Understanding the mechanism of non-polar Diels-Alder reactions. A comparative ELF analysis of concerted and stepwise diradical mechanisms

Investigation of thia-Diels-Alder Reactions by Ultrafast Transient Absorption Spectroscopy and DFT Calculations

The thia-Diels-Alder reaction represents a versatile synthetic method for the preparation of six-membered sulfur-containing compounds. However, the mechanism of the thia-Diels-Alder reactions remains unclear. In this work, time-resolved spectroscopic experiments and DFT calculations demonstrate that phenacyl sulfide undergoes Norrish II cleavage to produce thioaldehyde, and ortho-hydroxy benzhydryl alcohol occurs in a dehydration reaction to generate o-QMs using diphenylphosphate as the catalyst. Then, the thia-Diels-Alder reaction takes place between thioaldehyde and o-QMs by an asynchronous concerted mechanism. The illustration of the thia-Diels-Alder reaction mechanism not only provides important support for organic synthesis and drug design but also enhances fundamental insights into reaction pathways and catalytic processes in the field of chemical synthesis.

Energetics and electronics of polar Diels-Alder reactions at the atomic level: QTAIM and IQA analyses of complete IRC paths

The mechanism of Diels-Alder reactions between cyclopentadiene and several cyanoethylenes was studied by means of Density Functional Theory calculations using QTAIM and IQA (Interacting Quantum Atoms) analyses along complete IRC paths. Each geometry from the IRC had its wavefunction computed and the topology of the electronic density for it was then evaluated. By means of IQA, the global energetic profile was partitioned among the various atoms in the molecule, providing insight into what atoms are the main ones responsible for the magnitude of the energy barriers. The (a)synchronicity of the reaction mechanisms featuring non-symmetrically substituted dienophiles was characterized, from QTAIM, by the electron densities and Laplacians over the LCP's as well as by the different atomic energy barriers obtained from IQA. The magnitude of the atomic barrier nicely explains the (a)synchronicity of the reaction mechanisms, and the degree of (a)synchronicity is nicely revealed by the difference between the earlier and later bond breaking and bond formations. The main conclusion is that important energetic and electronic changes are occurring before and after the position of the transition state structure, mainly for those asynchronous mechanisms, and although these provide essential insight into the reaction mechanism, most studies cannot assess this kind of information because they are focusing solely on reactants, transition states, and products. We advocate that the additional computational effort required for such analyses is more than compensated by the great amount of useful information it provides.

Multiselective Diels-Alder Reaction of α-Arylacroleins Catalyzed by Boron Tribromide-Assisted Chiral Phosphoric Acids

A multiselective Diels-Alder (DA) reaction of α-arylacroleins with cyclopentadiene using BBr₃-assisted chiral BINOL-derived phosphoric acid catalysts has been developed. This unusual exo- and enantioselective DA reaction can be multicontrolled by the chiral cavity of the in situ-formed acid-base cooperative catalysts, in particular, suppressing the competitive hetero Diels-Alder (HDA) reaction effectively.

Enhanced Sampling Simulation Reveals How Solvent Influences Chirogenesis of the Intra-Molecular Diels-Alder Reaction

The timescale involved in chemical reactions is quite often beyond that of normal molecular dynamics simulations. Here, we combine metadynamics with selective integrated tempering sampling to simulate an intra-molecular Diels-Alder reaction in explicit solvents. Based on a one-dimensional collective variable obtained from harmonic linear discriminant analysis, four chiral isomers of products were observed in the simulation. Analyses of reactive trajectories showed that this reaction follows a concerted mechanism in all four solvents. In addition, the hydrogen bond between the reactant and water solvent plays an important role in the water-accelerated reaction mechanism. The dynamics of chirality formation varies significantly with solvents. The chirality of products forms significantly before the transition state, especially in ionic liquid.

