A Molecular Electron Density Theory Study of the Role of the Copper Metalation of Azomethine Ylides in [3 + 2] Cycloaddition Reactions

Mechanisms and origins of stereoselectivity in the NHC-catalyzed oxidative reaction of enals and pyrroles: a density functional theory study

The mechanism, chemoselectivity and stereoselectivity in the NHC-catalyzed reaction between enals and pyrroles for the synthesis of 5,6-dihydroindolizine were studied using DFT calculations. The cycle for catalytic generation of 5,6-dihydroindolizine proceeds via seven steps: (1) addition of the NHC to enal, (2) formation of a Breslow intermediate through [1,2]-proton transfer, (3) oxidation, (4) Michael addition, (5) [2+2] cycloaddition, (6) liberation of NHCs and (7) decarboxylation. Our results show that the presence of DMAP·H+ lowers the barrier for [1,2]-proton transfer. In addition, NHC·H+ plays a key role in decarboxylation. Michael addition which involves the formation of a new C-C bond was identified to be the chemo- and stereoselectivity-determining step, leading to the experimentally observed 5S,6R-dihydroindolizine. Analysis of the noncovalent interactions revealed that the observed stereoselectivity is attributed to the differential weak interactions (CH⋯π, LP⋯π and CH⋯CH) involved in the transition states during the Michael addition step. The computational results not only rationalize experimental observations but also provide some useful information for the future design of new catalytic processes.

Mechanisms and Stereoselectivities in the NHC-Catalyzed [4 + 2] Annulation of 2-Bromoenal and 6-Methyluracil-5-carbaldehyde

To disclose the reaction mechanism and selectivity in the NHC-catalyzed reaction of 2-bromoenal and 6-methyluracil-5-carbaldehyde, a systematic computational study has been performed. According to DFT computations, the catalytic cycle is divided into eight elementary steps: nucleophilic attack of the NHC on 2-bromoenal, 1,2-proton transfer, C-Br bond dissociation, 1,3-proton transfer, addition to 6-methyluracil-5-carbaldehyde, [2 + 2] cycloaddition, NHC dissociation, and decarboxylation. The Bronsted acid DABCO·H+ plays a crucial role in proton transfer and decarboxylation steps. The addition to 6-methyluracil-5-carbaldehyde determines both chemoselectivity and stereoselectivity, leading to R-configured carbocycle-fused uracil, in agreement with experimental results. NCI analysis indicates that the CH···N, CH···π, and LP···π interactions should be the key factor for determining the stereoselectivity. ELF analysis shows the main role of the NHC in promoting C-Br bond dissociation. The mechanistic insights obtained in the present work may guide the rational design of potential NHC catalysts.

Electron-rich benzofulvenes as effective dipolarophiles in copper(i)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides

A series of benzofulvenes without any electron-withdrawing substituents were employed as 2π-type dipolarophiles for the first time to participate in Cu(i)-catalyzed asymmetric 1,3-dipolar cycloaddition (1,3-DC) reactions of azomethine ylides. An intrinsic non-benzenoid aromatic characteristic from benzofulvenes serves as a key driving force for activation of the electron-rich benzofulvenes. Utilizing the current methodology, a wide range of multi-substituted chiral spiro-pyrrolidine derivatives containing two contiguous all-carbon quaternary centers were formed in good yield with exclusive chemo-/regioselectivity and high to excellent stereoselectivity. Computational mechanistic studies elucidate the origin of the stereochemical outcome and the chemoselectivity, in which the thermostability of these cycloaddition products is the major factor.

