Combined activation strain model and energy decomposition analysis methods: a new way to understand pericyclic reactions

Carbene-Decorated Geometrically Constrained Borylenes for Bond Activations

While metal-ligand cooperativity is well-known, studies on element-ligand cooperativity involving main group species are comparatively much less explored. In this study, we computationally designed a few geometrically constrained borylenes supported by different carbenes. Our density functional theory studies indicate that they possess enhanced nucleophilicity as well as electrophilicity, thus rendering them promising candidates for exhibiting borylene-ligand cooperativity. The cooperation between the boron and adjacent carbene centers facilitates different bond activation processes, including the cycloaddition of acetylene across the boron-carbene bond as well as B-H/Si-H bond activation reactions, which have been analyzed in detail. To the best of our knowledge, the borylenes proposed in this study represent the first examples of theoretically proposed geometrically constrained bis(carbene)-stabilized borylenes capable of cooperative activation of enthalpically strong bonds.

Exploring the Impact of Elements on the Reactivity of a Straightforward Procedure for Generating Vinyl-Carbazole Derivatives via a Frustrated Lewis Pair Mechanism

The effect of chemical element on the reactivity for carbazolation reaction of phenylacetylene utilizing G13(C6F5)3 (Lewis acid) and G15-carbazole (Lewis base) was theoretically investigated using density functional theory (M06-2X-D3/def2-TZVP), where G13 represents Group 13 elements and G15 represents Group 15 elements. Through activation strain model (ASM) analysis, it is apparent that the reactivity of the entire carbazolation reaction is chiefly governed by the structural strain energy of the alkyne fragment. In other words, if G13(C6F5)3 or G15-carbazole features an atomic radius that is either too small (e.g., B atom) or too large (e.g., Tl or Bi atom), it results in inadequate orbital overlap between the reactants due to the impact of steric effects. This, in turn, results in an elevation of the activation energy for such reactions, thereby impeding the alkyne from undergoing the carbazole catalytic reaction. In light of the above analyses, our theoretical findings suggest that, except for Tl(C6F5)3, the other four Lewis acid catalysts (B(C6F5)3, Al(C6F5)3, Ga((C6F5)3, and In((C6F5)3) demonstrate effectiveness in catalyzing the carbazolation reaction of alkyne alongside with N-carbazole. Additionally, it is anticipated that, among the five categories of G15-carbazole molecules studied, only N-carbazole can participate in the carbazolation reaction with alkyne catalyzed by B(C6F5)3, considering both kinetic and thermodynamic factors at room temperature. Our theoretical investigations, as outlined in this study, indicate that the carbazolation reaction of the alkyne, catalyzed by G13(C6F5)3 and G15-carbazole, follows Hammond's postulate. To put it more plainly, when the transition state of the chemical reaction occurs earlier, it results in a decrease in activation energy.

Understanding the CO capture reaction through electronic structure analysis of four-membered-ring group-13/N- and B/group-15-based Lewis acid-base pairs

Incomplete combustion yields a significant byproduct, known for its high toxicity to humans: gas phase carbon monoxide (CO). This study utilized several advanced theoretical methods to examine the factors contributing to the activation energy involved in CO capture by a frustrated Lewis pair (FLP) and to forecast the potential success of the CO capture reaction. The current theoretical findings indicate that among the four-membered-ring Group-13/N-FLP and B/Group-15-FLP molecules, only the B/N-based FLP-type molecule effectively captures CO, considering both thermodynamics and kinetics. According to the results obtained through energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV), it can be concluded that the donor-acceptor (singlet-singlet) model, rather than the electron-sharing (triplet-triplet) model, effectively characterizes the electronic structures in the CO trapping reaction involving four-membered-ring G13/G15-FLPs. Theoretical findings, derived from EDA-NOCV and frontier molecular orbital theory, demonstrate that the CO capture reaction by G13/G15-FLP involves two distinct bonding interactions. The first interaction is characterized by FLP-to-CO forward bonding, with the lone pair of G15 (G13/G15-FLP) donating to the empty p-π* orbital of carbon (CO), which predominates. The second interaction involves CO-to-FLP backward bonding, where the empty σ* orbital of G13 (G13/G15-FLP) accepts the lone pair of carbon (CO), albeit to a lesser extent. In summary, our theoretical findings indicate that the G13-C and G15-C bonds in the G15/G15-TS species with a four-membered ring can be classified as two dative single bonds. The importance of the interaction between Lewis bases and CO surpasses that of the interaction between Lewis acids and CO. Theoretical evidences in this study demonstrate a linear connection between the G13-G15 bond length within the four-membered-ring G13/G15-FLP and the activation barrier linked to CO capture. The activation strain model analysis in this study suggests that the activation energy required for bond formation primarily depends on the geometric deformation energy of G13/G15-FLP in capturing CO. Our DFT investigation shows that Hammond's postulate is obeyed by the CO catching reaction of the four-membered-ring G13/N-FLP, meaning that an earlier transition state is associated with a lower activation barrier, but not with the CO catching reaction of the four-membered-ring B/G15-FLP.

