Searching for zwitterionic intermediates in Hetero Diels–Alder reactions between methyl α,p-dinitrocinnamate and vinyl-alkyl ethers

On the Question of Zwitterionic Intermediates in the [3+2] Cycloaddition Reactions between Aryl Azides and Ethyl Propiolate

The molecular mechanism of the [3+2] cycloaddition reactions between aryl azides and ethyl propiolate was evaluated in the framework of the Molecular Electron Density Theory. It was found that independently of the nature of the substituent within the azide molecule, the cycloaddition process is realized via a polar but single-step mechanism. All attempts of localization as postulated earlier by Abu-Orabi and coworkers' zwitterionic intermediates were not successful. At the same time, the formation of zwitterions with an "extended" conformation is possible on parallel reaction paths. The ELF analysis shows that the studied cycloaddition reaction leading to the 1,4-triazole proceeds by a two-stage one-step mechanism. It also revealed that both zwitterions are created by the donation of the nitrogen atom's nonbonding electron densities to carbon atoms of ethyl propiolate.

Understanding the Molecular Mechanism of Thermal and LA-Catalysed Diels-Alder Reactions between Cyclopentadiene and Isopropyl 3-Nitroprop-2-Enate

The molecular mechanism of the Diels-Alder reaction with the participation of cyclopentadiene and isopropyl 3-nitroprop-2-enate was examined based on wb97xd/6-311+G(d) (PCM) quantum chemical calculations. It was found that the type of mechanism for the conversion of addends depends significantly on the reaction conditions. In less-polar environments, a one-step polar mechanism is realised. In more polar solvents, the formation of "extended"-type zwitterionic intermediates is possible. In contrast, in the presence of an LA-type catalyst, the one-step mechanisms are replaced by respective stepwise mechanisms with zwitterionic or heterocyclic intermediates.

Understanding the kinetics and atmospheric degradation mechanism of chlorotrifluoroethylene (CF2CFCl) initiated by OH radicals

The atmospheric degradation of chlorotrifluoroethylene (CTFE) by OH˙ was investigated using density functional theory (DFT). The potential energy surfaces were also defined in terms of single-point energies derived from the linked cluster CCSD(T) theory. With an energy barrier of -2.62 to -0.99 kcal mol^-1 using the M06-2x method, the negative temperature dependence was determined. The OH˙ attack on C_α and C_β atoms (labeled pathways R1 and R2, respectively) shows that reaction R2 is 4.22 and 4.42 kcal mol^-1, respectively, more exothermic and exergonic than reaction R1. The main pathway should be the addition of OH˙ to the β-carbon, resulting in ˙CClF-CF₂OH species. At 298 K, the calculated rate constant was 9.87 × 10^-13 cm³ molecule^-1 s^-1. The TST and RRKM calculations of rate constants and branching ratios were performed at P = 1 bar and in the fall-off pressure regime over the temperature range of 250-400 K. The formation of HF and ˙CClF-CFO species via the 1,2-HF loss process is the most predominant pathway both kinetically and thermodynamically. With increasing temperature and decreasing pressure, the regioselectivity of unimolecular processes of energized adducts [CTFE-OH]˙ gradually decreases. Pressures greater than 10^-4 bar are often adequate for assuring saturation of the estimated unimolecular rates when compared to the RRKM rates (in high-pressure limit). Subsequent reactions involve the addition of O₂ to the [CTFE-OH]˙ adducts at the α-position of the OH group. The [CTFE-OH-O₂]˙ peroxy radical primarily reacts with NO and then directly decomposes into NO₂ and oxy radicals. "Carbonic chloride fluoride", "carbonyl fluoride", and "2,2-difluoro-2-hydroxyacetyl fluoride" are predicted to be stable products in an oxidative atmosphere.

