Competition between the one-step and two-step, zwitterionic mechanisms in the [2+3] cycloaddition of gem-dinitroethene with (Z)-C,N-diphenylnitrone: a DFT computational study

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

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

On the Question of Zwitterionic Intermediates in the [3+2] Cycloaddition Reactions: A Critical Review

New discoveries require a fundamental revision of the view on the mechanism of the 32CAreaction (according to the older nomenclature defined as 1,3-dipolar cycloaddition reactions). The view of the one-step, “concerted” mechanism of such processes developed in the 20-century is very popular today, both in academic literature and among organic chemists who do not specialize in such transformations. Meanwhile, more and more reports bring examples of reactions that clearly cannot be treated as processes without intermediates. However, these examples are documented very differently. In addition to comprehensive studies using many complementary research techniques, there are also reports in which the presence of intermediates in the cycloaddition environment is postulated on the basis of very unreliable premises. This review is an attempt at a critical analysis and systematization of data in the presented area.

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.

An expedient synthesis of new imino-thiazolidinone grafted dispiro-pyrrolidine-oxindole/indeno hybrids via a multicomponent [3+2] cycloaddition reaction in a deep eutectic solvent

A facile and selective synthesis of novel factionalized dispiro-pyrrolidines via a three component [3+2] cycloaddition reaction using a deep eutectic solvent.

Role of Water in the Reaction Mechanism and endo/exo Selectivity of 1,3-Dipolar Cycloadditions Elucidated by Quantum Chemistry and Machine Learning

Asymmetric 1,3-dipolar cycloadditions of azomethine ylides with activated olefins are among the most important and versatile methods for the synthesis of enantioenriched pyrroline and pyrrolidine derivatives. Despite both theoretical and practical importance, the role of water molecules in the reactivity and endo/exo selectivity remains unclear. To explore how water accelerates the reactions and improves the endo/exo selectivity of the cycloadditions of 1,3-dipole phthalazinium-2-dicyanomethanide (1) and two dipolarophiles, an ab initio-quality neural network potential that overcomes the computational bottleneck of explicitly considering water molecules was used. It is demonstrated that not only the nature of both the dipolarophile and the 1,3-dipole, but also the solvent medium, can perturb or even alter the reaction mechanism. An extreme case was found for the reaction of 1,3-dipole 1 with methyl vinyl ketone, in which the reaction mechanism changes from a concerted to a stepwise mode on going from MeCN to H₂ O as solvent, with formation of a zwitterionic intermediate that is a very shallow minimum on the energy surface. Thus, high stereocontrol can still be expected despite the stepwise nature of the mechanism. The results indicate that water can induce global polarization along the reaction coordinate and highlight the role of microsolvation effects and bulk-phase effects in reproducing the experimentally observed aqueous acceleration and enhanced endo/exo selectivity.

Understanding the mechanism of the 1,3-dipolar cycloaddition reaction between a thioformaldehyde S-oxide and cyclobutadiene: Competition between the stepwise and concerted routes

Changing the mechanism of the widely used 1,3-dipolar cycloaddition reaction from its usual asynchronous one-step pattern to the rarely observed stepwise form leads to the emergence of intermediates, side products, and other impurities. Thus, it is crucial to determine the nature of the mechanism of the 1,3-dipolar cycloaddition reaction between a special 1,3-dipole and a specified dipolarophile (by theoretical methods) before using them for synthesizing a desired product. In this study, therefore, we have investigated the possibility of some probable intermediates emergence in the 1,3-dipolar cycloaddition reaction between cyclobutadiene and thioformaldehyde S-oxide. The results showed that emergence of Int (B) (−52.1 kcal mol−1) via transition state (B-1) is favorable both thermodynamically and kinetically (in comparison with all other stepwise routes). That is, developing probable impurities should not be neglected at least in the cases of the reactions between some thioformaldehyde S-oxide and some dipolarophiles.