Collective Synthesis of Illudalane Sesquiterpenes via Cascade Inverse Electron Demand (4 + 2) Cycloadditions of Thiophene S,S-Dioxides

Thiophene S,S-dioxides are underutilized tools for the de novo construction of benzene rings in organic synthesis. We report a collective synthesis of nine illudalane sesquiterpenes using bicyclic thiophene S,S-dioxides as generalized precursors to the indane core of the natural products. Exploiting furans as unusual dienophiles in this inverse electron demand Diels-Alder cascade, this concise and convergent approach enables the synthesis of these targets in as little as five steps. Theoretical studies rationalize the reactivity of thiophene S,S-dioxides with both electron-poor and electron-rich dienophiles and reveal reaction pathways involving either nonpolar pericyclic or bifurcating ambimodal cycloadditions. Overall, this work demonstrates the wider potential of thiophene S,S-dioxides as convenient and flexible precursors to polysubstituted arenes.

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.

Highs and Lows of Bond Lengths: Is There Any Limit?

Two distinct points on the potential energy curve (PEC) of a pairwise interaction, the zero‐energy crossing point and the point where the stretching force constant vanishes, allow us to anticipate the range of possible distances between two atoms in diatomic, molecular moieties and crystalline systems. We show that these bond‐stability boundaries are unambiguously defined and correlate with topological descriptors of electron‐density‐based scalar fields, and can be calculated using generic PECs. Chemical databases and quantum‐mechanical calculations are used to analyze a full set of diatomic bonds of atoms from the s‐p main block. Emphasis is placed on the effect of substituents in C−C covalent bonds, concluding that distances shorter than 1.14 Å or longer than 2.0 Å are unlikely to be achieved, in agreement with ultra‐high‐pressure data and transition‐state distances, respectively. Presumed exceptions are used to place our model in the correct framework and to formulate a conjecture for chained interactions, which offers an explanation for the multimodal histogram of O−H distances reported for hundreds of chemical systems.

Mechanistic and Kinetic Factors of ortho-Benzyne Formation in Hexadehydro-Diels-Alder (HDDA) Reactions

With the rapid development of the hexadehydro-Diels-Alder reaction (HDDA) from its first discovery in 1997, the question of whether a concerted or stepwise mechanism better describes the thermally activated formation of ortho-benzyne from a diyne and a diynophile has been debated. Mechanistic and kinetic investigations were able to show that this is not a black or white situation, as minor changes can tip the balance. For that reason, especially, linked yne-diynes were studied to examine steric, electronic, and radical-stabilizing effects of their terminal substituents on the reaction mechanism and kinetics. Furthermore, the influence of the nature of the linker on the HDDA reaction was explored. The more recently discovered photochemical HDDA reaction also gives ortho-arynes, which display the same reactivity as the thermally generated ones, but their formation might not proceed by the same mechanism. This minireview summarizes the current state of mechanistic understanding of the HDDA reaction.

Are There Only Fold Catastrophes in the Diels-Alder Reaction Between Ethylene and 1,3-Butadiene?

This work revisits the topological characterization of the Diels-Alder reaction between 1,3-butadiene and ethylene. In contrast to the currently accepted rationalization, we here provide strong evidence in support of a representation in terms of seven structural stability domains separated by a sequence of 10 elementary catastrophes, but all are only of the fold type (F and F^†), that is, C₄H₆ + C₂H₄:1-7-[FF]F[F^†F^†][F^†F^†][FF]F^†-0: C₆H₁₀. Such an unexpected finding provides fundamental new insights opening simplifying perspectives concerning the rationalization of the CC bond formation in pericyclic reactions in terms of the simplest Thom's elementary catastrophe, namely, the one-(state) variable, one-(control) parameter function.

Predicting the chemical reactivity of organic materials using a machine-learning approach

Stability and compatibility between chemical components are essential parameters that need to be considered in the selection of functional materials in configuring a system. In configuring devices such as batteries or solar cells, not only the functionality of individual constituting materials such as electrodes or electrolyte but also an appropriate combination of materials which do not undergo unwanted side reactions is critical in ensuring their reliable performance in long-term operation. While the universal theory that can predict the general chemical reactivity between materials is long awaited and has been the subject of studies with a rich history, traditional ways proposed to date have been mostly based on simple electronic properties of materials such as electronegativity, ionization energy, electron affinity and hardness/softness, and could be applied to only a small group of materials. Moreover, prediction has often been far from accurate and has failed to offer general implications; thus it was practically inadequate as a selection criterion from a large material database, i.e. data-driven material discovery. Herein, we propose a new model for predicting the general reactivity and chemical compatibility among a large number of organic materials, realized by a machine-learning approach. As a showcase, we demonstrate that our new implemented model successfully reproduces previous experimental results reported on side-reactions occurring in lithium–oxygen electrochemical cells. Furthermore, the mapping of chemical stability among more than 90 available electrolyte solvents and the representative redox mediators is realized by this approach, presenting an important guideline in the development of stable electrolyte/redox mediator couples for lithium–oxygen batteries.

