaction mechanism of ethylene oligomerization on the K-LTA zeolite surface must be clarified. DFT calculations showed that ethylene could be physically adsorbed on Si–OH, Al–OH, the incomplete α- and β-cage on [100]-D4R-e surface. Butoxide forms through ethylene dimerization by stepwise and concerted mechanisms first. Then the reaction of ethylene and butoxide to n-hexyloxide leads to the continued growth of the carbon chain. The oligomerization reaction barriers of rate-determining steps are between 1.65∼1.75 eV, which is more difficult to occur than other zeolites. The ethylene oligomerization reaction can be divided into stepwise polymerization path, concerted polymerization path and concerted butene path. kMC simulation found that the dominant reaction paths are different under different temperatures. When the regeneration temperature reaches 473 K, trace n-butene is generated. At 513 K, ethylene oligommerization occurs and carbon deposition is formed. The results put forward higher requirements for strictly controlling the temperature of the dryer to ensure the excellent performance of K-LTA zeolite.

Unveiling the correlation between the catalytic efficiency and acidity of a metal-free catalyst in a hydrogenation reaction. A theoretical case study of the hydrogenation of ethene catalyzed by a superacid arising from a superhalogen.

A systematic quantum-chemical study of the hydrogenation of ethene, catalyzed by strong acids HX (X = F, Cl, Br) and superacids HA (A = MgX3, Mg2X5; X = F, Cl, Br) arising from octet superhalogens is explored. Two possible paths are proposed, concerted and stepwise, and the calculated results show that the concerted path is more favorable than the stepwise path. Compared to the hydrogenation reaction without any catalyst, the improvement of the catalytic efficiency of the superacid HA (A = MgX3, Mg2X5) is high, up to 38.8 to 59.4%. Compared to the strong acid HX (X = F, Cl, Br), the barrier energy is significantly reduced and the improvement of the catalytic efficiency of the superacid HA reaches 23.1 to 31.7%. In particular, for HMg2Br5, the barrier energy of the hydrogenation of ethene is only 36 kcal mol-1, which shows that the reaction could proceed under experimental conditions. In addition, the results show that the catalytic efficiency of the superacid HA is related to the acidity of the superacid. In general, the greater the acidity, the lower the barrier energy and the easier the hydrogenation reaction. From the analysis of the bond order, the newly formed C-H bond of the transition state (TS3) in the concerted path, in which the H atom comes from the superacid catalyst, directly affects the barrier energy of the entire reaction. For the more acidic catalyst, this H atom is provided more easily, and then the formed C-H bond in the transition state is stronger. Consequently, this stronger bond leads to a more stable transition state, and hence to a lower energy barrier as well as a higher efficiency of the superacid catalyst. Therefore, a positive correlation between the acidity of the metal-free catalyst and its catalytic efficiency is expected in the hydrogenation reaction.

Application of Multiconfiguration Pair-Density Functional Theory to the Diels-Alder Reaction.

Transition states for Diels-Alder reactions are strongly correlated, as evidenced by high-to-very-high M diagnostics, and therefore they require treatment by multireference methods. Multiconfiguration pair-density functional theory (MC-PDFT) combines a multiconfiguration wave function with a functional of the electron density and the on-top pair density to calculate the electronic energy for strongly correlated systems at a much lower cost than wave function methods that do not employ density functionals. Here we apply MC-PDFT to the Diels-Alder cycloaddition reaction of 1,3-butadiene with ethylene, where two kinds of reaction paths have been widely studied: concerted synchronous paths and diradical stepwise paths. The lowest-energy reaction path is now known to be a concerted synchronous one, and a method's ability to predict this is an important test. By comparison to the best available theoretical results in the literature, we test the accuracy of MC-PDFT with several choices of on-top functional for geometries and enthalpies of stable structures along both paths and for the transition state geometries. We also calculate the Arrhenius activation energies for both paths and compare these to experiment. We also compare to Kohn-Sham density functional theory (KS-DFT) with selected exchange-correlation functionals. CAS-PDFT gives consistently good energies and geometries for both the concerted and stepwise mechanisms, but none of the KS-DFT functionals gives accurate activation energies for both. The stepwise transition state is very strongly correlated, and MC-PDFT can treat it, but KS-DFT (which involves a single-configuration treatment) has larger errors. The results confirm that using a multiconfigurational reference function for strongly correlated transition states can significantly improve the reliability and that MC-PDFT can provide good accuracy at a much lower computational cost than competing multireference methods.

An Overlooked Pathway in 1,3-Dipolar Cycloadditions of Diazoalkanes with Enamines.

