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Keywords = biradical-zwitterions

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18 pages, 2229 KiB  
Article
The Puzzle of the New Type of Intermediate in the Course of [2 + 2] Cycloaddition with the Participation of Conjugated Nitroalkenes: MEDT Computational Study
by Radomir Jasiński and Agnieszka Kącka-Zych
Molecules 2025, 30(11), 2410; https://doi.org/10.3390/molecules30112410 - 30 May 2025
Viewed by 691
Abstract
The phenomena of regio- and stereoselectivity and the molecular mechanism of the [2 + 2] cycloaddition reaction between (E)-2-arylnitroethenes and the ynamine molecular system were analyzed using wb97xd/6-311 + G(d) (PCM) quantumchemical calculations. It was found that, independently of the stepwise nature of [...] Read more.
The phenomena of regio- and stereoselectivity and the molecular mechanism of the [2 + 2] cycloaddition reaction between (E)-2-arylnitroethenes and the ynamine molecular system were analyzed using wb97xd/6-311 + G(d) (PCM) quantumchemical calculations. It was found that, independently of the stepwise nature of the cycloaddition, the full retention of the stereoconfiguration of the nitroalkene can be interpreted and explained. Next, the analysis of the electronic properties of the localized reaction intermediate suggests its possible zwitterionic nature. Additionally, the solvent and the substituent effect on the reaction course were also evaluated. In consequence, the proposed mechanism can be treated as general for some groups of [2 + 2] cycloaddition processes. Lastly, for the model process, the full Bonding Evolution Theory (BET) analysis along the reaction coordinate was performed. It was found that the [2 + 2] cycloaddition reaction between (E)-2-phenylonitroethene and ynamine begins with the formation of two pseudoradical centers at the C2 and C3 atoms. First, a C2-C3 single bond is formed in phase V by combining two pseudoradical centers, while the formation of a second C4-C1 single bond begins at the last, eleventh phase of the reaction path. A BET analysis of intermediate (I) allows it to be classified as a compound with a pseudoradical structure. Next to zwitterions and biradicals, it is evidently new type of intermediate on the path of the [2 + 2] cycloaddition reaction. Full article
(This article belongs to the Section Organic Chemistry)
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12 pages, 4015 KiB  
Article
Photoinduced Processes in Lysine-Tryptophan-Lysine Tripeptide with L and D Tryptophan
by Aleksandra A. Ageeva, Roman S. Lukyanov, Sofia O. Martyanova, Ilya M. Magin, Alexander I. Kruppa, Nikolay E. Polyakov, Victor F. Plyusnin, Alexander B. Doktorov and Tatyana V. Leshina
Int. J. Mol. Sci. 2023, 24(4), 3331; https://doi.org/10.3390/ijms24043331 - 7 Feb 2023
Cited by 2 | Viewed by 2407
Abstract
Optical isomers of short peptide Lysine-Tryptophan-Lysine (Lys-{L/D-Trp}-Lys) and Lys-Trp-Lys with an acetate counter-ion were used to study photoinduced intramolecular and intermolecular processes of interest in photobiology. A comparison of L- and D-amino acid reactivity is also the focus of scientists’ attention in various [...] Read more.
Optical isomers of short peptide Lysine-Tryptophan-Lysine (Lys-{L/D-Trp}-Lys) and Lys-Trp-Lys with an acetate counter-ion were used to study photoinduced intramolecular and intermolecular processes of interest in photobiology. A comparison of L- and D-amino acid reactivity is also the focus of scientists’ attention in various specialties because today, the presence of amyloid proteins with D-amino acids in the human brain is considered one of the leading causes of Alzheimer’s disease. Since aggregated amyloids, mainly Aβ42, are highly disordered peptides that cannot be studied with traditional NMR and X-ray techniques, it is trending to explore the reasons for differences between L- and D-amino acids using short peptides, as in our article. Using NMR, chemically induced dynamic nuclear polarization (CIDNP) and fluorescence techniques allowed us to detect the influence of tryptophan (Trp) optical configuration on the peptides fluorescence quantum yields, bimolecular quenching rates of Trp excited state, and the photocleavage products formation. Thus, compared with the D-analog, the L-isomer shows a greater Trp excited state quenching efficiency with the electron transfer (ET) mechanism. There are experimental confirmations of the hypothesis about photoinduced ET between Trp and the CONH peptide bond, as well as between Trp and another amide group. Full article
(This article belongs to the Section Physical Chemistry and Chemical Physics)
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17 pages, 4728 KiB  
Review
On the Question of Stepwise [4+2] Cycloaddition Reactions and Their Stereochemical Aspects
by Radomir Jasiński
Symmetry 2021, 13(10), 1911; https://doi.org/10.3390/sym13101911 - 11 Oct 2021
Cited by 41 | Viewed by 6624
Abstract
Even at the end of the twentieth century, the view of the one-step [4+2] cycloaddition (Diels-Alder) reaction mechanism was widely accepted as the only possible one, regardless of the nature of the reaction components. Much has changed in the way these reactions are [...] Read more.
Even at the end of the twentieth century, the view of the one-step [4+2] cycloaddition (Diels-Alder) reaction mechanism was widely accepted as the only possible one, regardless of the nature of the reaction components. Much has changed in the way these reactions are perceived since then. In particular, multi-step mechanisms with zwitterionic or diradical intermediates have been proposed for a number of processes. This review provided a critical analysis of such cases. Full article
(This article belongs to the Special Issue Regio- and Stereoselectivity in Cycloaddition Reactions)
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