Periselectivity and ambimodal transition states in cycloadditions of tetrachloro-o-benzoquinone with 6,6-dimethylfulvene

The reaction mechanism of cycloadditions of tetrachloro-o-benzoquinone with 6,6-dimethylfulvene were systematically investigated with density functional theory calculations. It was found that conditional primary interactions stabilize the ambimodal transition states in the endo pathways. Ambimodal transition states lead to [6 + 4]/[4 + 2] adducts or [4 + 2]/[2 + 4] adducts, which interconvert through 3,3-sigmatropic shift reactions. The substituent effects on periselectivity were also investigated.

Metal Influence on Cyaphide-Azide 1,3-Dipolar Cycloaddition Reactions: Aromaticity and Activation Strain

The factors governing 1,3-dipolar cycloaddition reactions involving C≡P-containing compounds are computationally explored in detail using quantum chemical tools. To this end, the parent process involving tBuN3 and tBuCP is analyzed and compared to the analogous reaction involving organometallic cyaphide complexes (metal=Au, Pt, Ge, Mg), in order to understand the role of the metal fragment in such transformations. It is found that while the metal fragment does not significantly influence the aromaticity of the corresponding concerted transition states or the regioselectivity of the transformation, it may modify the reactivity of the cyaphide complexes (i. e. Ge and Mg cyaphide complexes are comparatively more reactive). The computed reactivity trends and the factors behind the regioselectivity of the cycloaddition reaction are quantitatively analyzed with the help of the activation strain model in combination with the energy decomposition analysis method.

Understanding the reactivity and selectivity of Diels-Alder reactions involving furans

The reactivity and endo/exo selectivity of the Diels-Alder cycloaddition reactions involving furan and substituted furans as dienes have been computationally explored. In comparison to cyclopentadiene, it is found that furan is comparatively less reactive and also less endo-selective in the reaction with maleic anhydride as the dienophile. Despite that, both the reactivity and the selectivity can be successfully modified by the presence of substituents at either 2- or 3-positions of the heterocycle. In this sense, it is found that the presence of strong electron-donor groups significantly increases the reactivity of the system while the opposite is found in the presence of electron-withdrawing groups. The observed trends in both the reactivity and selectivity are analyzed quantitatively in detail by means of the activation strain model of reactivity in combination with the energy decomposition analysis methods.

Theoretical Study on Cooperation Catalysis of Chiral Guanidine/ Copper(I) in Asymmetric Azide-Alkyne Cycloaddition/[2 + 2] Cascade Reaction

Density functional theory (DFT) calculations with BP86-D3(BJ) functionals were employed to reveal the mechanism and stereoselectivity of chiral guanidine/copper(I) salt-catalyzed stereoselective three-component reaction among N-sulfonyl azide, terminal alkyne, and isatin-imine for spiroazetidinimines that was first reported by Feng and Liu (Angew. Chem. Int. Ed. 2018, 57, 16852-16856). For the noncatalytic cascade reaction, the denitrogenation to generate ketenimine species was the rate-determining step, with an activation barrier of 25.8-34.8 kcal mol^-1. Chiral guanidine-amide promoted the deprotonation of phenylacetylene, generating guanidine-Cu(I) acetylide complexes as active species. In azide-alkyne cycloaddition, copper acetylene coordinated to the O atom of the amide moiety in guanidium, and TsN₃ was activated by hydrogen bonding, affording the Cu(I)-ketenimine species with an energy barrier of 3.5∼9.4 kcal mol^-1. The optically active spiroazetidinimine oxindole was constructed via a stepwise four-membered ring formation, followed by deprotonation of guanidium moieties for C-H bonding in a stereoselective way. The steric effect of the bulky CHPh₂ group and chiral backbone in the guanidine, combined with the coordination between the Boc group in isatin-imine with a copper center, played important roles in controlling the stereoselectivity of the reaction. The major spiroazetidinimine oxindole product with an SS configuration was formed in a kinetically more favored way, which was consistent with the experimental observation.

η6 -Metalated Aryl Iodides in Diels-Alder Cycloaddition Reactions: Mode of Activation and Catalysis

The potential application of η⁶ -metalated aryl iodides as organocatalyst has been explored by means of computational methods. It is found that the enhanced halogen bonding donor ability of these species, in comparison with their demetalated counterparts, translates into a significant acceleration of the Diels-Alder cycloaddition reaction involving cyclohexadiene and methyl vinyl ketone. The factors behind this acceleration, the endo-exo selectivity of the process and the influence of the nature of the transition metal fragment in the activity of these species are quantitatively explored in detail by means of the combination of the Activation Strain Model of reaction and the Energy Decomposition Analysis methods.

Fluoro, Trifluoromethyl, and Trifluoroacetyl Substituent Effects on Cycloaddition Reactivities: Computations and Analysis

The importance of fluoro and trifluoromethyl substituents in drug effectiveness prompted the computational exploration of fluorine-containing substituents in valuable synthetic cycloadditions. Diels-Alder or 1,3-dipolar cycloaddition reactions of typical reactants, cyclopentadiene, N-phenyldiazoacetamide, tetrazine, and N-phenylsydnone involving fluorine-containing substituents (F, CF₃, and COCF₃) were studied with M06-2X density functional theory. Inductive and conjugative effects influence normal and inverse electron-demand reactions differently. These results provide a guide to the design and use of cycloadditions for the introduction of fluoro and trifluoromethyl substituents in synthetic processes.