Mechanistic aspects of the synthesis of seven-membered internal nitronates via stepwise [4 + 3] cycloaddition involving conjugated nitroalkenes: Molecular Electron Density Theory computational study

The density functional theory computational study indicates the possibility of the synthesis of seven-membered internal nitronates via cycloaddition reactions involving Z-C-aryl-N-methylnitrones and E-2-aryl-1-cyano-1-nitroethenes. The detailed exploration of the reaction paths indicates a polar, stepwise reaction mechanism through the zwitterionic intermediate. Using bonding evolution theory (BET), we have deciphered the molecular mechanism of the [4 + 3] cycloaddition reaction between E-2-phenyl-1-cyano-1-nitroethene and Z-C-phenyl-N-methylnitrone. The BET study has revealed that the formation of two CO single bonds takes place in the same way, through the depopulation of NC and CC bonding regions and monosynaptic basins, respectively. The first O1C7 single bond was formed in the sixth phase, while the second C3O4 bond was formed in the last ninth phase.

Preparation and Characterization of Self-Healing Polyurethane Powder Coating Using Diels-Alder Reaction

Although powder coating systems offer many environmental, ecological and energy related benefits over liquid based coatings, in the case of uretdione based polyurethane systems, high curing temperature is still an issue. On the other hand, powder coating systems make it possible to reduce the costs and enhance the process of forming complex 3D structures using the deep drawing method by pre-coated metal substrates. During this processing method, there is a probability of micro crack formation in the coated film due to strain impact on the coating layer. A powder coating with self-healing ability is an ultimate solution to face not only this kind of fraction but also any other possible ones (such as defects caused by any impact on film surface during processing, transporting or even service). Here, a single molecule that is prepared via Diels–Alder cycloaddition reaction and retro Diels–Alder cleavage reaction was utilized as a self-healing additive to achieve self-healing ability in the powder coating system that is based on a commercially available uretdione cross-linker and OH-polyester resin. Coatings were prepared through melt mixing of components in a lab mixer, milling, sieving, and then application on the metal substrate through the electrostatic spraying method. To illustrate the role of self-healing additive, various concentrations (4 and 9% wt.) in combination with different curing temperatures (80 °C to 200 °C) were investigated. Both samples containing HA showed self-healing ability at elevated temperature around 120 °C for about 30 min with acceptable roughness and surface properties. Hardness measurement of cured film as well as thermal investigation indicate the chemical reaction of HA in a cross-linked network of cross-linker and resin. In addition, using HA leads to a 40 K drop in curing temperature of the system without using any catalyst. A 2.58% improvement in hardness values at a lower curing temperature and healing time of around 12.5 min at 120 °C to recover 100% of initial scratch (more than 10 cycles) in the sample containing 9% wt. HA was observed.

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.

Trends in the Diels-Alder reaction in polymer chemistry

The Diels-Alder (DA) reaction is regarded as quite a useful strategy in organic and macromolecular syntheses. The reversibility of this reaction and the advent of self-repair technology, as well as other applications in controlled macromolecular architectures and crosslinking, have strongly boosted the research activity, which is still attracting a huge interest in both academic and industrial research. The DA reaction is a simple and scalable toolbox. Though it is well-established that furan/maleimide is the most studied diene/dienophile couple, this perspective article reports strategies using other reversible systems with deeper features on other types of diene/dienophile pairs being either petro-sourced (cyclopentadiene, anthracene) or bio-sourced (muconic and sorbic acids, myrcene and farnesene derivatives, eugenol, cardanol). This review is composed of four sections. The first one briefly recalls the background on the DA reactions involving cyclodimerizations, dienes, and dienophiles, parameters affecting the reaction, while the second part deals with the furan/maleimide reaction. The third one deals with petro-sourced and bio-sourced (or products becoming bio-sourced) reactants involved in DA reactions are also listed and discussed. Finally, the authors' opinion is given on the potential future of the crosslinking-decrosslinking reaction, especially regarding the process (e.g., key temperatures of decrosslinking) or possibly monocomponents. It presents both fundamental and applied research on the DA reaction and its applications.