Stability and compatibility between chemical components are essential parameters that need to be considered in the selection of functional materials in configuring a system.

A molecular electron density theory study of the participation of tetrazines in aza-Diels–Alder reactions

The reactions of eight tetrazines of increased electrophilic character with nucleophilic tetramethyl ethylene (TME) and with electrophilic tetracyanoethylene (TCE) have been studied using Molecular Electron Density Theory. These reactions are domino processes comprising an aza-Diels–Alder (ADA) reaction followed by an extrusion of molecular nitrogen, yielding a dihydropyridazine. Analysis of the conceptual DFT (CDFT) indices showed an increase of the electrophilicity and a decrease of the nucleophilicity of tetrazines with an increase of the electron-withdrawing character of the substituent. A very good correlation between the global electron density transfer at the transition structures and the activation enthalpies for the ADA reactions involving TME was found. However, tetrazines have no tendency to react with electrophilic ethylenes such as TCE. Bonding Evolution Theory (BET) analysis of the ADA reaction of dinitro tetrazine with TME showed that the activation energy is mainly associated with the continuous depopulation of the C–C and C–N double bonds.

The electron-withdrawing substituents on the tetrazine favour aza-Diels–Alder reactions towards nucleophilic ethylenes, but they do not react with electrophilic ethylenes.

BET & ELF Quantum Topological Analysis of Neutral 2-Aza-Cope Rearrangement of γ-Alkenyl Nitrones

The 2-Aza-Cope rearrangement of γ-alkenyl nitrones is a rare example of the neutral thermal 2-aza-Cope process that usually takes place with cationic species. During the rearrangement, a redistribution of bonds and electronic density occurs in one kinetic step. However, the introduction of substituents with different steric requirements and electronic features might alter the activation energies and the synchronicity of the reaction. The electron localization function (ELF) analysis and its application to Bonding Evolution Theory (BET) analysis within the context of Molecular Electron Density Theory (MEDT) is an excellent tool to monitor the electron density along the reaction coordinate and thus investigate in detail bond breaking and formation and the corresponding energy barriers. By analyzing topological ELF calculations of seventeen 2-aza-Cope nitrone rearrangements with selected substituents, the main factors influencing the synchronicity of the process were investigated. This MEDT study results revealed that the rearrangement is a non-polar process mostly influenced by steric factors rather than by electronic ones, and confirms the pseudoradical character of the process rather than any pericyclic electron-reorganization.

Introducing topology to assess the synchronicity of organic reactions. Dual reactivity of oximes with alkenes as a case study

Quantum chemical topology analyses provide a new way of understanding the synchronicity of organic reactions.

Molecular Electron Density Theory: A Modern View of Reactivity in Organic Chemistry

A new theory for the study of the reactivity in Organic Chemistry, named Molecular Electron Density Theory (MEDT), is proposed herein. MEDT is based on the idea that while the electron density distribution at the ground state is responsible for physical and chemical molecular properties, as proposed by the Density Functional Theory (DFT), the capability for changes in electron density is responsible for molecular reactivity. Within MEDT, the reactivity in Organic Chemistry is studied through a rigorous quantum chemical analysis of the changes of the electron density as well as the energies associated with these changes along the reaction path in order to understand experimental outcomes. Studies performed using MEDT allow establishing a modern rationalisation and to gain insight into molecular mechanisms and reactivity in Organic Chemistry.