Nitronium salts as mild and inexpensive oxidizing reagents toward designing efficient strategies in organic syntheses; A mechanistic investigation based on the DFT insights

Today, introducing and evaluating the performance of novel reagents are an undeniable part of designing a successful synthetic strategy. Herein, we study the efficiency and mechanism of recently synthesized nitronium salts (e.g., NO₂FSO₃, NO₂CF₃SO₃, NO₂HS₂O₇, NO₂BF₄, NO₂PF₆, and NO₂HSO₄) in the oxidation reaction of ethanol to acetic acid, as a model of the primary alcohol transformations to linear carboxylic acid. An aldehyde molecule is the first produced species in this reaction which is converted to the acetic acid molecule in the presence of in situ-produced nitric acid. Concerning the proposed mechanism, among the studied nitronium salts, two different behaviors can be observed in the transition state of the step in which the aldehyde molecule is formed. The calculated barrier energies of this step have been scrutinized by powerful descriptors such as Quantum Theory of Atoms in Molecules (QTAIM), Natural Bond Orbital (NBO), Electrostatic Potential (ESP) surfaces, and Activation Strain Model (ASM). The outcomes of the studied descriptors illustrate that nitronium salts have different performances in progressing the formation of the aldehyde molecule. Indeed, the likeness of the transition state of this step to the products for NO₂FSO₃, NO₂CF₃SO₃, and NO₂HS₂O₇ species is more significant than the others. Accordingly, these reagents have more potential to apply as oxidizing agents in the primary alcohol transformations to linear carboxylic acid.

Rationalizing the Substituent Effects in Diels-Alder Reactions of Triazolinediones with Anthracene

In this work we tackle the problem of the substituent effects in the Diels-Alder cycloadditions between triazolinediones (TADs) and anthracene. Experiments showed that aryl TADs substituted with electron-withdrawing groups (EWG) are more reactive than those substituted with electron-donating (EDG) or alkyl groups. However, the molecular origin of this preference is not yet understood. By a combination of methods including the activation strain model (ASM), energy decomposition analysis (EDA), molecular orbital (MO) theory, and conceptual density functional theory (CDFT), we disclosed the substituent effects of TADs. First, ASM/EDA analysis revealed that the reactivity of alkyl and aryl-substituted TADs is controlled by interaction energies, ΔE_int, which are ultimately defined by orbital interactions between frontier molecular orbitals. Moreover, alkyl-TADs are also controlled by the extent of strain at the transition state. The MO analysis suggested that the rate acceleration for EWG-substituted TADs is due to a more favorable orbital interaction between the HOMO of anthracene and the LUMO of the TADs, which is corroborated by calculations of charge transfer at the transition states. From CDFT, the chemical potential of anthracene is higher than those of TADs, indicating a flow of electron density from anthracene to TADs, in agreement with the results from the electrophilicity index.

Mechanistic Study of Asymmetric Alkynylation of Isatin-Derived Ketimine Mediated by a Copper/Guanidine Catalyst

In this work, we performed a mechanistic study of asymmetric alkynylation of isatin-derived N-Boc ketimine that was first reported by Feng, Liu, and co-workers (Chem. Commun. 2018, 54, 678-681). Guanidine-amide promoted the formation of highly nucleophilic copper acetylene species by abstracting the terminal proton of phenylacetylene with an imine moiety. The guanidinium salt-Cu(I) complex was the most active species in the addition of the C═N bond, in which copper acetylene coordinated to the O atom of the amide moiety, and the isatin-derived ketimine substrate was activated by hydrogen bonding as well as tert-butoxycarbonyl···Cu(I) coordination. Due to weak interaction between Cu(I) and the Ph group in the amide of guanidine, as well as the repulsion between the tert-butyl group in ketimine and the cyclohexyl group in guanidine, the copper acetylene preferred to attack isatin-derived ketimine from the re-face, leading to the S-configuration product with excellent stereoselectivity. The affinity of the counterion for the Cu(I) center in the copper salt affected the deprotonation of phenylacetylene and the formation of guanidinium salt active species. In contrast to CuBr and CuCl, the combination of CuI with aniline-derived guanidine-amide exhibited high catalytic activity and a chiral induction effect, contributing to a high turnover frequency (9.70 × 10^-4 s^-1) in catalysis and ee%.

