A reexamination of the molecular mechanism of the Diels–Alder reaction between tetrafluoroethene and cyclopentadiene

A New Insight into the Molecular Mechanism of the Reaction between 2-Methoxyfuran and Ethyl (Z)-3-phenyl-2-nitroprop-2-enoate: An Molecular Electron Density Theory (MEDT) Computational Study

The molecular mechanism of the reaction between 2-methoxyfuran and ethyl (Z)-3-phenyl-2-nitroprop-2-enoate was investigated using wb97xd/6-311+G(d,p)(PCM) quantum chemical calculations. It was found that the most probable reaction mechanism is fundamentally different from what was previously postulated. In particular, six possible zwitterionic intermediates were detected on the reaction pathway. Their formation is determined by the nature of local nucleophile/electrophile interactions. Additionally, the channel involving the formation of the exo-nitro Diels-Alder cycloadduct was completely ruled out. Finally, the electronic nature of the five- and six-membered nitronates as potential TACs was evaluated.

A molecular electron density theory study of asymmetric Diels-Alder [4 + 2] reaction's mechanism of furan with three substituted alkynes (5-R substituted-3-(3-(phenylsulfonyl)-propioloyl)-oxazolidin-2-one)

CONTEXT

The [4 +2 ] cycloaddition reactions between furan and three substituted alkynes (5-R-substituted-3-(3-(phenylsulfonyl)-propioloyl)-oxazolidin-2-one) have been investigated using the MEDT approach. Reactivity indices, reaction pathways, and activation energies are calculated. In an investigation of conceptual DFT indices, furan acts as a nucleophile, while the three substituted alkynes (5-R-substituted-3-(3-(phenylsulfonyl)-propioloyl)-oxazolidin-2-one) function as electrophiles in this reaction. The cycloaddition is regioiselective, as demonstrated by the activation and reaction energies, in clear agreement with the experiment's results. Hetero Diels-Alder [4 + 2] cycloadditions occur following a non-concerted two stages one-step molecular mechanism.

METHODS

For the purpose of this study, all calculations were performed using the Gaussian 09 software. Optimization was achieved through Berny's computational gradient optimization method, employing the B3LYP functional and the 6-31G(d) basis set. Analysis of both local and global reactivity indices provided insights into the reactivity tendencies of the reactants, distinguishing between electrophilic and nucleophilic characteristics via Parr functions. Frequency calculations were employed to identify and characterize stationary points, with transition states indicated by a single imaginary frequency and positive values of all frequencies for reactants and product. The electron localization function (ELF) was investigated using the Multiwfn software within the context of topological analyses.

Fully Selective Synthesis of Spirocyclic-1,2-oxazine N-Oxides via Non-Catalysed Hetero Diels-Alder Reactions with the Participation of Cyanofunctionalysed Conjugated Nitroalkenes

Hetero Diels-Alder (HDA) reactions with the participation of E-2-aryl-1-cyano-1-nitroethenes and methylenecyclopentane were evaluated on the basis of experimental as well as quantumchemical data. It was found that contrary to most known HDA reactions, title processes are realised under non-catalytic conditions and with full regiocontrol. The DFT study shows, without any doubt, the polar but single-step reaction mechanism. Deeper exploration using Bonding Evolution Theory (BET) techniques gives a clear image of the sequences of electron density reorganisation along the reaction coordinate. The first C4-C5 bond is created in phase VII by merging two monosynaptic basins, while the second O1-C6 bond is created in the last phase by a donation of the nonbonding electron density of O1 to C6. Based on the research, we can conclude that the analysed reaction proceeds according to a two-stage one-step mechanism.

On the Question of Stepwise [4+2] Cycloaddition Reactions and Their Stereochemical Aspects

Even at the end of the twentieth century, the view of the one-step [4+2] cycloaddition (Diels-Alder) reaction mechanism was widely accepted as the only possible one, regardless of the nature of the reaction components. Much has changed in the way these reactions are perceived since then. In particular, multi-step mechanisms with zwitterionic or diradical intermediates have been proposed for a number of processes. This review provided a critical analysis of such cases.