Unveiling the high reactivity of experimental pseudodiradical azomethine ylides within molecular electron density theory

The [3+2] cycloaddition (32CA) reactions of N-methyl azomethine ylide (AY) with styrene, benzaldehyde and methyl 2-formyl-benzoate (MFB) were studied within molecular electron density theory (MEDT), at the ωB97X-D/6-311G(d) computational level, in order to characterize the reactivity of an experimental pseudodiradical TAC for the first time. ELF topological analysis indicates that AY presents a pseudodiradical structure. Analysis of CDFT reactivity indices allows classifying AY as a supernucleophile; while styrene is classified as a moderate electrophile, benzaldehyde and MFB are classified as strong electrophiles. The 32CA reaction with MFB is the most favorable one with a relatively low activation Gibbs free energy of 6.9 kcal mol^-1, being irreversible and completely endo stereo- and chemo-selective towards the carbonyl group, a behavior predicted by the analysis of the Parr functions. The bonding evolution theory (BET) study indicates that while the 32CA reaction of AY with styrene is characterized as a pdr-type 32CA reaction, the one involving benzaldehyde follows a pmr-type mechanism prompted by the presence of the carbonyl group. The present MEDT study describes in detail the tunable high reactivity of one of the few experimentally available pseudodiradical TACs, showing that the mechanism of 32CA reactions can be modified not only by changing the electronic structure of TACs through proper substitution but also by the nature of their opposing ethylene derivative.

A molecular electron density theory study on the Chichibabin reaction: The origin of regioselectivity

A Molecular Electron Density Theory (MEDT) study was performed on the nucleophilic amination of pyridine and benzene to elucidate the observed regioselectivity in the Chichibabin reaction. For this purpose, three possible reaction paths were considered between NaNH₂ and the 2-, 3- and 4-positions of pyridine. The reaction of NaNH₂ and benzene was also investigated. The results indicated that in each reaction, the first step involves the nucleophilic attack of the NH₂‾ ion toward the aromatic system to produce an anionic intermediate. The second step, proceeds via hydride elimination of the anionic intermediate to produce the corresponding aromatic amine. This step is more favourable both kinetically and thermodynamically for the reaction at the 2-position of pyridine relative to the 4-position. In contrast to the Parr functions analysis which indicates that the 4-position is activated more electrophilically in pyridine, the PES analysis reveals that the 2-position attack with the NH₂‾ ion is more favourable both thermodynamically and kinetically. The results for the reaction of sodium amide and benzene indicated that this reaction is not feasible due to the high activation barriers. In consistent with the experimental results, the ELF analysis indicated that in the first step of the reaction between pyridine and sodium amide, the formation of the C2-N bond takes place via a pseudoradical coupling between the C2 carbon atom of pyridine and the N atom of NaNH₂. These variations occur in the second-half of the first step of the reaction. NCI analysis on the reaction revealed that the more attractive forces between the fragments, make the transition states for the 2-position attack more stable relative to the others. Thus, the NCI analysis can satisfactorily describe the observed regioselectivity in the Chichibabin reaction.

Synthesis, Characterization, and Cytotoxicity of New Spirooxindoles Engrafted Furan Structural Motif as a Potential Anticancer Agent

A new series of spirooxindoles based on ethylene derivatives having furan aryl moiety are reported. The new hybrids were achieved via [3 + 2] cycloaddition reaction as an economic one-step efficient approach. The final constructed spirooxindoles have four contiguous asymmetric carbon centers. The structure of 3a is exclusively confirmed using X-ray single crystal diffraction. The supramolecular structure of 3a is controlled by O···H, H···H, and C···C intermolecular contacts. It includes layered molecules interconnected weak C-H···O (2.675 Å), H···H (2.269 Å), and relatively short Cl···Br interhalogen interactions [3.4500(11)Å]. Using Hirshfeld analysis, the percentages of these intermolecular contacts are 10.6, 25.7, 6.4, and 6.2%, respectively. The spirooxindoles along with ethylene derivatives having furan aryl moiety were assessed against breast (MCF7) and liver (HepG2) cancer cell lines. The results indicated that the new chalcone 3b showed excellent activity in both cell lines (MCF7 and HepG2) with IC₅₀ = 4.1 ± 0.10 μM/mL (MCF7) and 3.5 ± 0.07 μM/mL (HepG2) compared to staurosporine with 4.3 and 2.92 folds. Spirooxindoles 6d (IC₅₀ = 4.3 ± 0.18 μM/mL), 6f (IC₅₀ = 10.3 ± 0.40 μM/mL), 6i (IC₅₀ = 10.7 ± 0.38 μM/mL), and 6j (IC₅₀ = 4.7 ± 0.18 μM/mL) exhibited potential activity against breast adenocarcinoma, while compounds 6d (IC₅₀ = 6.9 ± 0.23 μM/mL) and 6f (IC₅₀ = 3.5 ± 0.11 μM/mL) were the most active hybrids against human liver cancer cell line (HepG2) compared to staurosporine [IC₅₀ = 17.8 ± 0.50 μM/mL (MCF7) and 10.3 ± 0.23 μM/mL (HepG2)]. Molecular docking study exhibited the virtual mechanism of binding of compound 3b as a dual inhibitor of EGFR/CDK-2 proteins, and this may highlight the molecular targets for its cytotoxic activity.