Pericyclic reaction benchmarks: hierarchical computations targeting CCSDT(Q)/CBS and analysis of DFT performance

Hierarchical, convergent ab initio benchmark computations were performed followed by a systematic analysis of DFT performance for five pericyclic reactions comprising Diels-Alder, 1,3-dipolar cycloaddition, electrocyclic rearrangement, sigmatropic rearrangement, and double group transfer prototypes. Focal point analyses (FPA) extrapolating to the ab initio limit were executed via explicit quantum chemical computations with electron correlation treatments through CCSDT(Q) and correlation-consistent Gaussian basis sets up to aug'-cc-pV5Z. Optimized geometric structures and vibrational frequencies of all stationary points were obtained at the CCSD(T)/cc-pVTZ level of theory. The FPA reaction barriers and energies exhibit convergence to within a few tenths of a kcal mol-1. The FPA benchmarks were used to evaluate the performance of 60 density functionals (eight dispersion-corrected), covering the local-density approximation (LDA), generalized gradient approximations (GGAs), meta-GGAs, hybrids, meta-hybrids, double-hybrids, and range-separated hybrids. The meta-hybrid M06-2X functional provided the best overall performance [mean absolute error (MAE) of 1.1 kcal mol-1] followed closely by the double-hybrids B2K-PLYP, mPW2K-PLYP, and revDSD-PBEP86 [MAE of 1.4-1.5 kcal mol-1]. The regularly used GGA functional BP86 gave a higher MAE of 5.8 kcal mol-1, but it qualitatively described the trends in reaction barriers and energies. Importantly, we established that accurate yet efficient meta-hybrid or double-hybrid DFT potential energy surfaces can be acquired based on geometries from the computationally efficient and robust BP86/DZP level.

Reactivity of the superhalogen/superalkali ion encapsulating C60 fullerenes

The Diels-Alder cycloaddition reaction between 1,3-cyclohexadiene and a series of C₆₀ fullerenes with encapsulated (super)alkali/(super)halogen species (Li⁺@C₆₀, Li₂F⁺@C₆₀, Cl^-@C₆₀, and LiF₂^-@C₆₀) was explored by means of DFT calculations. The reactivity of the ion encapsulating systems was compared to that of the parent C₆₀ fullerene. Significant enhancement in reactivity was found for cation-encapsulating Li⁺/Li₂F⁺@C₆₀ complexes. The cycloadduct formed by LiF₂^-@C₆₀ was found to be the most thermodynamically favorable among the studied ones. In contrast, encapsulation of Cl^- anions disfavors the cycloaddition reaction both kinetically and thermodynamically. Higher activation energy barrier and less stability of the reaction product in the case of Cl^-@C₆₀ were associated with the higher deformation energies of the fullerene cage and the lower interaction energy between the reactants in comparison with the other studied complexes.

Influence of the CH/B replacement on the Reactivity of Boranthrene and Related Compounds

The influence of the replacement of CH groups by boron atoms on the reactivity of planar polycyclic aromatic hydrocarbons has been explored by means of computational tools. To this end, [4 + 2]-cycloaddition reactions involving anthracene and neutral boranthrene with different dienophiles such as ethylene, acetylene, and CO₂ have been compared. In addition, the influence of additional fused aromatic rings (pentacene or borapentacene) on the reactivity of these species has been also explored. It was found that the B-doped systems are systematically much more reactive than their all-carbon counterparts from both kinetic and thermodynamic points of view. The observed trends in reactivity are quantitatively analyzed in detail using state-of-the-art methods, namely, the activation strain model of reactivity and the energy decomposition analysis method. Our calculations reveal the importance of molecular orbital interactions as the key factor responsible for the enhanced reactivity of the B-doped systems.

Origin of asynchronicity in Diels-Alder reactions

Asynchronicity in Diels-Alder reactions plays a crucial role in determining the height of the reaction barrier. Currently, the origin of asynchronicity is ascribed to the stronger orbital interaction between the diene and the terminal carbon of an asymmetric dienophile, which shortens the corresponding newly formed C-C bond and hence induces asynchronicity in the reaction. Here, we show, using the activation strain model and Kohn-Sham molecular orbital theory at ZORA-BP86/TZ2P, that this rationale behind asynchronicity is incorrect. We, in fact, found that following a more asynchronous reaction mode costs favorable HOMO-LUMO orbital overlap and, therefore, weakens (not strengthens) these orbital interactions. Instead, it is the Pauli repulsion that induces asynchronicity in Diels-Alder reactions. An asynchronous reaction pathway also lowers repulsive occupied-occupied orbital overlap which, therefore, reduces the unfavorable Pauli repulsion. As soon as this mechanism of reducing Pauli repulsion dominates, the reaction begins to deviate from synchronicity and adopts an asynchronous mode. The eventual degree of asynchronicity, as observed in the transition state of a Diels-Alder reaction, is ultimately achieved when the gain in stability, as a response to the reduced Pauli repulsion, balances with the loss of favorable orbital interactions.