A Molecular Electron Density Theory Study of the Reactivity and Selectivities in [3 + 2] Cycloaddition Reactions of C,N-Dialkyl Nitrones with Ethylene Derivatives

The zw-type [3 + 2] cycloaddition (32CA) reactions of C,N-dialkyl nitrones with a series of ethylenes of increased electrophilic character have been studied within the Molecular Electron Density Theory (MEDT) at the MPWB1K/6-311G(d,p) computational level. Both, reactivity and selectivities are rationalized depending on the polar character of the reaction. Due to the strong nucleophilic character of C,N-dialkyl nitrones, the corresponding zw-type 32CA reactions are accelerated with the increased electrophilic character of the ethylene, which also plays a crucial role in the reaction mechanism, thus determining the regio- and stereoselectivities experimentally observed. While, in the 32CA reactions with nucleophilic ethylenes, the reaction begins with the formation of the C-C single bond, determining the ortho regioselectivity, in the 32CA reactions with strong electrophilic ethylenes, the reaction begins with the formation of the C-O single bond involving the β-conjugated carbon of the ethylene, determining the meta regioselectivity. The present MEDT study also provides an explanation for the unexpected ortho regioselectivity experimentally found in the 32CA reactions involving weak electrophilic ethylenes such as ethyl acrylate and acrylonitrile.

On Searching for a Stepwise Channel for the Mechanism of a 1,3-Dipolar Cycloaddition between a Thiocarbonyl S-Oxide and C20 Fullerene using Born–Oppenheimer ab Initio QM/MM Molecular Dynamics

The probability of existence of a stepwise route which is in parallel with the well-known concerted pathway for the mechanism of 1,3-dipolar cycloaddition is debated by many researchers. As the route is stepwise, it would lead to emergence of at least a metastable intermediate which would produce some stereoisomers such as enantiomers and diastereomers. In 1986 the first clear stepwise example for a 1,3-dipolar cycloaddition was reported by Huisgen where an electron-poor alkene was reacted with a thiocarbonyl ylide. Since then, researchers have focused on the thiocarbonyl ylide 1,3-dipolar group in finding more stepwise examples. It was found that some reactions of a thiocarbonyl ylide with some dipolarophiles proceeded by a stepwise route, while others did not.

This situation led us to investigate the probability of existence a stepwise route in parallel with the concerted path for the reaction of methyl thiocarbonyl S-oxide and C20 fullerene as a chemically active and nano-dimension electronegative alkene. To give more reliable data, Born–Oppenheimer ab initio QM/MM molecular dynamics was used.

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

ABSTRACT

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

GRAPHICAL ABSTRACT

Regio- and stereoselective synthesis of novel isoxazolidine heterocycles by 1,3-dipolar cycloaddition between C-phenyl-N-methylnitrone and substituted alkenes. Experimental and DFT investigation of selectivity and mechanism

A series of isoxazolidine heterocycles was synthesized through the 1,3-dipolar cycloaddition (13DC) reaction of C-phenyl-N-methylnitrone with different substituted alkenes.

A DFT computational study of the molecular mechanism of [3 + 2] cycloaddition reactions between nitroethene and benzonitrile N-oxides

DFT calculations were performed to shed light on the molecular mechanism of [3 + 2] cycloadditions of simple conjugated nitroalkenes to benzonitrile N-oxides. In particular, it was found that these processes proceed by a one-step mechanism through asynchronous transition states. According to the latest terminology, they should be considered polar but not stepwise processes.

Beyond the Alternatives that Switch the Mechanism of the 1,3-Dipolar CyCloadditions from Concerted to Stepwise or Vice Versa: A Literature Review

For several decades, the concerted or stepwise character of the mechanism of the 1,3-dipolar cycloaddition reaction has been one of the most debated issues in the field of organic chemistry. The significance of this problem is due to the fact that in a catalyst-free 1,3-dipolar cycloaddition, when the mechanism switches from concerted to stepwise, the stereospecificity is lost and thus unwanted stereoisomers may emerge. The first proposals about the mechanism of the 1,3-dipolar reaction were due to Huisgen (concerted model) and subsequently by Firestone (two-step diradical channel) in the 1960s. After a decade of debate, most researchers accepted the concerted model for the reaction, but during these years, researchers reported some examples of the stepwise mechanism for catalyst-free 1,3-dipolar cycloadditions. This review attempts to find a number of factors that could influence the reaction channels and switch the mechanism from concerted to stepwise, or vice versa.