A Molecular Electron Density Theory Study of the [3+2] Cycloaddition Reaction of an Azomethine Ylide with an Electrophilic Ethylene Linked to Triazole and Ferrocene Units

The [3+2] cycloaddition (32CA) reaction of an azomethine ylide (AY) with an electrophilic ethylene linked to triazole and ferrocene units has been studied within the Molecular Electron Density Theory (MEDT) at the ωB97X-D/6-311G(d,p) level. The topology of the electron localization function (ELF) of this AY allows classifying it as a pseudo(mono)radical species characterized by the presence of two monosynaptic basins, integrating a total of 0.76 e, at the C1 carbon. While the ferrocene ethylene has a strong electrophilic character, the AY is a supernucleophile, suggesting that the corresponding 32CA reaction has a high polar character and a low activation energy. The most favorable ortho/endo reaction path presents an activation enthalpy of 8.7 kcal·mol^-1, with the 32CA reaction being exergonic by -42.1 kcal·mol^-1. This reaction presents a total endo stereoselectivity and a total ortho regioselectivity. Analysis of the global electron density transfer (GEDT) at the most favorable TS-on (0.23 e) accounts for the high polar character of this 32CA reaction, classified as forward electron density flux (FEDF). The formation of two intermolecular hydrogen bonds between the two interacting frameworks at the most favorable TS-on accounts for the unexpected ortho regioselectivity experimentally observed.

Synthesis and Structure Elucidation of Novel Spirooxindole Linked to Ferrocene and Triazole Systems via [3 + 2] Cycloaddition Reaction

In the present work, a novel heterocyclic hybrid of a spirooxindole system was synthesized via the attachment of ferrocene and triazole motifs into an azomethine ylide by [3 + 2] cycloaddition reaction protocol. The X-ray structure of the heterocyclic hybrid (1″R,2″S,3R)-2″-(1-(3-chloro-4-fluorophenyl)-5-methyl-1H-1,2,3-triazole-4-carbonyl)-5-methyl-1″-(ferrocin-2-yl)-1″,2″,5″,6″,7″,7a″-hexahydrospiro[indoline-3,3″-pyrrolizin]-2-one revealed very well the expected structure, by using different analytical tools (FTIR and NMR spectroscopy). It crystallized in the triclinic-crystal system and the P-1-space group. The unit cell parameters are a = 9.1442(2) Å, b = 12.0872(3) Å, c = 14.1223(4) Å, α = 102.1700(10)°, β = 97.4190(10)°, γ = 99.1600(10)°, and V = 1484.81(7) ų. There are two molecules per unit cell and one formula unit per asymmetric unit. Hirshfeld analysis was used to study the molecular packing of the heterocyclic hybrid. H···H (50.8%), H···C (14.2%), Cl···H (8.9%), O···H (7.3%), and N···H (5.1%) are the most dominant intermolecular contacts in the crystal structure. O···H, N···H, H···C, F···H, F···C, and O···O are the only contacts that have the characteristic features of short and significant interactions. AIM study indicated predominant covalent characters for the Fe–C interactions. Also, the electron density (ρ(r)) at the bond critical point correlated inversely with the Fe–C distances.