Mechanism and Selectivity of Cyclopropanation of 3-Alkenyl-oxindoles with Sulfoxonium Ylides Catalyzed by a Chiral N,N'-Dioxide-Mg(II) Complex

The mechanism and stereoselectivity of an asymmetric cyclopropanation reaction between 3-alkenyl-oxindole and sulfoxonium ylide catalyzed by a chiral N,N'-dioxide-Mg(II) complex were explored using the B3LYP-D3(BJ) functional and the def2-TZVP basis set. The noncatalytic reaction occurred via a stepwise mechanism, with activation barriers of 21.6-23.5 kcal mol^-1. The C₂-C_α bond formed followed by the carbanion S_N2 substitution, constructing a three-membered ring in spiro-cyclopropyl oxindoles, accompanied by the release of dimethylsulfoxide. The electron-withdrawing N-protecting t-butyloxy carbonyl (Boc) and acetyl (Ac) groups in isatin enhanced the local electrophilicity of the C2 atom and the repulsion between the two COPh groups in the reactants, contributing to high reactivity as well as good diastereoselectivity results. The N-Boc-3-phenacylideneoxindole coordinated to the chiral ligand (L-PiPr₂) in a bidentate fashion, forming a hexacoordinate-Mg(II) complex as the reactive species. The origin of enantioselectivity was from the shielding effect of 2,6-diisopropylphenyl groups in the ligand toward the si-face of oxindole. The repulsion between the SO(CH₃)₂ and COPh groups in 3-alkenyl-oxindole and the neighboring ortho-iPr group in the ligand directed the re-face of ylide to attack the re-face of oxindole preferably, contributing to the high diastereoselectivity of the product. A metal-ion-ligand matching relationship was important for a good asymmetric induction effect of the chiral N,N'-dioxide-metal catalyst. A large chiral cavity in the Zn(II) catalyst weakened the shielding effect of 2,6-diisopropylphenyl groups in the ligand toward the prochiral face of oxindole, leading to inferior enantioselectivity observed in the experiment.

Lewis Acid-Catalyzed Diels-Alder Reactions: Reactivity Trends across the Periodic Table

The catalytic effect of various weakly interacting Lewis acids (LAs) across the periodic table, based on hydrogen (Group 1), pnictogen (Group 15), chalcogen (Group 16), and halogen (Group 17) bonds, on the Diels-Alder cycloaddition reaction between 1,3-butadiene and methyl acrylate was studied quantum chemically by using relativistic density functional theory. Weakly interacting LAs accelerate the Diels-Alder reaction by lowering the reaction barrier up to 3 kcal mol-1 compared to the uncatalyzed reaction. The reaction barriers systematically increase from halogen Collapse

Key Words

• Diels-Alder reaction
• Lewis acids
• activation strain model
• density functional calculations
• reactivity

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MESH Headings Collapse

Grants

• Netherlands Organization for Scientific Research (NWO)
• Dutch Astrochemistry Network (DAN)
• PID2019-106184GB-I00 Spanish MICIIN
• RED2018-102387-T Spanish MICIIN
• Nederlandse Organisatie voor Wetenschappelijk Onderzoek
• Dutch Astrochemistry Network
• Nederlandse Organisatie voor Wetenschappelijk Onderzoek

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Affiliation(s)

Pascal Vermeeren Department of Theoretical ChemistryAmsterdam Institute of Molecular and Life Sciences (AIMMS)Amsterdam Center for Multiscale Modeling (ACMM)Vrije Universiteit AmsterdamDe Boelelaan 10831081 HVAmsterdam (TheNetherlands
Marco Dalla Tiezza Department of Theoretical ChemistryAmsterdam Institute of Molecular and Life Sciences (AIMMS)Amsterdam Center for Multiscale Modeling (ACMM)Vrije Universiteit AmsterdamDe Boelelaan 10831081 HVAmsterdam (TheNetherlands
Michelle van Dongen Department of Theoretical ChemistryAmsterdam Institute of Molecular and Life Sciences (AIMMS)Amsterdam Center for Multiscale Modeling (ACMM)Vrije Universiteit AmsterdamDe Boelelaan 10831081 HVAmsterdam (TheNetherlands
Israel Fernández Departamento de Química Orgánica I and Centro de Innovación en Química Avanzada (ORFEO-CINQA)Facultad de Ciencias QuímicasUniversidad Complutense de Madrid28040MadridSpain
F. Matthias Bickelhaupt Department of Theoretical ChemistryAmsterdam Institute of Molecular and Life Sciences (AIMMS)Amsterdam Center for Multiscale Modeling (ACMM)Vrije Universiteit AmsterdamDe Boelelaan 10831081 HVAmsterdam (TheNetherlands Institute for Molecules and MaterialsRadboud UniversityHeyendaalseweg 1356525 AJNijmegen (TheNetherlands
Trevor A. Hamlin Department of Theoretical ChemistryAmsterdam Institute of Molecular and Life Sciences (AIMMS)Amsterdam Center for Multiscale Modeling (ACMM)Vrije Universiteit AmsterdamDe Boelelaan 10831081 HVAmsterdam (TheNetherlands

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A novel partitioning scheme for the application of the distortion/interaction - activation strain model to intramolecular reactions