Methyl diazoacetate reacts with 1‐(N‐pyrrolidino)cycloalkenes to give products of 1,3‐dipolar cycloadditions and azo couplings. The kinetics and mechanisms of these reactions were investigated by NMR spectroscopy and DFT calculations. Orthogonal π‐systems in the 1,3‐dipoles of the propargyl‐allenyl type allow for two separate reaction pathways for the (3+2)‐cycloadditions. The commonly considered concerted pathway is rationalized by the interaction of the enamine HOMO with LUMO+1, the lowest unoccupied orbital of the heteropropargyl anion fragment of methyl diazoacetate. We show that HOMO/LUMO(π*N=N) interactions between enamines and methyl diazoacetate open a previously unrecognized reaction path for stepwise cycloadditions through zwitterionic intermediates with barriers approximately 40 kJ mol−1 lower in energy in CHCl3 (DFT calculations) than for the concerted path.

Computational Screening and Experimental Synthesis of Doped TiO2 for Propane Dehydrogenation

Low-cost, environmentally friendly, and highly efficient catalysts are urgently needed for the direct propane dehydrogenation (PDH) to propylene. Studies have shown that defective TiO2 obtained by in situ reduction shows excellent catalytic PDH performance. Doping metals in the lattice of TiO2 is a plausible method for tuning the vacancy content and electronic property. In this paper, doping with fifth period transition metals was screened using a computational method, followed by experimental synthesis to further improve the PDH activity of TiO2-based catalysts. The TiO2 (101) surface with doping metals was first studied to determine the positive effect on the vacancy formation. Then, the two disputed reaction paths, that is, the concerted and stepwise path, were compared extensively on all of the doping models. Step one is found to be more preferable for all. The microkinetic modeling was employed to obtain the turnover frequency (TOF), based on which the linear relationship is found for TOF and binding energy of dissociated H2 or co-adsorption energy of 2-propyl&H. The abovementioned screening process shows Ru-doped TiO2, which shows the highest PDH activity and is justified by experimental results.

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.

Hydrogenations of Isocyanic Acid: A Computational Study on Four Possible Concerted Paths for Formamide Formation

Interstellar formations and significances of formamide as a possible prebiotic precursor, for many complex organic molecules of life are well studied in recent times. In this work, computational studies (B3LYP, wB97xD, MP2, and CCSD(T)) have been carried out to study the formation mechanism of formamide, from HNCO + H2, via concerted paths. Beside the well-studied, Direct Route, three new routes, imine, carbene and Oxime intermediate routes have been investigated. Carbene and oxime routes were found to be endothermic and hence may not be useful in interstellar circumstances. On the other hand, imine route, being slightly exothermic, may be considered as an auxiliary channel to the direct route. Detailed studies for these two routes have been carried out. Also, based only on a qualitative analysis of possibility of tunnelling, we have suggested the possible usefulness of these two reaction paths in cryogenic conditions of interstellar molecular clouds. We hope this study will be useful to our future understanding about the astrochemistry of formamide and origin of life.

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.

A density functional theory study of the synthesis of 3-phospholene oxide through the McCormack cycloaddition reaction

The [4 + 2] cycloaddition reaction (McCormack reaction) of 1,3-butadiene with dichloromethylphosphine (DCMP) yielding 3-phospholene oxide has been studied by density functional theory (DFT) at the M06-2x/6-311++G(2d,2p) computational level. This reaction is a cheletropic process that comprises two steps with the formation an ionic intermediate in the first step and subsequently, its hydrolysis, in the second step. For the first step of this reaction, three paths were considered as a stepwise pathway with a diradical intermediate and an assumed path with a trigonal intermediate that compared with the concerted path in terms of energy. Analysis of the reactivity indices showed that the 1,3-butadiene and DCMP behaved as nucleophile and electrophile, respectively.

Molecular Organization of Reagents in the Kinetics and Catalysis of Liquid-Phase Reactions: XIII. Cyclic Transition States Involving Solvent Molecules in the Mechanism of Aminolysis of Cyclocarbonates in an Alcohol Medium

Trends in the influence of a proton-donor solvent on the reaction of aminolysis of ethylene carbonate, which proceeds through concerted and stepwise mechanisms, are studied. The study is based on kinetic data of model reactions in alcohol solutions and results of DFT quantum chemical calculations of reaction paths that involve hydrogen-bonded transition states and intermediates containing different numbers of solvent molecules in the proton-transfer and stabilization cycles. The calculated data on the activation energies of the reaction, which includes cycles of different sizes, indicate that the stepwise mechanism is preferable compared to the concerted one when the reaction proceeds in methanol. The activation barrier of the stepwise path of the reaction is lower by 2.9 kcal/mol than that for the concerted path. Only the proton-transfer cycle is essential for the progress of the reaction, and the presence of a stabilization cycle does not lead to a decrease in the activation barrier.