Selective sensing of ozone and the chemically active gaseous species of the troposphere by using the C20 fullerene and graphene segment

OZONE is a key species in forming a layer in the atmosphere of earth that brings vita for our planet and supports the complex life. This three-atom molecule in the ozone-layer, is healing the earth's ecosystem by protecting it from dangerous rays of the sun. Until this molecule is in the stratosphere, it would support the natural order of the life; but, when it appears in our environment, damages will begin against us. In this project, we have tried to find a new way for beaconing ozone species in our environment via physical adsorption by the C₂₀ fullerene and graphene segment as a sensor. To find the selectivity of this nano-sized segment in sensing ozone (O₃), compared to the usual chemically active gasses of the troposphere like O₂, N₂, CO₂, H₂O, CH₄, H₂, and CO, the density of state (DOS) plots were analyzed, for each interacting species. The results showed that ozone could significantly change the electrical conductivity of C₂₀ fullerene, for each adsorption step. Thus, this fullerene could clearly sense ozone in different adsorption steps; while, the graphene segment could do this only at the second step adsorption (/ΔE_g-B/=0.016eV) (at the first adsorption step the /ΔE_g-A/ is 0.00eV).

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

Abstract

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

Graphical abstract

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

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

Effect of Steric Congestion on the Stepwise Character and Synchronicity of a 1,3-Dipolar Reaction of a Nitrile Ylide and an Olefin

It has been hypothesised that steric congestion could play a key role in directing the 1,3-dipolar reaction mechanism in its shift from a concerted to a stepwise model. An example is given of the influence of steric congestion on the concertedness of a 1,3-dipolar reaction mechanism. Keeping other influences untouched, including the effects of electron-withdrawing or electron-donating groups, the nature of the dipole or dipolarophile, solvent, as well as the catalytic effects which have been kept fixed, this study focuses on the steric effect. At least in the case of the 1,3-dipolar cycloaddition of nitrile ylide and common olefins, when steric congestion serves as the only influence, it is unable to change the reaction mechanism from concerted into stepwise.

A polar nature of benzoic acids extrusion from nitroalkyl benzoates: DFT mechanistic study

Using DFT calculations at various theory levels, quantum-chemical simulations of decomposition paths were performed for a series of nitroalkyl benzoates. It was discovered, that these reactions proceed via polar, but one-step mechanism. It turned out that depending on the nature of the substituent in the ester molecule and on medium polarity, the studied reactions may take place via transition states with varied synchronicity—from E1-like structures, to E1cb-like structures. A purely ionic, two-stage mechanism was not identified in any of the cases.

Nitroacetylene as dipolarophile in [2 + 3] cycloaddition reactions with allenyl-type three-atom components: DFT computational study

Abstract

[2 + 3] Cycloaddition reactions of nitroacetylene with allenyl-type three-atom components take place according to the polar, but a one-step mechanism. Alternatively to cycloadducts, during the reaction between the aforementioned reagents, zwitterionic structures with “extended” conformation may be formally created. However, this route is supported by neither kinetic nor thermodynamic factors.

Graphical Abstract

Understanding the molecular mechanism in a regiospecific [3 + 2] cycloaddition reaction including C–O and C–S interactions: an ELF topological analysis

Regiospecific 32CA reaction of nitrone 10 toward thioketone 11 takes place via a non-concerted one-step mechanism confirmed by ELF analysis.