The distortion/interaction or activation strain model, developed by Houk and Bickelhaupt, relates chemical reactivity to the reagents deformations and reciprocal electronic influences. However, in its original formulation, it struggles to elucidate the mechanistic insights of intramolecular reactions, those unimolecular processes in which two parts of a molecule, the reaction centers, linked by a connector, are brought together to yield a different chemical species. Here we present a modification of the distortion/interaction procedure for its application on intramolecular reactions. This new procedure allows the calculation of the influence exerted by the connector over the reaction pathway in an indirect way, from the distortions of the two reaction centers and their interaction energy. This procedure does not include additional, undesired interactions and offers the possibility of calculating very large connectors in a computationally inexpensive way. We applied this methodology in the normal electron-demand Diels–Alder reaction of 1,3,8-nonatriene derivatives, with different functionalizations and connector lengths. In-depth analysis of the IRC showed that the reaction pathway can be subdivided in three main regions, what we called the oncoming, conversion and relaxation phases, each of them characterized by different evolutions of the distortion and interaction energies, and with clear geometry changes. We suggest that this new formulation can provide additional information for intramolecular reactions, especially to those processes for which the connector is said to play a crucial role in the observed reaction rates.

Catalysis by Bidentate Iodine(III)-Based Halogen Donors: Surpassing the Activity of Strong Lewis Acids

The poorly understood mode of activation and catalysis of bidentate iodine(III)-based halogen donors have been quantitatively explored in detail by means of state-of-the-art computational methods. To this end, the uncatalyzed Diels–Alder cycloaddition reaction between cyclohexadiene and methyl vinyl ketone is compared to the analogous process mediated by a bidentate iodine(III)-organocatalyst and by related, highly active iodine(I) species. It is found that the bidentate iodine(III)-catalyst accelerates the cycloaddition by lowering the reaction barrier up to 10 kcal mol^–1 compared to the parent uncatalyzed reaction. Our quantitative analyses reveal that the origin of the catalysis is found in a significant reduction of the steric (Pauli) repulsion between the diene and dienophile, which originates from both a more asynchronous reaction mode and a significant polarization of the π-system of the dienophile away from the incoming diene. Notably, the activity of the iodine(III)-catalyst can be further enhanced by increasing the electrophilic nature of the system. Thus, novel systems are designed whose activity actually surpasses that of strong Lewis acids such as BF₃.

Asymmetric Cyanation of Activated Olefins with Ethyl Cyanoformate Catalyzed by Ti(IV)-Catalyst: A Theoretical Study

The reaction mechanism and origin of asymmetric induction for conjugate addition of cyanide to the C=C bond of olefin were investigated at the B3LYP-D3(BJ)/6-31+G**//B3LYP-D3(BJ)/6-31G**(SMD, toluene) theoretical level. The release of HCN from the reaction of ethyl cyanoformate (CNCOOEt) and isopropanol (HOiPr) was catalyzed by cinchona alkaloid catalyst. The cyanation reaction of olefin proceeded through a two-step mechanism, in which the C-C bond construction was followed by H-transfer to generate a cyanide adduct. For non-catalytic reaction, the activation barrier for the rate-determining C-H bond construction step was 34.2 kcal mol−1, via a four-membered transition state. The self-assembly Ti(IV)-catalyst from tetraisopropyl titanate, (R)-3,3′-disubstituted biphenol, and cinchonidine accelerated the addition of cyanide to the C=C double bond by a dual activation process, in which titanium cation acted as a Lewis acid to activate the olefin and HNC was orientated by hydrogen bonding. The steric repulsion between the 9-phenanthryl at the 3,3′-position in the biphenol ligand and the Ph group in olefin raised the Pauli energy (ΔE≠Pauli) of reacting fragments at the re-face attack transition state, leading to the predominant R-product.

Stereoretention in styrene heterodimerisation promoted by one-electron oxidants

Radical cations generated from the oxidation of C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C π-bonds are synthetically useful reactive intermediates for C–C and C–X bond formation. Radical cation formation, induced by sub-stoichiometric amounts of external oxidant, are important intermediates in the Woodward–Hoffmann thermally disallowed [2 + 2] cycloaddition of electron-rich alkenes. Using density functional theory (DFT), we report the detailed mechanisms underlying the intermolecular heterodimerisation of anethole and β-methylstyrene to give unsymmetrical, tetra-substituted cyclobutanes. Reactions between trans-alkenes favour the all-trans adduct, resulting from a kinetic preference for anti-addition reinforced by reversibility at ambient temperatures since this is also the thermodynamic product; on the other hand, reactions between a trans-alkene and a cis-alkene favour syn-addition, while exocyclic rotation in the acyclic radical cation intermediate is also possible since C–C forming barriers are higher. Computations are consistent with the experimental observation that hexafluoroisopropanol (HFIP) is a better solvent than acetonitrile, in part due to its ability to stabilise the reduced form of the hypervalent iodine initiator by hydrogen bonding, but also through the stabilisation of radical cationic intermediates along the reaction coordinate.

A computational study details the mechanism, catalytic cycle and origins of stereoselectivity underlying hole-catalyzed intermolecular alkene heterodimerisation to give unsymmetrical, tetra-substituted cyclobutanes.