Analysis of the possibility and molecular mechanism of carbon dioxide consumption in the Diels-Alder processes

The large and significant increase in carbon dioxide concentration in the Earth’s atmosphere is a serious problem for humanity. The amount of CO2 is increasing steadily which causes a harmful greenhouse effect that damages the Earth’s climate. Therefore, one of the current trends in modern chemistry and chemical technology are issues related to its utilization. This work includes the analysis of the possibility of chemical consumption of CO2 in Diels-Alder processes under non-catalytic and catalytic conditions after prior activation of the C=O bond. In addition to the obvious benefits associated with CO2 utilization, such processes open up the possibility of universal synthesis of a wide range of internal carboxylates. These studies have been performed in the framework of Molecular Electron Density Theory as a modern view of the chemical reactivity. It has been found, that explored DA reactions catalyzed by Lewis acids with the boron core, proceeds via unique stepwise mechanism with the zwitterionic intermediate. Bonding Evolution Theory (BET) analysis of the molecular mechanism associated with the DA reaction between cyclopentadiene and carbon dioxide indicates that it takes place thorough a two-stage one-step mechanism, which is initialized by formation of C–C single bond. In turn, the DA reaction between cyclopentadiene and carbon dioxide catalysed by BH3 extends in the environment of DCM, indicates that it takes place through a two-step mechanism. First path of catalysed DA reaction is characterized by 10 different phases, while the second by eight topologically different phases.

Theoretical Insight into the Reversal of Chemoselectivity in Diels-Alder Reactions of α,β-Unsaturated Aldehydes and Ketones Catalyzed by Brønsted and Lewis Acids

Experimentally, a reversal of chemoselectivity has been observed in catalyzed Diels–Alder reactions of α,β-unsaturated aldehydes (e.g., (2E)-but-2-enal) and ketones (e.g., 2-hexen-4-one) with cyclopentadiene. Indeed, using the triflimidic Brønsted acid Tf2NH as catalyst, the reaction gave a Diels–Alder adduct derived from α,β-unsaturated ketone as a major product. On the other hand, the use of tris(pentafluorophenyl)borane B(C6F5)3 bulky Lewis acid as catalyst gave mainly the cycloadduct of α,β-unsaturated aldehyde as a major product. Our aim in the present work is to put in evidence the role of the catalyst in the reversal of the chemoselectivity of the catalyzed Diels–Alder reactions of (2E)-but-2-enal and 2-Hexen-4-one with cyclopentadiene. The calculations were performed at the ωB97XD/6-311G(d,p) level of theory and the solvent effects of dichloromethane were taken into account using the PCM solvation model. The obtained results are in good agreement with experimental outcomes.

Theoretical Studies of DENOx SCR over Cu-, Fe- and Mn-FAU Catalysts

Ab initio calculations based on the density functional theory were used. A cluster model of the faujasite zeolite structure (Al2Si22O66H36) with metal particles adsorbed above the aluminium centres was used. The NO and NH3 adsorption processes, individual and co-adsorption, have been studied over metal nanoparticles bound into zeolite clusters. Several configurations, electronic structure (charges, bond orders) and vibration frequencies have been analyzed to determine feasible pathways for the deNOx reaction. The M2O dimers (M = Cu, Mn or Fe) were considered in relation to the previous studies of iron complexes.

2,3-Dimethoxy-2,3-dimethyl-1,4-dioxane as a useful precursor to 2,3-dimethylene-1,4-dioxane for [4+2] cycloaddition reaction

2,3-Dimethoxy-2,3-dimethyl-1,4-dioxane readily prepared from biacetyl serves as a stable precursor to 2,3-dimethylene-1,4-dioxane which undergoes a [4+2] cycloaddition reaction with dienophiles to give functionalized cyclohexene derivatives. The cycloaddition adducts obtained by the present procedure are transformed into potentially useful intermediates for biologically important materials.

2,3-Dimethoxy-2,3-dimethyl-1,4-dioxane serves as a stable precursor to 2,3-dimethylene-1,4-dioxane which undergoes a cycloaddition with dienophiles. The adducts are transformed into useful intermediates for biologically important materials.

The mechanism of conversion of substituted glycals to chiral acenes via Diels-Alder reaction: a computational study

Synthesis of linearly fused aromatic systems using a glycal-based diene with an aryne is a long-standing topic of interest in glycal chemistry. We have examined the mechanistic pathways for the transformation of substituted glycals to chiral fused aromatic cores via Diels-Alder (DA) reaction using the SMDACN-M06-2X/6-31G(d) level of theory. The DA reactions of E (1a) and Z (1a') forms of C-2 alkenyl glycal and an aryl glycal (1b) as a diene were examined with a benzyne intermediate generated as a dienophile. The computational results reveal that 1a and 1b can only be transformed into the fused aromatic cores by the base-catalyzed reaction because a [1,5] sigmatropic hydrogen shift is not feasible. The activation free energy barrier for the base-catalyzed proton abstraction process is 4.2 kcal mol-1 and there is almost no barrier for stereoisomeric 1a DA-complexes. The activation free energy barrier values for stereoisomeric 1b DA-complexes for the base-catalyzed proton abstraction process are 10.8 and 12.4 kcal mol-1. The appropriate orientation of glycal-ring-oxygen and hydrogen at the 5th position of Z (1a') forms of C-2 alkenyl glycal facilitates the [1,5] sigmatropic hydrogen shift; however, the base-catalyzed reaction is energetically more favored than the former case. The rate-determining step for 1a and 1a' is the ring-opening step (18.2 and 19.5 kcal mol-1 for the S-stereoisomer), whereas the DA adduct formation step is the rate-determining step for 1b (16.1 kcal mol-1 for the S-stereoisomer). The structural analysis reveals the formation of the preferred S-stereoisomer over the R-stereoisomer with the respective dienes.