N-Heterocyclic Carbene Organocatalysis: With or Without Carbenes?

In this work the mechanism of the aldehyde umpolung reactions, catalyzed by azolium cations in the presence of bases, was studied through computational methods. Next to the mechanism established by Breslow in the 1950s that takes effect through the formation of a free carbene, we have suggested that these processes can follow a concerted asynchronous path, in which the azolium cation directly reacts with the substrate, avoiding the formation of the carbene intermediate. We hereby show that substituting the azolium cation, and varying the base or the substrate do not affect the preference for the concerted reaction mechanism. The concerted path was found to exhibit low barriers also for the reactions of thiamine with model substrates, showing that this path might have biological relevance. The dominance of the concerted mechanism can be explained through the specific structure of the key transition state, avoiding the liberation of the highly reactive, and thus unstable carbene lone pair, whereas activating the substrate through hydrogen‐bonding interactions. Polar and hydrogen‐bonding solvents, as well as the presence of the counterions of the azolium salts facilitate the reaction through carbenes, bringing the barriers of the two reaction mechanisms closer, in many cases making the concerted path less favorable. Thus, our data show that by choosing the exact components in a reaction, the mechanism can be switched to occur with or without carbenes.

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.

Force-Induced Dissolution of Imaginary Mode in Mechanochemical Reaction: Dibenzophenazine Synthesis

Dibenzophenazine was recently synthesized mechanochemically at a high yield. On the basis of experimental kinetics and theoretical vibronic coupling density analysis, this efficient synthesis has been expected to involve two types of reaction mechanisms. In this study, the reaction pathways, including the catalytic effect of water, are determined using density functional theory. The lowest-energy path involves stepwise formation of two C–N bonds, and a concerted path with a higher barrier is also found. The highest-energy point along the latter path is not a transition state but a higher-order saddle point. Therefore, the concerted path is not generally accepted as a reaction path. However, even a small mechanical force dissolves imaginary frequencies so that the higher-order saddle point is changed to a transition state. This mechanical dissolution of imaginary frequencies is a new feature of theoretical mechanochemistry.

Theoretical insights into the effect of solvents on the [4 + 2] cycloaddition of singlet oxygen to substituted anthracenes: A change from a stepwise process to a concerted process

AbstractThe [4 + 2] cycloadditions of singlet oxygen to 9,10‐diphenylanthracene (1) and the meta and para isomers of 9,10‐dipyridylanthracene (2m/p) and 9,10‐methoxyphenylanthracene (3m/p) have been studied by density functional calculations in the gas phase at the UB3LYP/6‐31G* level and for the first time in solvents at the conductor‐like polarizable continuum model (CPCM) UM062X/6‐31G* level. The differences in calculated transition state (TS) energies derived from this method are in line with experimentally observed reactivity orders in solution. For the gas‐phase reaction, the first TS of the stepwise pathway (TS1) has biradical character, and its energy lies below the energy of the TS of the concerted path (TSconc). In contrast, in the solvent acetonitrile, TS1 resembles a zwitterion and lies significantly higher than the TSconc. Thus, a concerted mechanism applies in solvents, and the energy gap between the TS of the two processes decreases with decreasing polarity. A change from a pyridyl against a methoxyphenyl substituent in the para position causes a maximal reduction of the activation barrier by approximately 1.7 kcal/mol, resulting in a fivefold increased reactivity.

A reaction force perspective of a model amide bond formation reaction

A reaction force approach of amide bond formation between ammonia and formic acid is demonstrated along with each atom’s contribution to structural changes and electronic reordering of the chemical reaction. The B3LYP/6-31G(d,p) level of density functional theory based calculations were carried out to explore transition states (TSs) of stepwise and concerted amide bond formation reaction pathways. Various stages that characterize structural and electronic properties in the progress of reaction are identified from reaction force calculations on energetics of geometries obtained from intrinsic reaction coordinate (IRC) pathways. The reaction works in preparative region of the pathway are less than that in relaxation stage, reflecting favorable formation of product geometries (Formamide and water). More than 80% of the activation energy comes from structural changes in preparative region. A comparison of reaction force profiles and reaction works in preparative region of TS1 and TS2 of stepwise path and TS3 of concerted path discloses that both the stepwise and concerted mechanisms are equally competent in terms of kinetic feasibility. The atomic resolution of pathways unveils that, hydrogen and oxygen forming the water molecule contribute significantly to the negative reaction energy of product geometries. The preferential contribution of the atoms comes from enhanced negative charge of the oxygen and positive charge on hydrogen in the progress of reaction.