Understanding the reactivity of polycyclic aromatic hydrocarbons and related compounds

This perspective article summarizes recent applications of the combination of the activation strain model of reactivity and the energy decomposition analysis methods to the study of the reactivity of polycyclic aromatic hydrocarbons and related compounds such as cycloparaphenylenes, fullerenes and doped systems. To this end, we have selected representative examples to highlight the usefulness of this relatively novel computational approach to gain quantitative insight into the factors controlling the so far not fully understood reactivity of these species. Issues such as the influence of the size and curvature of the system on the reactivity are covered herein, which is crucial for the rational design of novel compounds with tuneable applications in different fields such as materials science or medicinal chemistry.

Cooperative Catalysis of Chiral Guanidine and Rh2(OAc)4 in Asymmetric O-H Insertion of Carboxylic Acid: A Theoretical Investigation

We reported a mechanistic study on asymmetric O-H insertion reaction of α-diazoester with carboxylic acid using Rh₂(OAc)₄/chiral guanidine-amide as the cocatalyst by density functional theory [B3LYP-D3(BJ)/def2-TZVP//B3LYP-D3(BJ)/[6-31G**, SDD] (SMD, Et₂O)]. The catalytic reaction included two stages: (i) formation of Rh-carbene species, subsequently by the construction of C-O bond forming enol and (ii) chiral guanidinium salt-assisted H-transfer to the enol. In cooperative catalysis, Rh₂(OAc)₄ helped to form an enol intermediate via high-reactivity Rh-carbene species, while the in situ-formed guanidium carboxylate acted as a chiral proton shuttle to construct a hydrogen bonding net for the stereo-determinant protonation. The repulsions between the phenyl group of the enol intermediate and the cyclohexyl as well as the ortho-substituted isopropyl group of chiral guanidine played important roles in controlling stereoselectivity. A disadvantageous steric arrangement in si-face attack weakened the stabilizing electrostatic and orbital interaction of reacting species in the H-transfer step, enhancing the pathway to form a predominant product with R-configuration in the two competing pathways. A model was proposed to explain the asymmetric induction of chiral guanidine-amide in protonation.

Remarkable Reactivity of Boron-Substituted Furans in the Diels-Alder Reactions with Maleic Anhydride

The reactivity of boron-substituted furans as dienes in the Diels-Alder reaction with maleic anhydride has been investigated. Gratifyingly, the furans with boryl substituents at C-3 gave the exo cycloadduct exclusively with excellent yields. In particular, the potassium trifluoroborate exhibited outstanding reactivity at room temperature. Theoretical calculations suggested that the trifluoroborate group is highly activating and also that the thermodynamics is the main factor that determines whether the products can be obtained efficiently or not.

exo/endo Selectivity Control in Diels-Alder Reactions of Geminal Bis(silyl) Dienes: Theoretical and Experimental Studies

The factors controlling the reactivity and exo/ endo selectivity of Diels-Alder reactions of geminal bis(silyl) dienes catalyzed by AlEt₂Cl are studied at the B3LYP-D3(BJ)(SMD,CH₂Cl₂)/6-31++G**//B3LYP-D3(BJ)(SMD,CH₂Cl₂)/6-31+G* theoretical level. The reaction proceeds via a two-stage one-step mechanism, and the AlEt₂Cl as a Lewis acid catalyst enhances the electrophilicity of the carbonyl compound by coordination, consequently accelerating a cycloaddition reaction with a low energy barrier. A geminal bis(silyl) group of the diene and an α-substituent in α,β-unsaturated carbonyl compounds adjust the interaction energy (Δ E_int) as well as the deformation energy (Δ E_strain) of the diene and dienophile, affecting the barrier height and the diastereochemical outcome accordingly. The steric repulsion between the geminal bis(silyl) group and Al(III) catalyst increases the Pauli repulsion energy (Δ E_Pauli) and strain energy of dienophile fragment (Δ E_{strain(dienophile-LA)}) in the endo pathway, ensuring the exo selectivity. The introduction of a halogen atom (Cl or Br) or methyl group at the α-position of α,β-unsaturated carbonyl compounds increases the deformation energy of the diene fragment. Meanwhile, the noncovalent interactions (that is, dispersion and electrostatic interaction) stabilize the endo transition state, leading to predominant endo products. The theoretical predictions of the exo/ endo selectivity for Diels-Alder reactions of the substituted α,β-unsaturated carbonyl compound with Cl or Br atoms by the DFT method are also well confirmed by experiment.

Molecular design principles towards exo-exclusive Diels–Alder reactions

The exo selective Diels–Alder reactions, reported as special cases, usually involve catalytic reaction conditions and specific cyclic structural motifs on the diene and/or the dienophile. Here we report a systematic computational investigation on the substituent effect for simple, linear dienes and dienophiles towards exo control in Diels–Alder reactions under thermal conditions. Through detailed characterization of reaction pathways for Diels–Alder cycloadditions between linear dienes and dienophiles with various substituents, we summarize a set of design principles aiming for an optimal and nearly-exclusive exo selectivity. These results shall lead to valuable guidelines and more versatile strategies in organic synthesis that are in accordance with the principles of green chemistry.