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.

Is the pericyclic transition structure of aza-Diels-Alder reaction aromatic?

The notion of aromaticity is an elusive concept, typically delineated as an electron delocalization pattern within a cyclic structure, and is characterized by unusual stability, reactivity, and other properties. Recently, the aromatic concept has been extended to inorganic molecules and saturated systems. In our work, the so-called pericyclic transition state structures (TSs) involved in aza-Diels-Alder reactions are investigated. Interestingly, geometries and energy barriers evidence that these reactions are highly energetic and are extremely asynchronous. Additionally the localized orbital locator, ELF and QTAIM topological analyses have proven that at least one pair of atoms is not bound at the TS configuration. Moreover, HOMA and PDI indexes show that these TSs where aromaticity is not important. Therefore, these results provoke us to rule out the aromatic character of these so-called pericyclic TSs structures.

Formation Mechanism of Benzo(a)pyrene: One of the Most Carcinogenic Polycyclic Aromatic Hydrocarbons (PAH)

The formation of polycyclic aromatic hydrocarbons (PAHs) is a strong global concern due to their harmful effects. To help the reduction of their emissions, a crucial understanding of their formation and a deep exploration of their growth mechanism is required. In the present work, the formation of benzo(a)pyrene was investigated computationally employing chrysene and benz(a)anthracene as starting materials. It was assumed a type of methyl addition/cyclization (MAC) was the valid growth mechanism in this case. Consequently, the reactions implied addition reactions, ring closures, hydrogen abstractions and intramolecular hydrogen shifts. These steps of the mechanism were computed to explore benzo(a)pyene formation. The corresponding energies of the chemical species were determined via hybrid density funcional theory (DFT), B3LYP/6-31+G(d,p) and M06-2X/6-311++G(d,p). Results showed that the two reaction routes had very similar trends energetically, the difference between the energy levels of the corresponding molecules was just 6.13 kJ/mol on average. The most stable structure was obtained in the benzo(a)anthracene pathway.

Understanding of the molecular mechanism of the phenylsulfenic acid elimination from nitroalkyl systems

The molecular mechanism of the phenylsulfenic acid elimination from nitroalkyl systems has been explored using M06-2X/6-31++G(d,p) theoretical level. It was found that independently of environment polarity as well as degree of screening of nitroalkyl moiety, these types of reactions proceed via one-step, polar mechanism with asynchronous transition state. According to actual state of knowledge this type of the mechanism should not be treated as "pericyclic". Simultaneously, all attempts for localization of stationary structures which can be related to the hypothetical, stepwise mechanism were not successful.

Regiospecific formation of the nitromethyl-substituted 3-phenyl-4,5-dihydroisoxazole via [3 + 2] cycloaddition

ABSTRACT

5-(Nitromethyl)-3-phenyl-4,5-dihydroisoxazole was obtained as a product of a high-yielding [3 + 2] cycloaddition reaction of in situ-generated benzonitrile N-oxide and 3-nitroprop-1-ene. For the first time, the regiochemistry of this reaction was unambiguously proven by X-ray structural analysis. The quantum-chemical calculation performed at the M06-2X/6-31G(d) (PCM) theoretical level affords a basis for explaining the course of reaction as well as the nature of transition states. Next, further DFT calculations together with spectral data shed light on structural aspects of the product.

GRAPHICAL ABSTRACT

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.