First efficient synthesis of SF5-substituted pyrrolidines using 1,3-dipolar cycloaddition of azomethine ylides with pentafluorosulfanyl-substituted acrylic esters and amides

Di-, tri- and tetrasubstituted pentafluorosulfanylated pyrrolidines have been efficiently synthetized via 1,3-dipolar cycloaddition of azomethine ylides.

In the searching for zwitterionic intermediates on reaction paths of [3 + 2] cycloaddition reactions between 2,2,4,4-tetramethyl-3-thiocyclobutanone S-methylide and polymerizable olefins

DFT calculations show that the [3 + 2]-cycloaddition of 2,2,4,4-tetramethyl-3-thiocyclobutanone S-methylide with nitroethene takes place according to a polar, two-step mechanism with a zwitterionic intermediate.

Features of styryl dye crystal packings and their influence on [2 + 2] photocycloaddition reaction with single crystal retention

Only in two of three crystal modifications the initial dye is subjected to the SC–SC [2 + 2]-photocycloaddtion under visible light irradiation.

A new mechanistic insight on β-lactam systems formation from 5-nitroisoxazolidines

DFT calculations at various theory levels unequivocally show that conversion of 5-nitroisoxazolidines into β-lactams does not take place via a ring opening step (as hitherto suggested), but takes place according to a three-step, domino-type reaction.

Inter molecular azide–diisocyanate coupling: new insights for energetic solid propellants

The paper brings to light a fundamental understanding of azide–diisocyanate reaction in polymers containing hydroxyl and azide groups. This is a competitive pathway for conventional urethane reactions which has been overlooked previously.

Regio- and stereoselectivity of polar [2+3] cycloaddition reactions between (Z)-C-(3,4,5-trimethoxyphenyl)-N-methylnitrone and selected (E)-2-substituted nitroethenes

[2+3] Cycloaddition reactions of the highly reactive (Z)-C-(3,4,5-trimethoxyphenyl)-N-methylnitrone with (E)-2-R-nitroethenes proceed under mild conditions and yield mixtures of stereoisomeric 2-methyl-3-(3,4,5-trimethoxyphenyl)-4-nitro-5-R-isoxazolidines. The effect of regiospecificity of the cycloadditions may be accounted for by the theory of electrophilicity indexes. Stereoselectivity, however, is determined by a compilation of steric and secondary orbital effects.

A computational investigation into the substituent effect on the chemo- and stereoselectivity of crossed intermolecular radical anion [2 + 2] cycloadditions of enones

The chemoselectivity of cycloaddition is caused by the bulky group on the C atoms which form σ bond in the first step. The stereoselectivity mainly caused by the difference in steric interaction between the trans and cis transition states.

An experimental and theoretical study of the polar [2+3] cycloaddition reactions between 1-chloro-1-nitroethene and (Z)-C-aryl-N-phenylnitrones

Abstract Kinetic studies and B3LYP/6-31g(d) calculations indicate the polar nature of [2+3] cycloadditions between 1-chloro-1-nitroethene to (Z)-C-aryl-N-phenylnitrones. This is clearly confirmed by the activation parameters and the substituent and solvent effects. Graphical abstract

The reaction mechanism of the [2+3] cycloaddition between α-phenylnitroethene and (Z)-C,N-diphenylnitrone in the light of a B3LYP/6-31G(d) computational study

Abstract The analysis of reactivity indices suggests the polar nature of the [2+3] cycloaddition of a-phenylnitroethene to (Z)-C,N-diphenylnitrone. Similar conclusions can be drawn from the investigation of the reaction pathways using the B3LYP/6-31g(d) algorithm. This shows that the cycloaddition mechanism depends on the polarity of the reaction medium. A one-step mechanism is followed in the gas phase and toluene in all the theoretically possible pathways. In more polar media (nitromethane, water), a zwitterionic, two-step rather than a one-step mechanism occurs in the pathway leading to 3,4-trans-2,3,5-triphenyl-4-nitroisoxazolidine. Graphical abstract