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WHY DIELS-ALDER REACTIONS ARE NON-CONCERTED PROCESSES
2014
ABSTRACT Two exhaustive topological analyses using the Electron Localisation Function (ELF) along the non-polar Diels-Alder reactions of 1,3-butadiene [J. Phys. Chem. A 107 , 6014 (2003)] and cyclopentadiene [Org. Biomol. Chem. 8 , 5495, (2010)] with ethylene established that the bonding changes along these reactions are non-concerted. Herein, the ELF bonding changes along the intramolecular Diels-Alder reaction of 1-( hex-5-enyl)cyclohexa-1,3-diene is analysed. The geometrical restrictions imposed by the tether break the synchronicity in the single bond formation in this non-polar reaction. The present ELF topological analysis along this intramolecular Diels-Alder reaction supports the ear…
An Understanding of the Electrophilic/Nucleophilic Behavior of Electro-Deficient 2,3-Disubstituted 1,3-Butadienes in Polar Diels−Alder Reactions. A D…
2008
The electrophilic/nucleophilic behavior of dimethyl 2,3-dimethylenesuccinate 1, an electron-deficient 2,3-disubstituted 1,3-butadiene, in polar Diels-Alder reactions has been studied using DFT methods at the B3LYP/6-31G(d) level of theory. The electronic nature of bonding of the transition structures involved in the cycloaddition reactions of the diene 1 toward the nucleophilically activated dienophile 6 and the strong electrophilically activated dienophile 7 has been carefully examined within the natural bond orbital (NBO) and the topological analysis of the electron localization function (ELF) frameworks. Additionally, a study of the global electrophilicity pattern of the reagents at the …
ChemInform Abstract: Reactions of Pyrano(3,4-b)indol-3-ones with Dienophiles: Consecutive (4 + 2) Cycloaddition/Cycloreversion/1,2 Elimination.
1990
Methylpyrano[3,4-b]indol-3-ones 1 react with selected dienophiles (as acetylene equivalents) in a consecutive Diels-Alder/cycloreversion/1,2 elimination reaction sequence to furnish the 1-methyl- and 1,4-dimethylcarbazoles 4.
Understanding the reactivity of captodative ethylenes in polar cycloaddition reactions. A theoretical study
2008
The electrophilic/nucleophilic character of a series of captodative (CD) ethylenes involved in polar cycloaddition reactions has been studied using DFT methods at the B3LYP/6-31G(d) level of theory. The transition state structures for the electrophilic/nucleophilic interactions of two CD ethylenes toward a nucleophilically activated ethylene, 2-methylene-1,3-dioxolane, and an electrophilically activated ethylene, 1,1-dicyanoethyelene, have been studied, and their electronic structures have been characterized using both NBO and ELF methods. Analysis of the reactivity indexes of the CD ethylenes explains the reactivity of these species. While the electrophilicity of the molecules accounts for…
ChemInform Abstract: Regio- and Stereoselective Synthesis of Spiropyrrolizidines and Piperazines Through Azomethine Ylide Cycloaddition Reaction.
2016
A series of original spiropyrrolizidine derivatives has been prepared by a one-pot three-component [3 + 2] cycloaddition reaction of (E)-3-arylidene-1-phenyl-pyrrolidine-2,5-diones, l-proline, and the cyclic ketones 1H-indole-2,3-dione (isatin), indenoquinoxaline-11-one and acenaphthenequinone. We disclose an unprecedented isomerization of some spiroadducts leading to a new family of spirooxindolepyrrolizidines. Furthermore, these cycloadducts underwent retro-1,3-dipolar cycloaddition yielding unexpected regioisomers. Upon treatment of the dipolarophiles with in situ generated azomethine ylides from l-proline or acenaphthenequinone, formation of spiroadducts and unusual polycyclic fused pip…
Dehydrotriphenylene zum Aufbau gewinkelter molekularer Bandstrukturen
1998
Dehydrotriphenylenes for the Generation of Bent Molecular Ribbons The synthetic sequences 1 11 and 17 23a lead to highly reactive mono- and bisdienophiles whose cycloaddition processes with in situ generated isobenzofuran 13 were studied: 11 + 13 15 and 23a + 2×13 24a.
ChemInform Abstract: Stereoselective Synthesis of Quinolizidine Alkaloids: (-)-Lasubin II.
2008
Initially, lasubin II was synthesized diastereoselectively in racemic form [3]. Asymmetric syntheses of lasubin II were achieved based on stereoselective transformations of enantiomerically pure substrates [4] or, for example, via a diastereoselective aza-Diels-Alder reaction using a resolved chiral arylaldehyde tricarbonylchromium complex [5]. Recently, an enantioselectively catalyzed aza-Diels-Alder reaction [6] and a [2+2+2] cycloaddition reaction [7] were successfully used for the synthesis of enantiomerically pure lasubin II.
Stereoselective Synthesis of Quinolizidine Alkaloids: (–)-Lasubin II
2008
Initially, lasubin II was synthesized diastereoselectively in racemic form [3]. Asymmetric syntheses of lasubin II were achieved based on stereoselective transformations of enantiomerically pure substrates [4] or, for example, via a diastereoselective aza-Diels-Alder reaction using a resolved chiral arylaldehyde tricarbonylchromium complex [5]. Recently, an enantioselectively catalyzed aza-Diels-Alder reaction [6] and a [2+2+2] cycloaddition reaction [7] were successfully used for the synthesis of enantiomerically pure lasubin II.
Nanocomposite, an innovative route to large structure and infrastructure protection. A case of study: CNT-doped polyurea
2010
<p>In this work we focus on experimental studies of polymer-nanocomposites intended for passive protection of civil structures. The material consists of a polyurea-matrix enhanced by different quantities of multiwalled carbon nanotubes. Several tensile and cyclic tests are performed to evaluate and characterize the mechanical behaviour of this new material. Furthermore, we subject the material to accelerated thermal aging and study its performance deterioration which is fairly unknown. The results on the unaged specimens are intuitive and indicate that an arbitrary amount of CNTs may result in suboptimal performance. More specifically, 1% in weight of CNTs yields the best mechanical p…