A Molecular Electron Density Theory Study of the [3+2] Cycloaddition Reaction of Pseudo(mono)radical Azomethine Ylides with Phenyl Vinyl Sulphone

The [3+2] cycloaddition (32CA) reaction of an azomethine ylide (AY), derived from isatin and L-proline, with phenyl vinyl sulphone has been studied within Molecular Electron Density Theory (MEDT) at the ωB97X-D/6-311G(d,p) level. ELF topological analysis of AY classifies it as a pseudo(mono)radical species with two monosynaptic basins at the C1 carbon, integrating a total of 0.76 e. While vinyl sulphone has a strong electrophilic character, AY is a supernucleophile, suggesting a high polar character and low activation energy for the reaction. The nucleophilic Parr functions indicate that the pseudoradical C1 carbon is the most nucleophilic center. The 32CA reaction presents an activation Gibbs free energy of 13.1 kcal·mol−1 and is exergonic by −26.8 kcal·mol−1. This reaction presents high endo stereoselectivity and high meta regioselectivity. Analysis of the global electron density transfer (GEDT) at the most favorable meta/endo TS, 0.31 e, accounts for the high polar character of this 32CA reaction, classified by forward electron density flux (FEDF). A Bonding Evolution Theory (BET) study along the most favorable meta/endo reaction path characterizes this 32CA reaction, taking place through a non-concerted two-stage one-step mechanism, as a pseudo(mono)radical-type 32CA reaction, in agreement with the ELF analysis of the AY.

Unveiling the intramolecular [3 + 2] cycloaddition reactions of C,N-disubstituted nitrones from the molecular electron density theory perspective

The zwitterionic type intramolecular [3 + 2] cycloaddition reactions of nitrones show remarkable substituent effects and varying reactivities in the cases of cyclic and acyclic alkenes.

Synthesis of Spirooxindole Analogs Tethered Pyrazole Scaffold as Acetylcholinesterase Inhibitors

A new series of spirooxindole analogs tethered pyrazole scaffold constructed via [3+2] cycloaddition (32CA) reaction starting from the new chalcone named (E)‐3‐(5‐chloro‐3‐methyl‐1‐phenyl‐1H‐pyrazol‐4‐yl)‐1‐(5‐methyl‐1‐phenyl‐1H‐pyrazol‐4‐yl)prop‐2‐en‐1‐one which confirmed by single crystal X‐ray diffraction analysis. Synthesized spirooxindole analogs were evaluated for their neuroprotection through the inhibition of acetylcholine esterase enzyme using Ellman's method. Compounds 9 w, 9 e and 9 x showed the strongest acetylcholine esterase inhibition (AChEI) with IC50 values of 5.7, 7.8 and 8.3 μM, respectively. Obviously, the incorporation of NO2 group into isatin 5th position and N‐methylglycine (sarcosine) play a crucial role for the activity which lead to compound 9 w had the most potent inhibitory activity with IC50 value of 5.7 μM. Molecular docking was used to study their interaction with the active site of hAChE. These 32CA reactions takes place via a one‐step mechanism with a high polar character as a consequence of the supernucleophilic character of azomethine yildes and the strong electrophilic character of ethylenes.