Molecular mechanism of Hetero Diels-Alder reactions between (E)-1,1,1-trifluoro-3-nitrobut-2-enes and enamine systems in the light of Molecular Electron Density Theory

The molecular mechanism of the reaction of (E)-1,1,1-trifluoro-3-nitrobut-2-ene 1 with 3,3-dimethyl-2-morpholinobutene 2 has been studied within the Molecular Electron Density Theory (MEDT). This theoretical study confirm the possibility of the formation of zwitterionic structures in the first reaction stage. Interestingly, that localized zwitterions are however not common intermediates for identified in the postreaction mixture products. The further Bonding Evolution Theory (BET) study show, that the key, HDA reaction takes place in one-step but in a non-concerted manner since three stages are clearly identified. First the C5-C6 double bond breaks, then the C1-C2 bond breaks, and in the last phase we observed the formation of the C1-C6 single bond and V(C2) and V' (C2) pseudoradical centers in Z1 molecule. In turn, the molecular mechanism of the conversion of zwitterion Z1 to product 3 can be divided also in three groups in which we observed the disappearance of the two pseudoradical centers and formation O4-C5 single and C3-N3 double bonds, respectively.

A DFT study on the molecular mechanism of the conjugated nitroalkenes polymerization process initiated by selected unsaturated nucleophiles

The participation of the nitroethene and its α-substituted analogs as model nitrofunctionalized monomers, and (Z)-C,N-diphenylnitrone and methyl vinyl ether as initiators in the polymerization reactions, has been analyzed in the framework of the density functional theory calculations at the M06-2X(PCM)/6-311 + G(d) level. Our computational study suggests the zwitterionic mechanism of the polymerization process. The exploration of the nature of critical structures shows that the first reaction stage exhibits evidently polar nature, whereas additions of further nitroalkene molecules to the polynitroalkyl molecular system formed should be considered as moderate polar processes. The more detailed view on the molecular transformations gives analysis based on the bonding evolution theory. This study shows that the case of polymerization reaction between nitroethene and (Z)-C,N-diphenylnitrone allows for distinguishing eleven topologically different phases, while, in the case of polymerization reaction nitroethene and methyl vinyl ether, we can distinguish nine different phases.

Prediction of the [4 + 2]- and [5 + 4]-cycloaddition reactions in zig-zag carbon nanotubes via an ambimodal transition state: density functional theory calculations

A unique type of chemical reaction known as an ambimodal reaction has drawn tremendous attention owing to its intriguing feature of forming multiple (two or more) products from the same (single) transition state. In contrast to conventional reactions, bifurcation of the potential energy surface takes place in ambimodal reactions. Density functional theory (DFT) based calculations were performed to probe the Diels–Alder (DA) cycloaddition reactions of various carbon nanotubes (CNTs) with 1,3-butadiene. The present investigation reveals the possibility of ambimodal transition state formation on a potential energy surface (PES) corresponding to an unusual [5 + 4]-cycloadduct along with the conventional [4 + 2]-cycloadduct. The ground state of the [5 + 4]-cycloadduct obtained from butadiene and the H-terminated CNTs is a triplet (³T) state, but on the other hand the [4 + 2]-cycloadduct is a singlet (¹S) state. The [5 + 4]-adduct is energetically more stable in comparison with the [4 + 2]-adduct. The possibility of the formation of the [5 + 4]-adduct is validated using frontier molecular orbitals. The length of the nanotube significantly influences the overall kinetics and thermodynamics of the reaction.

A DFT study has been performed to unveil the ambimodal reaction in H-terminated CNTs.

Biosynthesis of Grandione: An Example of Tandem Hetero Diels-Alder/Retro-Claisen Rearrangement Reaction?