Molecular Reactivity and Absorption Properties of Melanoidin Blue-G1 through Conceptual DFT

This computational study presents the assessment of eleven density functionals that include CAM-B3LYP, LC-wPBE, M11, M11L, MN12L, MN12SX, N12, N12SX, wB97, wB97X and wB97XD related to the Def2TZVP basis sets together with the Solvation Model Density (SMD) solvation model in calculating the molecular properties and structure of the Blue-G1 intermediate melanoidin pigment. The chemical reactivity descriptors for the system are calculated via the conceptual Density Functional Theory (DFT). The choice of the active sites related to the nucleophilic, electrophilic, as well as radical attacks is made by linking them with the Fukui function indices, the electrophilic Parr functions and the condensed dual descriptor Δ f ( r ) . The prediction of the maximum absorption wavelength tends to be considerably accurate relative to its experimental value. The study found the MN12SX and N12SX density functionals to be the most appropriate density functionals in predicting the chemical reactivity of the studied molecule.

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.

A clean and simple method for deprotection of phosphines from borane complexes

Simple, efficient, clean, and stereospecific protocols of protection of phosphorus atom with borane and deprotection from the borane complexes of the tertiary phosphines in mild conditions are reported. The proposed protection/deprotection reactions tolerate a range of functional groups and lead to pure products with excellent yield with no need for application of chromatographic or crystallisation purification procedures. For the first time mechanisms of the reactions of phosphine protection and deprotection have been studied based on experimental kinetic data as well as quantumchemical calculations, which allows designing reaction conditions suitable for a given substrate.

Diastereoselective [3 + 2] vs [4 + 2] Cycloadditions of Nitroprolinates with α,β-Unsaturated Aldehydes and Electrophilic Alkenes: An Example of Total Periselectivity

Diastereoselective multicomponent reactions of enantioenriched 4-nitroprolinates, obtained by enantiocatalyzed 1,3-dipolar cycloaddition (1,3-DC) of imino esters and nitroalkenes, with α,β-unsaturated aldehydes and electrophilic alkenes proceed with total periselectivity depending on the structure of the aldehyde employed. This process evolves through a [3 + 2] 1,3-DC when cinnamaldehyde is used in the presence of an azomethine ylide, giving the corresponding highly substituted pyrrolizidines with endo selectivity. However, in the case of the α,β-unsaturated aldehyde, which contains a hydrogen atom at the γ position, an amine-aldehyde-dienophile (AAD) [4 + 2] cycloaddition takes place by formation of an intermediate 1-amino-1,3-diene, affording highly functionalized cyclohexenes with high endo diastereoselectivity. This AAD process only occurred when a nitro group is bonded to the 4-position of the initial enantiomerically enriched pyrrolidine ring. DFT calculations have been carried out with the aim of explaining this different behavior between pyrrolidines with or without a nitro group, demonstrating the strong nitro-group-dependent periselectivity. The results of these computational studies also support the experimentally obtained absolute configuration of the final adducts.

Unexpected course of reaction between (E)-2-aryl-1-cyano-1-nitroethenes and diazafluorene: why is there no 1,3-dipolar cycloaddition?

ABSTRACT

Reactions between (E)-2-aryl-1-cyano-1-nitroethenes and diazafluorene lead to acyclic 2,3-diazabuta-1,3-diene derivatives, instead of the expected pyrazoline systems. DFT calculations suggest that this is a consequence of formation of zwitterionic structure in the first stage of the reaction. It must be noted that this is a specific property of the (E)-2-aryl-1-cyano-1-nitroethenes group, in contrast to most other conjugated nitroalkenes.

GRAPHICAL ABSTRACT

Computational Prediction of the Protonation Sites of Ac-Lys-(Ala)n-Lys-NH2 Peptides through Conceptual DFT Descriptors

Six density functionals (M11, M11L, MN12L, MN12SX, N12, and N12SX) in connection with the Def2TZVP basis set and the SMD solvation model (water as a solvent) have been assessed for the calculation of the molecular structure and properties of several peptides with the general formula Ac-Lys-(Ala)n-Lys-NH2, with n = 0 to 5. The chemical reactivity descriptors for these systems have been calculated through Conceptual density functional theory (DFT). The active sites for nucleophilic and electrophilic attacks have been chosen by relating them to the Fukui function indices, the condensed dual descriptor Δf(r), and the electrophilic Parr functions. The results allowed the prediction of the protonation sites of the peptides and rendered a qualitative explanation of the difference in pKa of the two Lys groups.