A mechanistic insight into the effect of piperidine as an organocatalyst on the [3 + 2] cycloaddition reaction of benzalacetone with phenyl azide from a computational study

Several transition structures (TSs) for catalyst-free [3 + 2] cycloaddition and two plausible mechanistic pathways for the organocatalyzed [3 + 2] cycloaddition (32CA) between benzalacetone and phenyl azide were located by quantum chemistry methods. Calculations were carried out with B3LYP, MPWB1K and M06-2X functionals using 6-31G(d) and 6-311G(d,p) basis sets in gas and solvent phases. The calculated activation barriers imply that the lowest barrier pathway is the catalyzed process producing 3-regioisomers through the iminium intermediate and not through the dienamine route. Electronic displacements along the reaction path have been examined using a topological analysis of the electron-localization function (ELF). ELF topological analyses along the intrinsic reaction coordinates (IRC) of both catalyzed and uncatalyzed 32CA reactions indicated that while the first C1-N1 single bond is formed as a dative bond, the formation of the second C2-N3 bond takes place via a C-to-N coupling between the interacting centers of the reagents. Moreover, the ELF analyses imply that the reaction mechanism is a two-stage one-step process in the presence of a piperidine organocatalyst, while bond formation in an uncatalyzed process is almost synchronous.

Dimerization of quercetin, Diels-Alder vs. radical-coupling approach: a joint thermodynamics, kinetics, and topological study

Quercetin is a prototypical antioxidant and prominent member of flavonoids, a large group of natural polyphenols. The oxidation of quercetin may lead to its dimerization, which is a paradigm of the more general polyphenol oligomerization. There exist two opposing mechanisms to describe the dimerization process, namely radical-coupling or Diels-Alder reactions. This work presents a comprehensive rationalization of this dimerization process, acquired from density functional theory (DFT) calculations. It is found that the two-step radical-coupling pathway is thermodynamically and kinetically preferred over the Diels-Alder reaction. This is in agreement with the experimental results showing the formation of only one isomer, whereas the Diels-Alder mechanism would yield two isomers. The evolution in bonding, occurring during these two processes, is investigated using the atoms in molecules (AIM) and electron localization function (ELF) topological approaches. It is shown that some electron density is accumulated between the fragments in the transition state of the radical-coupling reaction, but not in the transition state of the Diels-Alder process. Graphical Abstract Quantum chemistry calculations of the dimerization process of quercetin show that a radical coupling approach is preferred to a Diels-Alder type reaction, in agreement with experimental results. Analysis of the bonding evolution highlights the reaction mechanism.

Applications of the Conceptual Density Functional Theory Indices to Organic Chemistry Reactivity

Theoretical reactivity indices based on the conceptual Density Functional Theory (DFT) have become a powerful tool for the semiquantitative study of organic reactivity. A large number of reactivity indices have been proposed in the literature. Herein, global quantities like the electronic chemical potential μ, the electrophilicity ω and the nucleophilicity N indices, and local condensed indices like the electrophilic P k + and nucleophilic P k - Parr functions, as the most relevant indices for the study of organic reactivity, are discussed.

Curly arrows meet electron density transfers in chemical reaction mechanisms: from electron localization function (ELF) analysis to valence-shell electron-pair repulsion (VSEPR) inspired interpretation

The displacement of the nuclei along the reaction path provides an explanatory interpretation of the electron density transfers making possible to understand chemical reactions.

Revealing Stepwise Mechanisms in Dipolar Cycloaddition Reactions: Computational Study of the Reaction between Nitrones and Isocyanates

The mechanism of cycloaddition reactions of nitrones with isocyanates has been studied using density functional theory (DFT) methods at the M06-2X/cc-pVTZ level of theory. The exploration of the potential energy surfaces associated with two reactive channels leading to 1,2,4-oxadiazolidin-5-ones and 1,4,2-dioxazolidines revealed that the cycloaddition reaction takes place through a concerted mechanism in gas phase and in apolar solvents but a stepwise mechanism in polar solvents. In stepwise mechanisms, the first step of the reaction is a rare case in which the nitrone oxygen acts as a nucleophile by attacking the central carbon atom of the isocyanate (interacting with the π-system of the C═O bond) to give an intermediate. The corresponding transition structure is stabilized by an attractive electrostatic interaction favored in a polar medium. The second step of the reaction is the rate-limiting one in which the formation of 1,2,4-oxadiazolidin-5-ones or 1,4,2-dioxazolidines is decided. Calculations indicate that formation of 1,2,4-oxadiazolidin-5-ones is favored both kinetically and thermodynamically independently of the solvent, in agreement with experimental observations. Noncovalent interactions (NCI) and topological analysis of the gradient field of electron localization function (ELF) bonding confirmed the observed interactions.