Organics

21 pages, 2778 KiB

Open AccessReview

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

by Radomir Jasiński and Ewa Dresler

Organics 2020, 1(1), 49-69; https://0-doi-org.brum.beds.ac.uk/10.3390/org1010005 - 26 Nov 2020

Cited by 46 | Viewed by 5204

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, [...] Read more.

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. Full article

(This article belongs to the Special Issue Cycloaddition Reaction in Organic Synthesis)

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13 pages, 5374 KiB

Open AccessArticle

Participation of Phosphorylated Analogues of Nitroethene in Diels–Alder Reactions with Anthracene: A Molecular Electron Density Theory Study and Mechanistic Aspect

by Agnieszka Kącka-Zych

Organics 2020, 1(1), 36-48; https://0-doi-org.brum.beds.ac.uk/10.3390/org1010004 - 23 Nov 2020

Cited by 14 | Viewed by 3510

Abstract

The structure and the contribution of the bis(2-chloroethyl) 2-nitro 1a and 2-bromo-2-nitroethenylphosphonates 1b with anthracene 2 in the Diels–Alder (DA) reactions have been studied within the Molecular Electron Density Theory (MEDT) at the B3LYP functional together with 6-31G(d), 6-31+G(d) and 6-31+G(d,p) basic sets. [...] Read more.

The structure and the contribution of the bis(2-chloroethyl) 2-nitro 1a and 2-bromo-2-nitroethenylphosphonates 1b with anthracene 2 in the Diels–Alder (DA) reactions have been studied within the Molecular Electron Density Theory (MEDT) at the B3LYP functional together with 6-31G(d), 6-31+G(d) and 6-31+G(d,p) basic sets. Analysis of the Conceptual Density Functional Theory (CDFT) reactivity indices indicates that 1a and 1b can be classified as a strong electrophile and marginal nucleophile, while 2 is classified as a strong electrophile and strong nucleophile. The studied DA reactions take place through a one-step mechanism. A Bonding Evolution Theory (BET) of the one path associated with the DA reaction of 1a with 2 indicates that it is associated with non-concerted two-stage one-step mechanism. BET analysis shows that the first C2-C3 single bond is formed in Phase VI, while the second C1-C6 single bond is formed in the Phase VIII. The formation of both single bonds occurs through the merging of two C2 and C3, C1 and C6 pseudoradical centers, respectively. Full article

(This article belongs to the Special Issue Cycloaddition Reaction in Organic Synthesis)

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17 pages, 3924 KiB

Open AccessArticle

Understanding the Origin of the Regioselectivity in Non-Polar [3+2] Cycloaddition Reactions through the Molecular Electron Density Theory

by Luis R. Domingo, Mar Ríos Gutiérrez and Jorge Castellanos Soriano

Organics 2020, 1(1), 19-35; https://0-doi-org.brum.beds.ac.uk/10.3390/org1010003 - 13 Nov 2020

Cited by 13 | Viewed by 3795

Abstract

The regioselectivity in non-polar [3+2] cycloaddition (32CA) reactions has been studied within the Molecular Electron Density Theory (MEDT) at the B3LYP/6-311G(d,p) level. To this end, the 32CA reactions of nine simplest three-atom-components (TACs) with 2-methylpropene were selected. The electronic structure of the reagents [...] Read more.

The regioselectivity in non-polar [3+2] cycloaddition (32CA) reactions has been studied within the Molecular Electron Density Theory (MEDT) at the B3LYP/6-311G(d,p) level. To this end, the 32CA reactions of nine simplest three-atom-components (TACs) with 2-methylpropene were selected. The electronic structure of the reagents has been characterized through the Electron Localisation Function (ELF) and the Conceptual DFT. The energy profiles of the two regioisomeric reaction paths and ELF topology of the transition state structures are studied to understand the origin of the regioselectivity in these 32CA reactions. This MEDT study permits to conclude that the least electronegative X1 end atom of these TACs controls the asynchronicity in the C−X (X=C, N, O) single bond formation, and consequently, the regioselectivity. This behaviour is a consequence of the fact that the creation of the non-bonding electron density required for the formation of the new single bonds has a lower energy demand at the least electronegative X1 atom than at the Z3 one. Full article

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16 pages, 3069 KiB

Open AccessFeature PaperArticle

Understanding the Reactivity of Trimethylsilyldiazoalkanes Participating in [3+2] Cycloaddition Reactions towards Diethylfumarate with a Molecular Electron Density Theory Perspective

by Luis R. Domingo, Nivedita Acharjee and Haydar A. Mohammad-Salim

Organics 2020, 1(1), 3-18; https://0-doi-org.brum.beds.ac.uk/10.3390/org1010002 - 16 Oct 2020

Cited by 11 | Viewed by 3095

Abstract

A Molecular Electron Density Theory (MEDT) study is presented here for [3+2] cycloaddition (32CA) reactions of three trimethylsilyldiazoalkanes with diethyl fumarate. The presence of silicon bonded to the carbon of these silyldiazoalkanes changes its structure and reactivity from a pseudomonoradical to that of [...] Read more.

A Molecular Electron Density Theory (MEDT) study is presented here for [3+2] cycloaddition (32CA) reactions of three trimethylsilyldiazoalkanes with diethyl fumarate. The presence of silicon bonded to the carbon of these silyldiazoalkanes changes its structure and reactivity from a pseudomonoradical to that of a zwitterionic one. A one-step mechanism is predicted for these polar zw-type 32CA reactions with activation enthalpies in CCl₄ between 8.0 and 19.7 kcal·mol⁻¹ at the MPWB1K (PCM)/6-311G(d,p) level of theory. The negative reaction Gibbs energies between −3.1 and −13.2 kcal·mole⁻¹ in CCl₄ suggests exergonic character, making the reactions irreversible. Analysis of the sequential changes in the bonding pattern along the reaction paths characterizes these zw-type 32CA reactions. The increase in nucleophilic character of the trimethylsilyldiazoalkanes makes these 32CA reactions more polar. Consequently, the activation enthalpies are decreased and the TSs require less energy cost. Non-covalent interactions at the TSs account for the stereoselectivity found in these 32CA reactions involving the bulky trimethylsilyl group. Full article

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