[3+2] Cycloaddition Reaction for the Stereoselective Synthesis of a New Spirooxindole Compound Grafted Imidazo[2,1-b]thiazole Scaffold: Crystal Structure and Computational Study

A new spirooxindole hybrid engrafted imidazo[2,1-b]thiazole core structure was designed and achieved via [3+2] cycloaddition reaction approach. One multi-component reaction between the ethylene derivative based imidazo[2,1-b]thiazole scaffold with 6-Cl-isatin and the secondary amine under heat conditions afforded the desired compound in a stereoselective manner. The relative absolute configuration was assigned based on single-crystal X-ray diffraction analysis. Hirshfeld calculations for 4 revealed the importance of the H…H (36.8%), H…C (22.9%), Cl…H (10.4%) and S…H (6.6%), as well as the O…H (4.7%), N…H (5.3%), Cl…C (1.6%), Cl…O (1.0%) and N…O (0.5%) contacts in the crystal stability. DFT calculations showed excellent straight-line correlations (R2 = 0.9776–0.9962) between the calculated and experimental geometric parameters. The compound has polar nature (3.1664 Debye). TD-DFT and GIAO calculations were used to assign and correlate the experimental UV-Vis and NMR spectra, respectively.

Construction of Spirooxindole Analogues Engrafted with Indole and Pyrazole Scaffolds as Acetylcholinesterase Inhibitors

Twenty-five new hits of spirooxindole analogs 8a-y engrafted with indole and pyrazole scaffolds were designed and constructed via a [3+2]cycloaddition (32CA) reaction starting from three components: new chalcone-based indole and pyrazole scaffolds 5a-d, substituted isatins 6a-c, and secondary amines 7a-d. The potency of the compounds were assessed in modulating cholinesterase (AChE) activity using Ellman's method. Compounds 8i and 8y showed the strongest acetylcholine esterase inhibition (AChEI) with IC50 values of 24.1 and 27.8 μM, respectively. Molecular docking was used to study their interaction with the active site of hAChE.

Insight into the Mechanism and Regioselectivity of Pd(OAc)2-Catalyzed C-O Bond Activation via a β-O Elimination Approach: A Computational Study

The density functional theory investigations were carried out to elucidate the mechanism and the origin of regioselectivity for the Pd(OAc)₂-catalyzed carbon-oxygen bond activation in the reaction between 4-phenoxy-N-(quinolin-8-yl) butanamide and N-methylindole. The reaction proceeded through four main stages in succession: C-H activation, β-O elimination, nucleo-palladation of the new C-C bond formation, and proto-depalladation steps. A total of six pathways were considered since there were two possible forms of C-O bond breaking in the β-O elimination step and six reaction channels of nucleophilic attack in the crucial nucleo-palladation step. The computational results indicate that the common first step (C-H bond activation step) occurs via a concerted metalation deprotonation (CMD) mechanism. The nucleo-palladation was the rate-determining step for all six reaction pathways. The results also show that the most favorable pathway for the whole reaction is the one (denoted as path b1) in which phenol was removed in the second stage and the hydrogen atom of N-methylindole attacked the oxygen atom of acetate group of the intermediate in the third stage. According to the analyses of noncovalent interaction (NCI) and the reduced density gradient (RDG), the most favored pathway benefits from the strong attractive interaction and weak repulsive interaction in its key transition state. Furthermore, structural, natural bond orbital charge, and energy analyses of the transition states reveal the origin of the regioselectivity. This is a good explanation of the experimental phenomenon and benefits future design of a new strategy for a similar reaction.

Stereoselective Synthesis of the Di-Spirooxindole Analogs Based Oxindole and Cyclohexanone Moieties as Potential Anticancer Agents