Exo-exclusive stereoselectivity for simple, terminal-substituted dienes and dienophiles may be achieved under thermal conditions through a delicate control of substituent identities.

Influence of the charge on the reactivity of azafullerenes

The influence of the charge on the Diels-Alder reactivity of azafullerenes (C59N+ and C59N-) has been computationally explored by means of density functional theory calculations. In addition, the regioselectivity of the process has been investigated and compared to the analogous cycloaddition reaction involving the parent neutral azahydro[60]fullerene C59NH. It is found that the [4+2]-cycloaddition reaction between C59N+ and cyclopentadiene, which occurs concertedly through a synchronous transition state, proceeds with a lower activation barrier and is more exothermic than the analogous process involving C59NH. In contrast, the anionic C59N- counterpart is clearly less reactive. This reactivity trend is quantitatively analyzed in detail by means of the activation strain model of reactivity in combination with the energy decomposition analysis method. It is found that the frontier molecular orbital interactions are not responsible for the observed reactivity trend but the Pauli repulsion between closed-shells mainly governs the transformation.

Mechanistic Insights into the Ni-Catalyzed Reductive Carboxylation of C-O Bonds in Aromatic Esters with CO2 : Understanding Remarkable Ligand and Traceless-Directing-Group Effects

The mechanism of the Ni⁰ -catalyzed reductive carboxylation reaction of C(sp² )-O and C(sp³ )-O bonds in aromatic esters with CO₂ to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C-O bond cleavage, reductive elimination, and/or CO₂ insertion. Of these steps, C-O bond cleavage was found to be rate-determining, and it occurred through either oxidative addition to form a Ni^II intermediate, or a radical pathway that involved a bimetallic species to generate two Ni^I species through homolytic dissociation of the C-O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp² )-O and C(sp³ )-O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni⁰ catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two Ni^I species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C-O bonds were also employed to investigate several experimentally observed phenomena, including ligand-dependent reactivity and site-selectivity.

Rationalizing the Regioselectivity of the Diels-Alder Biscycloaddition of Fullerenes

The physical factors governing the regioselectivity of the double functionalization of fullerenes have been explored by means of density functional theory calculations. To this end, the second Diels-Alder cycloaddition reactions involving 1,3-butadiene and the parent C₆₀ fullerene as well as the ion-encapsulated system Li⁺@C₆₀ have been selected. In agreement with previous experimental findings on related processes, it is found that the cycloaddition reaction, involving either C₆₀ or Li⁺@C₆₀, occurs selectively at specific [6,6]-bonds. The combination of the activation strain model of reactivity and the energy decomposition analysis methods has been applied to gain a quantitative understanding into the markedly different reactivity of the available [6,6]-bonds leading to the observed regioselectivity in the transformation.

Computational insight into the cooperative role of non-covalent interactions in the aza-Henry reaction catalyzed by quinine derivatives: mechanism and enantioselectivity

Density functional theory (DFT) calculations were performed to elucidate the mechanism and the origin of the high enantioselectivity of the aza-Henry reaction of isatin-derived N-Boc ketimine catalyzed by a quinine-derived catalyst (QN). The C-C bond formation step is found to be both the rate-determining and the stereo-controlled step. The results revealed the important role of the phenolic OH group in pre-organizing the complex of nitromethane and QN and stabilizing the in situ-generated nitronate and protonated QN. Three possible activation modes for C-C bond formation involving different coordination patterns of catalyst and substrates were studied, and it was found that both the ion pair-hydrogen bonding mode and the Brønsted acid-hydrogen bonding mode are viable, with the latter slightly preferred for the real catalytic system. The calculated enantiomeric excess (ee) favouring the S enantiomer is in good agreement with the experimental result. The high reactivity and enantioselectivity can be ascribed to the cooperative role of the multiple non-covalent interactions, including classical and non-classical H bonding as well as anionπ interactions. These results also highlight the importance of the inclusion of dispersion correction for achieving a reasonable agreement between theory and experiment for the current reaction.

Studies on the interactions of 5-R-3-(2-pyridyl)-1,2,4-triazines with arynes: inverse demand aza-Diels–Alder reaction versus aryne-mediated domino process

The interactions between substituted 5-R-3-(pyridyl-2)-1,2,4-triazines with in situ generated substituted aryne intermediates have been studied.

Factors Governing the Diels-Alder Reactivity of (2,7)Pyrenophanes

The physical factors governing the Diels-Alder reactivity of (2,7)pyrenophanes have been computationally explored using state-of-the-art Density Functional Theory calculations. It is found that the [4 + 2]-cycloaddition reactions between these cyclophanes and tetracyanoethylene, which occur concertedly through highly asynchronous transition states, proceed with lower activation barriers and are more exothermic than the analogous process involving the parent planar pyrene. The influence of the bent equilibrium geometry of the pyrenophane as a function of the length of the bridge as well as the nature of the tether on the transformation are analyzed in detail. By means of the Activation Strain Model of reactivity and the Energy Decomposition Analysis methods, a detailed quantitative understanding of the reactivity of this particular family of cyclophanes is presented.