Mechanistic theoretical studies about the feasibility of the traditional proposed mechanism of formation for icetexane diterpene dimer grandione were assessed using density functional method at the M06-2X/6-31G(d,p) level of theory. Bulk water solvent effects were taken into account implicitly using the polarizable continuum model (SCI-PCM). The results were compared with the selectivity found in the biomimetic synthesis performed by experimental research groups. The relative free energy calculation shows that the one-step H-DA formation mechanism nominated in the literature is not a viable mechanism. We found that an alternative competing Tandem pathway is consistent with the experimental trends. Thus, our results suggested that the compound grandione is formed via a H-DA/retro-Claisen rearrangement and not by the traditional H-DA mechanism proposed early in the experimental studies. The H-DA initial step produce a biecyclic adduct followed by a domino retro-Claisen rearrangement that releases the energy strain of the bicyclic intermediary. Steric issues and hyperconjugation interactions are the mainly factors driving the reaction nature and the selectivity in the formation reaction. Finally, the enzymatic assistance for dimer formation was analyzed in terms of the calculated transition state energy barrier.

Synthesis of exo-Imidazolidin-2-one Dienes, Their Isomerization, and Selectivity in Diels-Alder Cycloadditions

An efficient and alternative synthesis of exo-imidazolidin-2-one dienes is described. A condensation reaction was carried out with bis-imino derivatives, diacetyl, and triphosgene, affording symmetrically N, N-disubstituted dienes. The use of alkyl methyl α-diketones led to the formation of nonsymmetrical dienes, which underwent isomerization to provide more stable inner-outer-ring dienes under Lewis acid conditions. Evaluation was made of the reactivity as well as regio- and stereoselectivity of these dienes in Diels-Alder reactions. They proved to be highly reactive and selective. DFT calculations of the transition states accounted for their behavior.

Trapping of 1,2-cyclohexadiene: A DFT mechanistic study on the reaction of 1,2-cyclohexadiene with olefins and nitrones

The mechanistic aspects of cycloaddition reactions of 1,2-cyclohexadiene with olefins and nitrones have been investigated with DFT calculations. The results show that the cycloaddition reactions of 1,2-cyclohexadiene with olefins do not go through a concerted pathway (one-step mechanism) but rather a stepwise one involving the formation of a biradical intermediate which then closes to form final cycloadduct. Electron-withdrawing substituents on the 1,2-cyclohexadiene decrease the activation barrier of the biradical-forming step but increase the barrier of the product-forming step and product stability, while electron-donating substituents on the 1,2-cyclohexadiene increase the barriers for both the biradical-forming step and the product-forming step but decrease the product stability. In the reaction of 1,2-cyclohexadiene with nitrones, the four pathways investigated have activation barriers within 1 kcal/mol of one another, the lowest being 10.45 kcal/mol and the highest 11.04 kcal/mol, indicating that these reactions are very unselective. Electron-withdrawing groups on the nitrone increase the stability of the resulting products whereas electron-donating group on the nitrone decrease the stability of the resulting products. The [3 + 2] cycloadduct proceeds to the formation of a more stable formal [5 + 2] cycloadduct if a phenyl substituent is present on the nitrogen of the nitrone.

Alkylhalovinylboranes: a new class of Diels–Alder dienophiles

The Diels–Alder reactions of alkylhalovinylboranes have been investigated theoretically and experimentally. Alkylhalovinylboranes presented higher reactivity than the corresponding dialkylvinylboranes. Although endo/exo selectivities were high for the reactions with cyclopentadiene, facial selectivities for the chiral analogues were low. Our results demonstrate that the replacement of an alkyl group on the boron atom by a halogen increases the dienophilicity considerably.

The Diels–Alder reactions of alkylhalovinylboranes have been investigated theoretically and experimentally.

Investigation of the reaction mechanism of [4 + 2] cyclization of 2,3 dimethylbuta-1,3-diene to methylacrylate using the Michaelis-Menten equation

The cycloaddition reaction between 2,3‑dimethylbuta-1,3-diene and methylacrylate proceeds by the second order kinetics. The rate constants increase with the increase in the excess of one of the reactants. The change in the effective rate constants is described by the Michaelis–Menten equation indicating that the reaction proceeds through the initial equilibrium stage of formation of a molecular complex stabilized by van der Waals interaction which then transforms into the product. The limiting stage of the reaction is established and its mechanism is suggested.

A theoretical study of an electronically mismatched Diels–Alder cycloaddition

The stepwise mechanism of the Diels–Alder cycloaddition between two electon-rich components.

Biogenesis of Triterpene Dimers from Orthoquinones Related to Quinonemethides: Theoretical Study on the Reaction Mechanism

The biogenetic origin of triterpene dimers from the Celastraceae family has been proposed as assisted hetero-Diels-Alder reaction (HDA). In this work, computational calculation of HDA between natural quinonemethides (tingenone and isopristimerol) and hypothetical orthoquinones has been performed at the M06-2X/6-31G(d) level of theory. We have located all the HDA transition states supporting the biogenetic route via HDA cycloadditions. We found that all reactions take place through a concerted inverse electron demand and asynchronous mechanism. The enzymatic assistance for dimer formation was analyzed in terms of the calculated transition state energy barrier.