A new series of di-spirooxindole analogs, engrafted with oxindole and cyclohexanone moieties, were synthesized. Initially, azomethine ylides were generated via reaction of the substituted isatins 3a-f (isatin, 3a, 6-chloroisatin, 3b, 5-fluoroisatin, 3c, 5-nitroisatin, 3d, 5-methoxyisatin, 3e, and 5-methylisatin, 3f, and (2S)-octahydro-1H-indole-2-carboxylic acid 2, in situ azomethine ylides reacted with the cyclohexanone based-chalcone 1a-f to afford the target di-spirooxindole compounds 4a-n. This one-pot method provided diverse structurally complex molecules, with biologically relevant spirocycles in a good yields. All synthesized di-spirooxindole analogs, engrafted with oxindole and cyclohexanone moieties, were evaluated for their anticancer activity against four cancer cell lines, including prostate PC3, cervical HeLa, and breast (MCF-7, and MDA-MB231) cancer cell lines. The cytotoxicity of these di-spirooxindole analogs was also examined against human fibroblast BJ cell lines, and they appeared to be non-cytotoxic. Compound 4b was identified as the most active member of this series against prostate cancer cell line PC3 (IC50 = 3.7 ± 1.0 µM). The cyclohexanone engrafted di-spirooxindole analogs 4a and 4l (IC50 = 7.1 ± 0.2, and 7.2 ± 0.5 µM, respectively) were active against HeLa cancer cells, whereas NO2 substituted isatin ring and meta-fluoro-substituted (2E,6E)-2,6-dibenzylidenecyclohexanone containing 4i (IC50 = 7.63 ± 0.08 µM) appeared to be a promising agent against the triple negative breast cancer MDA-MB231 cell line. To explore the plausible mechanism of anticancer activity of di-spirooxindole analogs, molecular docking studies were investigated which suggested that spirooxindole analogs potentially inhibit the activity of MDM2.

Understanding the Participation of Fluorinated Azomethine Ylides in Carbenoid-Type [3 + 2] Cycloaddition Reactions with Ynal Systems: A Molecular Electron Density Theory Study

The carbenoid-type (cb-type) 32CA reaction of 1,1-difluoroated azomethine ylide (DFAY) with phenylpropynal has been studied using the molecular electron density theory (MEDT). Electron localization function (ELF) characterizes DFAY as a carbenoid species participating in cb-type 32CA reactions. The supernucleophilic character of DFAY and the strong electrophilic character of the ynal cause this polar 32CA reaction to have an unappreciable barrier; the reaction, which is highly exothermic, presents total chemo- and regioselectivity. ELF topological analysis of the bonding changes along the reaction establishes its non-concerted two-stage one-step mechanism, in which the nucleophilic attack of the carbenoid carbon of DFAY on the electrophilic carbonyl carbon of the ynal characterizes the cb-type reactivity of this three-atom component (TAC). The presence of two fluorines at DFAY modifies the pseudodiradical structure and reactivity of the simplest azomethine ylide to that of a carbenoid TAC participating in cb-type 32CA reactions toward electrophilic ethylenes.

Unveiling the Unexpected Reactivity of Electrophilic Diazoalkanes in [3+2] Cycloaddition Reactions within Molecular Electron Density Theory

The [3+2] cycloaddition (32CA) reactions of strongly nucleophilic norbornadiene (NBD), with simplest diazoalkane (DAA) and three DAAs of increased electrophilicity, have been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G (d,p) computational level. These pmr-type 32CA reactions follow an asynchronous one-step mechanism with activation enthalpies ranging from 17.7 to 27.9 kcal·mol−1 in acetonitrile. The high exergonic character of these reactions makes them irreversible. The presence of electron-withdrawing (EW) substituents in the DAA increases the activation enthalpies, in complete agreement with the experimental slowing-down of the reactions, but contrary to the Conceptual DFT prediction. Despite the nucleophilic and electrophilic character of the reagents, the global electron density transfer at the TSs indicates rather non-polar 32CA reactions. The present MEDT study establishes the depopulation of the N–N–C core in this series of DAAs with the increase of the EW character of the substituents present at the carbon center is responsible for the experimentally found deceleration.