A Quantum-Chemical DFT Approach to Elucidation of the Chirality Transfer Mechanism of the Enantioselective Suzuki–Miyaura Cross-Coupling Reaction

The DFT calculations of the simplified model of the asymmetric Suzuki–Miyaura coupling reaction were performed at the M062x/LANL2DZ theory level at first. It was found that enantioselective reactions mediated by the palladium complexes of chiral C,P-ligands follow a four-stage mechanism similar to that proposed previously as one of the most credible mechanisms. It should be underlined that the presence of substituents in the substrates and the chiral ligand at ortho positions determines the energies of possible diastereoisomeric transition states and intermediates in initial reaction steps. This suggests that, in practice, a sharp selection of theoretically possible paths of chirality transfer from the catalyst to the product should have a place and, therefore, the absolute configuration of the formed atropisomeric product is defined and can be predicted.

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.

Ionic liquid supported acid additive stabilizes the transition structure of organocatalytic asymmetric direct aldol reaction by proton donation: A quantum mechanical study

ONIOM studies were performed on the transition structure (TS) of organocatalytic direct aldol reaction by using ionic liquid supported benzoic acid (ILS-PhCO2H) as an additive. Results obtained from this computation suggest direct involvement of ILS-PhCO2H in the TS as a proton donor. It has also come out from the present study that, the counter ion of the ILS-acid additive may also play significant role to maintain the proper TS geometry by holding the organocatalyst and the acid additive close together during the course of reaction.

First example of stepwise, zwitterionic mechanism for bicyclo[2.2.1]hept-5-ene (norbornene) formation process catalyzed by the 1-butyl-3-methylimidazolium cations

Abstract

B3LYP/6-31++G(d) calculations indicated that the reaction of (2E)-3-phenyl-2-nitroprop-2-enenitrile with cyclopentadiene catalyzed by cations of 1,3-dialkylimidazolium ionic liquid shall not take place according to the classical scheme of one-step [2+4] Diels–Alder cycloaddition. Along the path finally leading to bicyclo[2.2.1]hept-5-ene (norbornene) with a nitro group in endo orientation, the process of bicarbocyclic skeleton formation shall take place according to the domino mechanism, via [2+4] heterocycloadduct. On the other hand, along the path leading finally to bicyclo[2.2.1]hept-5-ene with a nitro group in exo orientation, acyclic adduct with a zwitterionic nature is formed in the first reaction, which undergoes cyclisation next in the second step of the reaction.

Graphical abstract

On the Brassard's rule of regioselectivity in Diels–Alder reactions between haloquinones and polar dienes

DFT/MC/AIM computational methods and experimental tests were used to further explain the Brassard’s rule for the [4 + 2] polar Diels–Alder reaction between nucleophilic dienes and haloquinones, which proceeds with remarkably high regioselectivity.

How the mechanism of a [3 + 2] cycloaddition reaction involving a stabilized N-lithiated azomethine ylide toward a π-deficient alkene is changed to stepwise by solvent polarity? What is the origin of its regio- and endo stereospecificity? A DFT study using NBO, QTAIM, and NCI analyses

The regio- and endo stereospecific 32CA reaction of the stabilized N-lithiated azomethine ylide 2 toward the π-deficient alkene 3in THF occurs to afford cycloadduct 6via a stepwise mechanism, passing through the zwitterionic intermediate 4.

Understanding the kinetics of thermal decomposition of 2,3-epoxy-2,3-dimethylbutane using RRKM theory

The thermal decomposition kinetics of 2,3-epoxy-2,3-dimethylbutane have been studied computationally using density functional theory, along with various exchange–correlation functionals and an extremely large basis set.