Unprecedented Regio- and Stereoselective Synthesis of Pyrene-Grafted Dispiro[indoline-3,2'-pyrrolidine-3',3″-indolines]: Expedient Experimental and Theoretical Insights into Polar [3 + 2] Cycloaddition

A series of dispiro[indoline-3,2'-pyrrolidine-3',3″-indolines] was synthesized via a multicomponent polar [3 + 2] cycloaddition (32CA) reaction of isatin derivatives, sarcosine and (E)-3-(2-oxo-2-(pyren-1-yl)ethylidene)indolin-2-one derivatives. The regio- and stereochemistries of the cycloadducts were established on the basis of one-dimensional (1D) (1H-, 13C-, 13C-CRAPT NMR) and two-dimensional (2D) homonuclear and heteronuclear correlation NMR spectrometry experiments (1H-1H gDQFCOSY, 13C-1H-HSQCAD, 13C-1H-HMBCAD, 1H-1H-ROESYAD). The molecular mechanism and regio- and stereoselectivities of the cycloaddition (CA) reaction have been investigated utilizing a density functional theory (DFT) method and were thoroughly explained based on the transition-state stabilities and global/local electrophilicity/nucleophilicity reactivity indices of the reactants.

Which Stereoinductor Is Better for Asymmetric Functionalization of α-Amino Acids in a Nickel(II) Coordination Environment? Experimental and DFT Considerations

The results of extended comparative investigation of nickel(II) Schiff base complexes (containing various auxiliary chiral moieties) commonly used as a methodological platform for the asymmetric synthesis of tailor-made α-amino acids are provided. The following issues are addressed: 1) redox activity (determining the possibility for electrochemically induced reactions); 2) quantitative estimation of the reactivity of deprotonated complexes towards electrophiles; and 3) quantum-chemical estimation of noncovalent interactions in the metal coordination environment (which shed light on the origin of the stereochemical outcome observed for different stereoinductors). Possible mechanisms that determine the relationship between the stereochemical configuration of a molecule and its electronic structure are discussed. The DFT-calculated HOMO-LUMO energies and localization, as well as relative energies for the (S)- and (R)-alanine derivatives, that determine the stereoinduction efficiency in thermodynamically controlled reactions in nickel(II) coordination are provided. The computational data are supported by experimental results on the monobenzylation of glycine derivatives.

Elucidating the origin of selectivity of [3 + 2]-cycloaddition reactions between thioketone and carbohydrate-derived nitrones by the DFT

The mechanism and origin of selectivity for [3 + 2]-cycloaddition (32CA) reactions between thioketone and carbohydrate-derived nitrones in THF were investigated by using the density functional theory (DFT) at the M06-2X/6-311+G(d,p)//M06-2X/6-31+G(d,p) level of theory combined with the solvation SMD model. The calculated results revealed that the 32CA reactions proceed through the asynchronous one-step manner. For the chemoselectivity in thioketone, the C=S bond as a dipolarophile attacking three-atom-component (TAC) nitrone in reactivity was more preferential than the C=O bond. The theoretical results also confirmed the stereoselectivity of two 32CA reactions of thioketone with carbohydrate-derived nitrones with the anti-form product being more favored than the syn-form product, and the predicted anti/syn product ratios are in agreement with the experimental ones in literature. Furthermore, the analysis of the conceptual density functional theory reactivity indices showed that the 32CA reactions have polar character. Weak noncovalent interaction and Parr function analyses are used to reveal the origin of the stereoselectivity. Graphical abstract [3 + 2]-cycloaddition reactions between thioketone and carbohydrate-derived nitrones.

Understanding the Molecular Mechanism of the Rearrangement of Internal Nitronic Ester into Nitronorbornene in Light of the MEDT Study

The characterization of the structure of nitronic esters and their rearrangement into nitronorbornene reactions has been analyzed within the Molecular Electron Density Theory (MEDT) using Density Functional Theory (DFT) calculations at the B3LYP/6-31G(d) computational level. Quantum-chemical calculations indicate that this rearrangement takes place according to a one-step mechanism. The sequential bonding changes received from the Bonding Evolution Theory (BET) analysis of the rearrangement of internal nitronic ester to nitronorbornene allowed us to distinguish seven different phases. This fact clearly contradicts the formerly-proposed concerted pericyclic mechanism.